SP_Solutions_-Adi.pdf

SOUNDARYA EDUCATIONAL TRUST (R)
SOUNDARYA INSTITUTE OF MANAGEMENT & SCIENCE
Soundarya Nagar, Sidedahalli, Hessaraghatta Main Road, Bangalore – 73
Department of Computer Science
SOLUTION BANK
BCA602T – System Programming
2 MARKS QUESTIONS AND ANSWERS
UNIT - 1
1. List components of a system software. (Repeated twice)
Components of system software are:
 Assemblers.
 Compilers.
 Loaders.
 Macro processor.
 Operating System.
2. Differentiate between processor and I/O channel.
Processor I/O Channel
A processor is a device that performs a
sequence of operations specified by
instructions in memory.
I/O channels need processor in order to
execute I/O instructions that are stored in
memory.
It is the internal components of the
system.
It is the external components of the
system.
Through I/O channels processor retrieves
and stores the data.
I/O channels will act as inter mediator
between I/O devices and the CPU.
3. Explain PSW. (Repeated four times)
PSW stands for Program Status Word contains the information required for proper
execution of a given program. It contains the value of the location, protection information
and interrupt status. The size of a PSW is 8 bytes.
4. Explain the advantages of using a base register.
Advantages of base register:
1. It helps in the process of relocation of a program.
2. Efficient addressing of core.
3. Saves 8 bits per address reference.
5. What is LTORG?
LTORG is a pseudo-op which tells the assembler to place the encountered literals at an
earlier location.

6. Define complier, assembler.
Compiler: Compiler is a program that accepts a source program in high level language
and produces its corresponding object program.
Assembler: Assembler is a program that translates the assembly language program
(Source Code) into machine language program (Object Code).
7. What is instruction interpreter?
Interpreter is a translator used for translating programs in high level language into machine
language. It interrupts the instruction line by line.
8. What is system software?
System Software is computer software designed to operate the computer hardware and
provides and maintains a platform for running application software.
9. What is location counter? What is its purpose?
Location counter is also known as Program counter(PC) or Instruction Counter is a
hardware memory device which holds the location of the current instructions being
executed.
10. List any two advantages of assembly language.
Advantages of Assembly Language:
1. It used mnemonic code.
2. Address are symbolic not absolute.
3. Reading is easier.
4. Introduction of data to program is easier.
5. It is not required to keep track of memory location.
11. Define system programming.
System programming involves designing and writing computer programs that allow the
computer hardware to interface with the programmer and the user, leading to the effective
execution of application software on the computer system.
12. Define open and closed subroutine.
Open Subroutine: An open subroutine is one whose code will be inserted at the point of
function call within the main definition.
Closed Subroutine: A closed subroutine will be stored outside the main routine and there
is a transfer in control to the subroutine for processing it.
13. Explain BCT and LTORG pseudo-ops (Refer. 5).
BCT: Indicates branch on count. It is a RX type instruction, whose size is 4 bytes.
14. Define DC and DS.
DC: DD stands for Define Constant, is a declarative pseudo-op used to create a memory
area to hold a constant value. That is it constructs memory words containing constants.
DS: DS stands for Define Storage, is a declarative pseudo-op that reserves some storage
for the data and gives them a name.

UNIT - 2
1. List all the variable tables and fixed tables used by an assembler.
Variable Tables: Literal Table(LT), Symbol Table(ST) and Base Table(BT).
Fixed Tables: Machine Op Table(MOT) and Pseudo Op Table(POT).
2. Write the format of POT.
8- bytes per entry
Pseudo-op(5-bytes)
Character
Address of routine to process pseudo-
op (3-bytes = 24 bits address)
“DROPb”
“ENDbb”
“EQUbb”
“START”
“USING”
P1DROP
P1END
P1EQU
P1START
P1USING
3. What is a symbol table? Give its format.
Symbol Table: It is a variable table that contains labels and its value. ST for Pass1 and
Pass2 are the same and contains symbol, value, length and relocation fields. Size of the
table is 14 bytes per entry.
14 - bytes per entry
Symbol (8-bytes)
character
Value (4-bytes)
Hexadecimal
Length (1-byte)
Hexadecimal
Relocation(1-byte)
character
“JHONbbbb” 0000 01 “R”
“FOURbbbb” 000C 04 “R”
“FIVEbbbb” 0010 04 “R”
“TEMPbbbb” 0014 04 “R”
4. What is Declaration Statement? Give example.
In programming, a declaration is a statement describing an identifier, such as the name of
a variable or a function
5. Mention any two disadvantages of Radix Sort.
Disadvantages:
1. Radix Sort that can make it less preferable than other sorts.
2. The speed of Radix Sort largely depends on the inner basic operations.
6. What is sorting?
Sorting is the process of arranging items of the table in some sequence, either in ascending
or descending order.

UNIT - 3
1. What is ALA?(Repeated twice) Explain its format.
ALA is a Argument List Array maintains the details about the parameters. ALA is used
during pass1 and pass2, but the functions are reverse in both the process.
Argument List Array 8 bytes per entry
Index Argument
0 “LOOP1bbb”
1 “DATA1bbb”
2 “DATA2bbb”
3 “DATA3bbb”
2. Define AIF and AGO.
AIF:AIF is a conditional branch pseudo-op. The format is as follows: AIF<expression>
<sequencing symbol>. It does not appear in the expanded source code.
AGO: AGO is an unconditional branch pseudo-op. The format is as follows:
AGO<sequencing symbol>. It does not appear in the expanded source code.
3. Differentiate between a macro and subroutine.
Macro Subroutine
Macro can be called only in the program it
is defined.
Subroutine can be called from other
programs also.
Macro can have maximum 9 parameters. Can have any number of parameters.
Macro can be called only after its
definition.
This is not true for Subroutine.
A macro is defined inside:
DEFINE …
….
END-OF-DEFINITION.
Subroutine is defined inside:
FORM …..
…..
ENDFORM.
Macro is used when same thing is to be
done in a program a number of times.
Subroutine is used for modularization.
4. What is Macro call?
A macro call consists of a name optionally followed by an actual parameter list. The
number of parameters in the actual parameter list must be the same as the number of formal
parameters specified in the definition of the macro.
5. Define Macro definition table.(MDT)
MDT is used to store the body of the macro definitions. It contains text lines. The size
of macro definition table is 80 bytes per entry. Every line in the macro definition, except
MACRO is stored in the MDT.
6. Write the four-basic task that can be performed by macro-instruction processor.
Four basic task that can be performed by macro-instruction processor:
1. Define macro.
2. Include macro name in the source program.

3. Expand Macro definition.
4. Compile or Assemble the Mcro.
7. What is Macro? Write down its syntax.
Macros allow a programmer to define pseudo operations, typically operations that are
generally desirable, are not implemented as part of the processor instruction, and can be
implemented as a sequence of instructions. Each use of a macro generates new program
instructions, the macro has the effect of automating writing of the program.
Syntax:
MACRO Start definition
[ ] Macro name
________
________ Sequence to be abbreviated
________
MEND End of definition
8. What is MDT (Ref. 5) and MNT?
MNT stands for Macro Name Table is used to store the names of the defined macros. It
is similar to machine op-table(MOT) and POT of the assembler. Each MNT entry
consists of a character string whose size is 8 bytes and a pointer to the entry in the MDT
that corresponds to the beginning of the macro definition. The size of he MDT is 4
bytes. Therefore, the size of the MNT is 12bytes per entry.
UNIT - 4
1. List the functions of a loader.(Repeated four times
Functions of Loader:
I. Allocation: Allocate space in memory for the programs.
II. Linking: Resolve symbolic references between object decks.
III. Relocation: Adjust all address dependent locations, such as address constants, to
correspond to the allocated space.
IV. Loading: physically place the machine instructions and data into memory.
2. Mention the four types of cards used in a direct linking loader.
Four types of cards used in a direct linking loader.
1. External Symbol Dictionary(ESD).
2. The Text Card(TXT).
3. The Relocation and Linkage Directory(RLD).
4. The END card.
3. What are overlays?
Overlaying is a programming method that allows programs to be larger than the computer's
main memory. An embedded system would normally use overlays because of the limitation
of physical memory, which is internal memory for a system-on-chip, and the lack of virtual
memory facilities.

4. What is dynamic loading?
Dynamic loading is a mechanism by which a computer program can, at run time, load a
library (or other binary) into memory, retrieve the addresses of functions and variables
contained in the library, execute those functions or access those variables, and unload the
library from memory.
5. Define Relocation factor.
The translated and linked origins of program P be t_origin (p) and l_origin (p),
respectively. Consider a symbol symb in P. Let its translation time be t (symb) and link
time address be l (symb). The relocation factor of P is defined as relocation_factor =
l_origin (p) – t_origin (p). This value can be positive, negative or zero.
UNIT - 5
1. What is intermediate form?
The process of generating the object code for each construction after determining the
syntactic construction is known as intermediate form.
2. List the three tasks of lexical phase.
The three tasks of Lexical phase:
1. To parse the source program into the basic elements or tokens of the language.
2. To build a literal table and an identifier table.
3. To build a uniform symbol table.
3. What are the three classes of uniform symbols?
The three classes of uniform symbols are:
1. Identifier(IDN).
2. Terminal Symbol(TRM).
3. Literal(LIT).
4. Define local and global optimization (Repeated twice).
Global optimization: Removing or deleting the duplicate entries in the matrix and
modifying all references to the deleted entries is one type of machine independent
optimization.
5. What is a compiler?
A compiler is a special program that processes statements written in a
particular programming language and turns them into machine language or "code" that
a computer's processor uses.
6. What are tokens? Give an example.
Tokens in C. Tokens are the smallest elements of a program, which are meaningful to the
compiler. The following are the types of tokens: Keywords, Identifiers, Constant, Strings,
Operators, etc.
7. What is code generation phase?
In computing, code generation is the process by which a compiler's code generator converts
some intermediate representation of source code into a form (e.g., machine
code) that can be readily executed by a machine. Sophisticated compilers typically
perform multiple passes over various intermediate forms.
*****

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SOUNDARYA EDUCATIONAL TRUST (R)
SOUNDARYA INSTITUTE OF MANAGEMENT & SCIENCE
Soundarya Nagar, Sidedahalli, Hessaraghatta Main Road, Bangalore – 73
Department of Computer Science
SOLUTION BANK
BCA602T – System Programming
5 MARKS QUESTIONS AND ANSWERS
UNIT – 1
1. Explain different data formats used in IBM 360 with an example.
Data is storing in memory as group of bits. This group of bits can be characters,
numbers or special characters. The different formats present in IBM 360 are:
a. Short form fixed point numbers: in short form fixed point 16 bits is
allocated to represent an integer number, out of this the first 1 bit is used the
sign 0 for + and 1 for – negative. The machine interprets the contents of
these two bytes as an integer. It represents the first bit as sign and the
remaining 15 bits as binary number.
S Integer
0 1 15
S is Sign bit
Ex: Decimal number +257 is represented as
--------------16 bits ----------------
0 000 0001 0000 0001
+ve Binary equivalent of 257
b. Long form fixed point numbers: In long form fixed point 32 bits are
allocated, out of this first one bit is used for sign bit(+ or -). The m/c
interprets the contents of this four bytes as an integer. It interprets the first
bit as sign and the remaining 31 bits are as a binary number.
S Integer
0 1 31
S is Sign bit
Ex: Decimal number +257 is represented as

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0 000 0000 0000 0000 0000 0001 0000 0001
+ve Binary equivalent of 257
c. Packed decimal numbers: Decimal digits, packed two to a byte, appear in
fields of variable length and are accompanied by a sign in the right most
four bits. Instead of representing the binary numbers, numbers are
represented in binary coded decimal format.
----------------------1 to 16 bytes ---------------------------
D D D S
0 4 8
True form binary coded decimal digit (4 bits) sign bit (4 bits)
Ex: Decimal Number -021 is represented as
---------- 2 bytes --------------
BCD of 021 sign bit (-)
Note: sign bit contains the BCD of hexadecimal digits A, B, C, D, E and F.
The hex digits C, A, F and E indicate a positive number while D and B
indicates a negative number.
d. Unpacked decimal numbers: The size of the unpacked numbers varies
from 1 to 16 bytes out of which the last 1 byte is reserved for sign and data.
Instead of representing the binary number, numbers are represented in
binary coded decimal.
----------------------1 to 16 bytes ---------------------------
Z D Z D S D
0 4 8 12 16
Zone code True form binary coded decimal sign bit
e. Short form floating point numbers: In short form floating point 32 bits
are allocated. Out of this 1 bit is reserved for sign (+ or -). The floating-
point number is divided into exponential and fractional part. This
representation gives a precision of seven decimal places.
0 1 8 31
S C F
Sign Characteristic Fraction
0000 0010 0001 1101

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f. Long form floating point numbers: In long form floating point 64 bits are
allocated for the floating-point number which contains the exponential and
fractional part. The long length of the fractional part gives up to 17 decimals
of precision, thus eliminating most requirements for double precision
arithmetic.
0 1 8 63
S C F
Sign Characteristic Fraction
g. Logical data: Characters are stored as 8-bit ASCII code each character
occupies 1 byte. The length of character data, varies from 1 to 256 bytes.
1 to 256 bytes
Ch Ch Ch
0 8 15
Character codes(0-7 bits)
2. Draw the general machine structure of IBM 360/370 with a neat diagram.
The structure of CPU for a typical Von Neumann Machine is as follows:

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The structure above consists of :
1. Instruction Interpreter
2. Location Counter
3. Instruction Register
4. Working Registers
5. General Register
The Instruction Interpreter Hardware is basically a group of circuits that
perform the operation specified by the instructions fetched from the memory.
The Location Counter can also be called as Program/Instruction Counter
simply points to the current instruction being executed. The working registers
are often called as the "scratch pads" because they are used to store temporary
values while calculation is in progress.
This CPU interfaces with Memory through MAR & MBR. MAR (Memory
Address Register) - contains address of memory location (to be read from or
stored into). MBR (Memory Buffer Register) - contains copy of address
specified by MAR. Memory controller is used to transfer data between MBR
& the memory location specified by MAR. The role of I/O Channels is to input
or output information from memory.
3. Explain various instruction formats used in IBM 360/370 machine.
The 360/370 has five instruction formats. Each format is associated with a
single addressing mode and has a set of operations defined for that format.
While some operations are defined in multiple formats, most are not.
 RR (register-register ): It denotes register to register operation. That is both
the operands are registers.
 RX (register-indexed: It denotes a register and indexed storage operations.
That is one operand is a register and another one storage operand.
 RS (register-storage ): It denotes register and storage operation.
 SI (storage-immediate): It denotes a storage and immediate operand operation.
 SS (storage-storage): It denotes a storage operation.
The general formats are given in below figure.

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4. Explain any 5 pseudo ops, used in an assembly language program.
Pseudo op is an assemble language instruction that specifies an operation of
the assembler.
1. USING: It is a Pseudo-op that indicates to the assembler which general
purpose register to use as a base register and what its contents will be in
order to convert the addresses to base-displacement format. Since no
special register is set aside for addressing, the programmer must inform the
assembler which register to use as a base register and what value it contains
at execution time.
Syntax: USING <contents of base register> <GPR to be used as base
register >
EX: USING *, 5
2. START: It is Pseudo-op that tells the assembler where the beginning of the
program and allow the user to give a name to the program.
Ex: TEST START {TEST is the name of the program}
3. END: It is Pseudo-op that tells the assembler that the last statement of the
program has been reached.
Ex: END
4. EQU: It is Pseudo-op which allows the programmer to define variable. The
operand of the EQU statement can be any constants or valid arithmetic
expressions.
Ex: BASE EQU 15

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5. DC(Define Constant): DC is declarative Pseudo-op used to create a
memory area to hold a constant value. That is it constructs memory words
containing constants.
Syntax: <Label> DC ‘Constant’
Ex: FOUR DC F’4’
6. DS(Define Storage): DS is declarative Pseudo-op that reserves some
storage for the data and gives them a name.
Syntax: <Label> DS SIZE
Ex: BLOCK DS 100F
7. DROP: It is a Pseudo-op which indicates an unavailable base register.
Syntax: DROP <BR number>
Ex: DROP 15
8. LTORG: It is a Pseudo-op which tells the assembler to place the
encountered literals at an earlier location.
5. Explain open subroutine and closed subroutine with an example.
Open subroutine:
An open subroutine or macro description is one whose code is inserted (flow
continues) into the main program. Therefore, if four times were called the same
open subroutine, it would appear in the calling system in four different places.
Closed subroutine:
A closed subroutine can also be stored outside the main routine, and
monitor transfers to the subroutine. Associated with the closed subroutine are
the two tasks the main program should perform a transfer of control & transfer
of data.
Open Subroutine Closed Subroutine
Open subroutine of macro definition
is one whose code will be inserted at
the point of function call within the
main definition
A closed subroutine will be stored
outside the main routine and there is
a transfer in control to the subroutine
for processing it
If the macro definition is very large
and if you call these functions
frequently shortage of memory may
occur.
Large size macros can be executed,
any number of times

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Saves time because there is no
overhead of program control transfer
and return
There is an overhead of transferring
the control to the function and
returning back which takes time
Performs only one task i.e insert a
macro
Performs two tasks that is transfer of
control and transfer of data
Wastage of memory Saves memory
These are loaded into memory at
different memory locations. i.e based
on location of calling macro
These are loaded into memory at a
specific address.
6. Explain Long-way-no-lopping with example.
The straight forward approach is without using any looping statements.
The implementation is easy. Instructions are repeated for all the data items. It
is impossible to access the first data item and the last data item using register 1
as the base. Wastage of memory. Need of relocation. Instruction would
overlap data in the core. Suppose you wanted to process 300data items, then
the storage needed for the instructions.
=instruction * number of data
=instruction + size of instruction
= 3*300*4
=3600 bytes
7. Explain address modification using instruction as data.
In straight forward approach we used 3 instructions, i.e. load, add and store
which was repeated. The only change was I the offset of load and store
instructions, i.e. first offset was 904, then 908, then 912 etc.,
Another approach to the problem is to write a program consisting only
of those 3 instructions followed by a sequence of commands that would change
the offset of the load and store instruction by adding 4 to them.
Advantages:
1. Saves memory.
2. Address is modified easily using the instruction.
Disadvantages:
1. Treating instructions as data is not a good programming practise because
over a period of time, it is difficult to understand what the original
programme was.
2. Separate instructions should be used for increasing the displacement of load
and store instructions.

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8. Explain the use of literals in assembly language programs using example.
Literals: In many instructions, it is possible to use a shorthand notation in
place of a label for the source memory operand. A literal look like a DC
operand but is preceded by a ‘=’ sign. While using literals, the assembler
creates data areas containing the constants.
Ex:
L 3, TEN
TEN DC F ‘10’
Can be replaced by
L 3, =F’10’
Where =F’10’ is a literal and a data area whose size is 1 full word, containing
the value 10 is created. In the load instruction, the address part points to a full
word containing the value 10.
Literal Table: All the constants that have been specified through literals will
be placed in a table called literal table. Usually it is placed at the end of the
program. LTORG is a pseudo-op which tells the assembler to place the
encountered literals at an earlier location. It is used when the program is very
long. To avoid addressability problems in multimodular programs, LTORG
instruction is coded as the last instruction of each control section in the
program. So, each control section in the program will have its own literal pool
at the end of the module.
9. Differentiate between Pseudo-op and Machine-op with example.
Pseudo-op Please Refer Question no 4.
Machine-op: Machine-op represents to the assembler a machine instruction.
Machine instructions should be included in the program, to load the proper
value into the base register at execution time.
The different machine-op’s are:
1. BALR: It is a branch and link instruction. It is an instruction to the
computer to load a register with the next address and branch to the address
specified in the second field. If the second operand is register o, then
execution proceeds with the next instruction. BALR loads the base register
and is not an executable statement. It is an RR type instruction; whose
length is 2 bytes. Ex: BALR BASE, 0
2. BR: BR is a machine-op indicating branch to the location whose address is
in general register. The calling programs leaves their return address in the
corresponding general register. Ex: BR 14
3. BCT: It indicates branch on count. It is a RX type instruction; whose size
is 4 bytes. Ex: BCT 3, LOOP

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UNIT – 2
1. Draw an overview flowchart for Pass1 of an assembler.
In general, an assembler must do the following tasks:
1. Generate Instructions:
a. Evaluate the mnemonic of the operations field to produce its machine
code.
b. Evaluate the sub fields- find the value of each symbol, process literals
and assign addresses.
2. Process pseudo ops:
These tasks can be grouped into two passes, associated with each task is one
or more assembler modules. Pass is a sequential scan over the input.
Pass1: Purpose of pass1 is to define symbols and literals.
1. Determine length of machine instructions (MOTGET1)
2. Keep track of location counter (LC)
3. Remember values of symbols until pass2 (STSTO)
4. Process some pseudo-ops, eg, EQU, DS (POTGET1)
5. Remember literals (LITSTO)

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2. Explain radix sort with suitable example.
Radix exchange sort is a distributive sorting method used when the keys
are represented in binaries. Sorting is accomplished by considering groups
with the same (M) first bits and ordering that group with respect to the (M+1)s
bit. The ordering of a group on a given bit is accomplished by scanning down
from the top of the group for a one bit and up from the bottom for a zero bit,
these two are exchanged and the sort continues.
Ex: 1. sort array with respect to left most bit
1
1
0
1
0
2. Partition array
3. Recursion
Recursively sort top sub array ignoring left most bit.
Recursively sort bottom sub array ignoring left most bit
Sorting is done like partitioning in the quick sort.
The Method is Repeat
Scan top-down to find key starting with 1.
Scan bottom-up to find key starting with 0.
Exchange keys. Until scan indicated cross.
1
1
0
1
0
0
0
1
1
1
0
0
1
1
1
0
0
1
1
1
0
1
0
1
1
0
1
0
1
1
0
0
1
1
1
First exchange Second exchange

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3. Draw the detailed Pass 2 flowchart of an assembler.
Pass2: Generate code: The purpose of Pass2 is to
1. Process each card and to find the values for its operation code and its
operand field.
2. Structuring the generated code into the appropriate format for later
processing by the loader.
3. To print an assembly listing containing the original source and the
hexadecimal equivalent of the bytes generated.

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4. Explain databases used by Pass1 and Pass2 of an assembler.
Pass1 Database:
1. Input source program.
2. A Location Counter (LC), used to keep track of each instruction’s location.
3. A table, the Machine-operation Table (MOT), that indicates the symbolic
mnemonic for each instruction and its length.
4. A table, the Pseudo-Operation Table (POT), that indicates the symbol
mnemonic and action to be taken for each pseudo-op in pass1.
5. A table, the Symbol Table(ST), that is used to store label and its
corresponding value.
6. A table, the Literal Table(LT), that is used to store each literal encountered
and its corresponding assigned location.
7. A copy of the input to b used by pass2. This may be stored in a secondary
storage device.
Pass2 Database:
1. Copy of the source program input to pass1.
2. Location Counter(LC).
3. A table, the MOT that indicates for each instruction, symbolic mnemonic,
length, binary machine op code and format.
4. A table, the POT, that indicates for each pseudo-op the symbolic mnemonic
and the action to be taken in pass2.
5. The ST, prepared by pass1, containing each label and its corresponding
value.
6. A table the Base Table(BT), that indicates which registers are currently
specified as base register by USING pseudo-ops and what are specified
contents of these registers.
7. A work-space, INST, that is used to hold each instruction as its various parts
are being assembled together.
8. A work- space, PRINT LINE, used to produce a printed listing.
9. A work-space, PUNCH CARD, used prior to actual outputting for
converting the assembled instructions into the format needed by the loader.
10.An output deck of assembled instructions in the format needed by the
loader.

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5. Illustrate Binary Search with an example.
Binary search algorithm searches for an item only in an ordered list. It starts
with the middle entry in the table, and compares it with the search element.
This give rise to 3 conditions. They are
1. The search element may be equal to the middle element.
2. The search element may be greater than the middle element.
3. The search element may be less than the middle element.
The next actions taken for each of these out comes is as follows.
1. If the search element is equal to the middle element, the search element is
found.
2. If the search element is greater than the middle element, use the bottom half
of the given table as a new table to search.
3. If the search element is smaller than he middle element, use the top half of
the table as a new table to search.
This method, effectively divides the table in half on each pass, systematically
bracketing the item searched for. The search is terminated with a ’not found’
condition when the length of the last sub table searched is down to 1 and the
item has not been found.
For a binary search to work, it is mandatory for the target array to be sorted.
We shall learn the process of binary search with a pictorial example. The
following is our sorted array and let us assume that we need to search the
location of value 31 using binary search.
First, we shall determine half of the array by using this formula −
mid = low + (high - low) / 2
Here it is, 0 + (9 - 0 ) / 2 = 4 (integer value of 4.5). So, 4 is the mid of the
array.
Now we compare the value stored at location 4, with the value being
searched, i.e. 31. We find that the value at location 4 is 27, which is not a

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match. As the value is greater than 27 and we have a sorted array, so we also
know that the target value must be in the upper portion of the array.
We change our low to mid + 1 and find the new mid value again.
low = mid + 1
mid = low + (high - low) / 2
Our new mid is 7 now. We compare the value stored at location 7 with our
target value 31.
The value stored at location 7 is not a match, rather it is more than what we
are looking for. So, the value must be in the lower part from this location.
Hence, we calculate the mid again. This time it is 5.
We compare the value stored at location 5 with our target value. We find
that it is a match.
We conclude that the target value 31 is stored at location 5.

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6. List different types of sorting. Explain Interchange sort.
Different types of sorting:
1. Interchange sort.
2. Shell sort.
3. Bucket sort.
4. Radix exchange sort
5. Address calculations sort.
Interchange Sort: It is also known as bubble sort, sinking sort or shifting
sort. It sorts by comparing each adjacent pair of items in a list and swapping
the items if necessary, and repeating the pass through the list until no swap are
done. After each pass at least one item is added to the bottom of the list in a
perfect order. Thus, the name sinking sort, because the elements sinks down
to the proper position.
To arrange a list of items in ascending order using interchange sort, the
first element is compared with the second element. If the first element is
greater then the second, the elements are interchanged Next, the second
element is compared with the third and the process continues until all the
elements of the list are compared. After the end of this (pass1), the largest
element bubbles down to the last position.
In the second pass, the same procedure is followed leaving out the last
element since it is already placed in the correct position. This pass brings the
second largest number to the last but one position. This procedure is repeated
until there are no more elements for comparison.
7. Explain shell sort with an example.
Shell sort is a highly efficient sorting algorithm and is based on insertion sort
algorithm. This algorithm avoids large shifts as in case of insertion sort, if the
smaller value is to the far right and has to be moved to the far left.
Let us consider the following example to have an idea of how shell sort works.
We take the same array we have used in our previous examples. For our
example and ease of understanding, we take the interval of 4. Make a virtual
sub-list of all values located at the interval of 4 positions. Here these values
are {35, 14}, {33, 19}, {42, 27} and {10, 44}

16
We compare values in each sub-list and swap them (if necessary) in the
original array. After this step, the new array should look like this −
Then, we take interval of 1 and this gap generates two sub-lists - {14, 27, 35,
42}, {19, 10, 33, 44}
We compare and swap the values, if required, in the original array. After this
step, the array should look like this −
Finally, we sort the rest of the array using interval of value 1. Shell sort uses
insertion sort to sort the array.

17
Following is the step-by-step depiction −
We see that it required only four swaps to sort the rest of the array.

18
8. Explain Pass 2 overview of an assembler with flow-chart.
Pass2: Purpose of pass2 is to generate object program.
1. Look up value of symbols(STGET).
2. Generate instructions(MOTGET).
3. Generate data (for DS, DC and literals).
4. Process pseudo ops(POTGET2).

19
9. Draw the detailed Pass1 flowchart of an assembler.
Pass1: Define symbols: The important purpose of pass1 is to
1. Assign a location to each instruction and data and to define pseudo
instructions.
2. To define values for symbols appearing in the label field of the source
program.
Detailed Pass1 Flowchart

20
10.Give format of all five tables used in assembler.
Assembler consists of five tables. MOT, POT, ST, LT and BT.
MOT: Machine Operation Table:
MOT is a fixed table that contains the mnemonic of each instruction. Fixed
table is a table, whose contents are not filled in or altered during the assembly
process.
Format of MOT
-----------------------------6 bytes per entry -----------------------------------
Mnemonic
op-code
(4 bytes)
(Characters)
Binary op-
code (1 byte)
(Hexadecimal)
Instructions
Length
(2bits)
(Binary)
Instruction
format
(3bits)
(Binary)
Not
used in
this
design
(3 bits)
“Abbb”
“AHbb”
“ALbb”
“ALRb”
“ARbb”
-----
“MVCb”
----
5A
4A
5E
1E
1A
---
D2
---
10
10
10
01
01
---
11
----
001
001
001
000
000
---
100
---
b~ represents the character “blank”
codes: Instruction Length Instruction Format
01= 1 half-words = 2 bytes
MOT for both Pass1 and Pass2
POT: Pseudo-Operation Table
The POT is fixed table that contains the pseudo-ops and corresponding actions.
POT for pass1 and pass2 are the same and it contains the name and address.
Size of POT is 8 bytes per entry.
000=RR
001=RX
010=RS
011=SI
100=SS

21
Format for POT
--------------------8 bytes per entry------------------------------
Pseudo-op
(5 bytes)
(character)
Address of routine to process
pseudo-op
(3-bytes=24 bit address)
“DROPb”
“ENDbb”
“EQUbb”
“START”
“USING”
P1DROP
P1END
P1EQU
P1START
P1USING
POT is similar for Pass1 and Pass2
ST: Symbol Table
ST is a variable table that contains labels and its values. ST for pass1 and pass2
are the same and contains symbol, value, length and relocation fields. Size of
the table is14 bytes per entry.
ST Format:
---------------------14 bytes per entry-------------------------------
Symbol
(8 bytes)
Character
Value
(4 bytes)
Hexadecimal
Length
(1 byte)
Hexadecimal
Relocation
(1 byte)
Character
“JOHNbbbb” 0000 01 “R”
“FOURbbbb” 000C- 04 “R”
“FIVEbbbb” 0010- 04 “R”
“TEMPbbbb” 0014- 04 “R”
ST is similar for Pass1 and Pass2
LT: Literal Table
LT is a variable table that contains literals and its value. It is same like symbol
table but instead of symbols, here we have literals.
LT Format:
---------------------14 bytes per entry-------------------------------
Symbol
(8 bytes)
Character
Value
(4 bytes)
Hexadecimal
Length
(1 byte)
Hexadecimal
Relocation
(1 byte)
Character
F’4’ 4 04 “R”

22
BT: Base Table
BT is a variable table that specifies the base register. It is used only in pass2.
The size of it is 4 bytes per entry.
BT Format:
---------------------4 bytes per entry-------------------------------
Availability
indicator
(1byte)
Character
Designated relative-address
contents of base register
(3 bytes=24 bit address)
Hexadecimal
1 “N” ---
2 “N” ---
: :
14 “N” --
15 “Y” 00 00 00
Availability indicator, Y means register specified in USING pseudo-op and
N means register never specified in USING pseud-op.

23
UNIT – 3
1. Write an algorithm for Pass1 processing macro definitions.
Pass 1 Algorithm:
-by-line scan over its input.
as MNTC=1.
-op, the entire macro definition except this (MACRO)
line is stored in MDT.
location of MDT entry of the definition.
-op is encountered all the macro definitions have been
processed, so control is transferred to pass2.
2. Explain implementation of macro processor within assembler.
The macro processor can be implemented within an assembler using two
different ways,
1.It can be added as a pre-processor to an assembler, making a complete pass
over the input text before pass1 of the assembler.
2.It can be implemented within pass1 of the assembler.

24
Advantages of incorporating the macro processor into pass1 of assembler:
1.Many functions do not have to be implemented twice.
2.There is less over head during processing.
3.Functions are combined and it is not necessary to create intermediate files as
output from the macro processor and input to the assembler.
4.More flexibility is available to the programmer in that he/she may use all the
features of the assembler in conjunction with macros.
Disadvantages of incorporating the macro processor into pass1 of
assembler:
1.The program becomes too large to fit into the core of some machines.
2.The program becomes complex with macros.
3. Explain macro facility with macro instruction argument with an example.
Following are the important features of macro:
1. Macro instruction arguments 2. Conditional Macro Expansion
3. Macro calls within macros 4. Macro Instructions Defining Macros
Macro Instruction Arguments : The macro which we have seen before lacks
flexibility, because all of the calls to any given macro will be replaced by
identical blocks. There is no way for a specific macro call to modify the coding
that replace it.
In order to overcome this problem, we use macro instruction in macro calls.
The corresponding macro dummy arguments call appear is macro definitions.
Two methods of arguments:
1.Positional Arguments: These are matched with the dummy arguments
Ex: INCR A,B,C
2.Keyword Arguments: Allows reference to dummy arguments by name as
well as by position.
Ex: INCR &ARG1=A, &ARG2=B, &ARG3=C.

25
4. Explain macro definitions with an example.
Macro instruction definition defines a name for sequence of operations.
1. Macro definition starts with MACRO pseudo-op, it indicates the beginning
of a macro definition.
2. Following the MACRO pseudo-op is the name of the macro.
3. Next to macro name sequence of instructions to be abbreviated. These
instructions are compressed macro instructions.
4. The macro definition is terminated with MEND pseudo-op, which indicates
the end of macro definition.

26
5. Explain simple one pass macro processor.
A Single pass Algorithms for Macro Definitions within Macro.
case of macro definition within the macro, the inner macro is defined only
after the outer macro has been called.
for any use of inner macro, we have to repeat both macro definition
and the macro call passes.
order to avoid this repetition we can reduce all macro processing to single
pass.
macro definitions must be processed before calls because macros must
be defined to the processor before it can expand calls.
Additional Data Structure Included
MDI-Macro Definition Input indicator: It keep track of macrocalls.
MDI=“ON” while expanding a macro call otherwise MDI=“OFF”.
MDLC-Macro Definition Level Counter: It is used to keep track of macro
definitions. It is a counter for MACRO and MEND pseudo-ops, when MACRO
pseudo-op is encountered, MDLC is incremented by one and is decremented
by one when MEND pseudo-op is encountered.
MDT format:
Index Card
1
2
3
4
5
6
7
8
9
10
DEFINE &SUB
MACRO
#1 &Y
CNOP 0,4
BAL 1,*+8
DC A(&Y)
L 15=V(#1)
BALR 14.15
MEND
MEND

27
MNT Format:
Index Name MDT Index
1 DEFINE 1
6. Explain conditional macro expansion.
Conditional Macro Expansion
sequence of macro expansion scan be reordered based on some
conditions using conditional macro expansion.
allows conditional selection of the machine instructions that appear in
expansions of a macro call.
macro expansion two pseudo-op are used AIF and AGO.
AIF is a conditional branch pseudo-op.
Format: AIF<Expression> <Sequencing Symbol>
Ex: AIF(&COUNTEQ1)FIN
Where expression is a relational expression.
AGO is an unconditional branch pseudo-op.
Format: AGO <Sequencing Symbol> Ex: AGOFIN

28
7. Explain macro instructions defining macros with an example.
definition, which itself is a sequence of instruction, can be abbreviated
by using macro.
is important that inner macro is not defined until the outer macro is called.
user can call the above macro with the statement DEFINE MACRO
ADD1
this call is executed then the macro ADD1 will be defined. So after the
above call user can call the ADD1 macro as following:
ADD1DATA1
The microprocessor will generate the sequence as
L1,DATA1
A1,=F’10’
ST1,DATA1
8. Give the database specifications for Pass1 and Pass2 of macro processor.
Pass1–Databases:
1.The input macro source deck.
2.The output macro source deck copy for pass2.
3.MDT (Macro Definition Table) to store the body of the macro definitions.
4.MNT(Macro Name Table) to store the names of defined macros.
5.MDTC(Macro Definition Table Counter) to indicate the next available entry
in MDT.
6.MNTC(Macro Name Table Counter) to indicate the next available entry in
MNT.
7.ALA(Argument List Array) to substitute index markers for dummy
arguments before storing a macro definition.

29
Pass 2 –Databases:
1.Copy of the input macro source deck.
2.Output expanded source deck to be used as input to the assembler.
3.MDT created by Pass1.
4.MNT created by Pass1.
5.MDTP(Macro Definition Table Pointer) used to indicated the next line of text
to be used during macro expansion.
6.ALA to substitute macro call arguments for the index markers in the stored
macro definition.
9. Explain the four basic tasks of microprocessor.
Basic Tasks/ Functions of a Macro processor: There are four basic tasks or
functions that any macro instruction processor must perform.
Recognize macro definition: A macro instruction processor must recognize
macro definition identified by MACRO and MEND pseudo-op.
Save the definitions: The processor must store the macro-definitions which
will be needed at the time of expansion of calls.
Recognize the Macro call: The processor must recognize macro calls that
appear as operation mnemonics.
Expand calls and substitute arguments: The macro call is replaced by the
macro definition. The dummy arguments are replaced by the actual data.
10.Explain the terms macro definition, macro call, macro expansion with syntax
and example.
Macro definition Refer Question no 4
Macro Call
Once the macro has been defined, the use of macro name as an operation
mnemonic in an assembly program is equivalent to the corresponding
instruction sequence. i.e., in the program, the repeated instruction sequence are
written with the macro name. This is called as macro call or macro
invocation.

30
Ex:
Macro Expansion
Whenever there is a macro call the macro processor substitutes the macro
definition in the place of the macro call. This is called as Macro Expansion.
•The macro definition itself appear in the expanded source code.
•The definition is saved by the microprocessor.
EX:Macro will expand the code as following in the source code.

31
UNIT – 4
1. Explain the design of Absolute Loader with a neat diagram.
Absolute Loader:
In this scheme the object program is placed in secondary devices. The loader
only accepts the machine language text & places into core at the location
prescribed by the assembler.
Disadvantage is that the programmer must specify the load address in the
program & also if there are multiple subroutines, the programmer must
remember the address of each & use that in other subroutines to perform
subroutine linkages.
Design of Absolute Loader:
Absolute loader performs the tasks of allocation, relocation and linking.
Therefore, it is only necessary for the loader to read cards of the object deck
and move the text on the cards into the absolute locations specified by the
assembler.
There are two types of information that the object deck communicate from the
assembler to the loader.
1. It must convey the machine instructions that the assembler has created along
with the assigned core locations.
2. It must convey the entry point of the program, which is where the loader is to
transfer control when all instructions are loaded.

32
2. Write a note on direct linking loader.
• It is a general relocatable loader, and is perhaps the most popular loading
scheme presently used.
• It has advantage of allowing the programmer multiple procedure segments
and multiple data segments.
• Other segments provide flexible intersegment referencing and accessing
ability, while translation of the programs.
• The assemble must give the loader the following information with each
procedure or data segment:
1. The length of object code segment.
2. A list of all the external symbol (could be used by other segment).
3. A list of all the external symbol (used by the segment)
4. Information about address constants.
5. The machine code translation of the source program.

33
The assembler produces four types of cards in the object deck
• External Symbol Dictionary(ESD): It contains information about all
symbols that are defined in the program but referred somewhere.
• The Text Cards(TXT): It contains the actual object code translated version
of the source program.
• The Relocation and Linkage Directory(RLD): Contain information about
locations in the program whose contents depend on the address at which the
program is placed.
• The END card: Indicates the end of the object deck and specifies the
starting address for execution.
3. Draw a flow chart for Pass2 of Loader.
Detailed Pass2 flowchart of Loader:

34
4. Explain compile-and-go loader with a diagram.
Method of performing the loader functions is to have the assembler run
in one part of memory and place the assembled machine instructions and data,
as they are assembled, directly into their assigned memory locations. When
the assembly is completed, the assembler causes a transfer to the starting
instruction of the program. This is simple solution, involving no extra
procedures. It is used by the complier and several other language processors.
It is relatively easy to implement. The assembler simply places the code
into core, and the “loader” consists of one instruction that transfers to the
starting instruction of the newly assembled program is called “compile-and-
go or assemble-and-go”.
Advantages:
1. It is relatively easy to implement.
2. It is simple and easy to follow.
Disadvantages:
1. A portion of the memory is wasted.
2. It is necessary to retranslate the program every time.
3. It is very difficult to handle multiple segments, specifically when they are in
different languages, so it is very difficult to produce modular programs.

35
5. Explain the databases used by Pass1 and Pass2 of a direct linking loader.
Pass 1 Data Bases:
1. Input object decks.
2. A parameter, the Initial Program Load Address(IPLA) supplied by the
programmer or OS, that specifies the address to load the first segment.
3. A Program Load Address (PLA) counter, used to keep track of each
segment’s assigned location.
4. A table, the Global External Symbol Table(GEST), that is used to store each
external symbol and its corresponding assigned core address.
5. A copy of the input to be used later by pass 2. this may be stored on an
auxiliary storage device, such as magnetic tape, disk, drum or the original
object decks may be reread by the loader a second time for pass 2.
6. A printed listing, the load map, that specifies each external symbol and its
assigned value.
Pass 2 Data Bases:
1. Copy of object programs inputted to pass 1.
2. The Initial Program Load Address parameter (IPLA).
3. The Program Load Address Counter(PLA).
4. The Global External Symbol Table(GEST), prepared by pass 1, containing
each external symbol and its corresponding absolute address value.
5. An array, the Local External Symbol Array(LESA), which is used to
establish a correspondence between the ESD ID numbers, used on ESD and
RLD cards, and the corresponding external symbol’s absolute address
value.
6. Explain dynamic loading.
In each of the loader schemes we have assumed that all of the subroutines
needed are loaded into the memory at the same time. If the total amount of
memory required by all these subroutines exceeds the amount available, then
there is trouble. Then we use dynamic loading schemes to solve this problem.
Dynamic loading is also called load-on-call. This is done to save
memory. If all the subroutines are loaded simultaneously, a lot of space is taken
up. But only one is used at a time. So, here only the required subroutines are
loaded. To identify the call sequence, we use a data structure called OVERLAY
STRUCTURE. It defines mutually exclusive subroutines. So, only the ones
needed are loaded and a lot of memory is saved. In order for the overlay to
work, it is necessary for the module loader to load the various subroutines as
they are needed.

36
7. Describe four types of cards used in direct linking loader.
Four types of cards used in direct linking loader:
1. External Symbol Dictionary(ESD): It contains information necessary to
build the external symbol dictionary or symbol table. External symbols are
symbols that can be referred beyond the subroutine level. The normal labels
in the source program are used only by the assembler and information about
them if not included in the object deck.
There are 3 types of external symbol:
1. Segment Definition (SD) name on START or CSECT card.
2. Local Definition(LD) specified on ENTRY card.
3. External Reference(ER) specified on EXTRN card.
2. The Text Cards(TXT): It contains the block of data and the relative
address at which the data is to be placed. Once the loader has decided where
to load the program, it merely adds the Program Load Address(PLA) to the
relative address and moves the data into the resulting location. The data on
the TXT card may be instructions, nonrelocated data, or initial values of
address constants.
3. The Relocation and Linkage Directory(RLD): Contain information about
1. The location and length of each address constants that needs to be
changed for relocation or linking.
2. The external symbol by which the address constant should be modified.
3. The operation to be performed.
4. The END card: Indicates the end of the object deck and specifies the start
of execution point for entire program. This address is recorded on the END

37
card. There is a final card required to specify the end of a collection of
object decks. The 360 loaders usually use either a Loader Terminate(LDT)
or End of File (EOF).
8. Write a short note on Relocating loaders.
Relocating Loader:
o Loaders that allow for program relocation are called
relocating loaders or relative loaders.
o Two methods for specifying relocation as part of the object program
Transfer Vector (Modification records)
o Suitable for a small number of relocations required
o When relative or immediate addressing modes are extensively used
Relocation bits
o Suitable for a large number of relocations required
o When only direct addressing mode can be used in a machine with
fixed instruction format (e.g., the standard SIC machine)
Example:
In the main memory, already OS occupies the memory from 0 to 200KB.Now the object
module OM has to be placed over the OS from 200KB onwards. But OM has been placed
from 0 to 200KB.Now there is an overlap, this leads to problems and confusions. The
module OM has to be addressed from 200KB to 400KB.So the starting address of OM is
200.This is called relocation.
Advantages:
• Relocation bits are used to solve the problem of relocation , transfer vector is
used to solve the problem of linking & program length information to solve
allocation.
Disadvantages:
1. Not suited for loading external data. Transfer vector increases the size.
2. Does not facilitate access to data segments that can be shared
3. It is necessary to allocate, relocate, link and load all the subroutines each time,
in order to execute a program.

38
9. Define binder. What are the classes of binders? Explain.
Binders: It is a program similar to direct-linking loader in binding subroutine
together, but rather than placing the relocated and linked text directory into
memory, it outputs the text as a file or card deck. This output file is in a format
ready to be loaded and typically called a load module.
The binder essentially performs the functions of allocation, relocation
and linking the module loader merely the function of loading.
There are two major classes of binders:
1. Core Image Builder: the simplest type produces a load module that looks
very much like a single absolute loader deck. This means that the specific
core allocation of the program is performed at the time that the subroutines
are bound together. Since this kind of module looks like an actual “snapshot
of image” of a section of core, it is called core image module and the
corresponding binder is called a core image builder.
2. Linkage Editor: A more sophisticated binder, called a linkage editor, can
keep track of the relocation information so that the resulting load module,
as an ensemble, can be further relocated and thereby loaded anywhere in
core. In this case the module loader must perform additional allocation and
relocation s=as well as loading, but it does not have to worry about the
complex problems of linking.
10.Explain the use of EXTERN and ENTRY statements.
ENTRY and EXTRN statements Consider an application program AP
consisting of a set of programs unit’s SP = {P (i)}. A program unit P (i)
interacts with another program unit P (j) using addresses of P (j)’s instructions
and data in its own instructions. To realize such interactions, P (j) and P (i)
must contain public definitions and external references as defined in the
following.
Public definition: a symbol pub_symb defined in a program unit which may
be referenced in other program units. This is denoted with the keyword
ENTRY and in the e.g. it is TOTAL.
External reference: a reference to a symbol ext_symb which is not defined in
the program unit containing the reference. This is denoted with the keyword
EXTRN and in the e.g. it is MAX and ALPHA.
These are collectively called subroutine linkages. They link a subroutine which
is defined in another program.
EXTRN: EXTRN followed by a set of symbols indicates that they are
definedin other programs, but referred in the present program.
ENTRY: It indicates that symbols that are defined in the present program are
referred in other programs.

39
UNIT – 5
1. What are the functions of analysis and synthesis phases of compiler.
Analysis phase reads the source program and splits it into multiple tokens and
constructs the intermediate representation of the source program.
And also checks and indicates the syntax and semantic errors of a source
program.
It collects information about the source program and prepares the symbol
table. Symbol table will be used all over the compilation process.
This is also called as the front end of a compiler.
Synthesis phase: It will get the analysis phase input (intermediate
representation and symbol table) and produces the targeted machine level
code.
This is also called as the back end of a compiler.
A compiler can have many phases and passes.
 Pass : A pass refers to the traversal of a compiler through the entire
program.
 Phase : A phase of a compiler is a distinguishable stage, which takes
input from the previous stage, processes and yields output that can be
used as input for the next stage. A pass can have more than one phase.

40
2. Write a note on assembly phase of compiler.
The process of generating the actual code is known as Assembly phase.
1. The code generation phase is producing assembly language, the compiler is
generating references to actual code in place of literals and variables
assigned by a storage allocation routine.
2. The assembly phase must resolve the label references in the object program,
format the appropriate information for the loader. If the code generator
simply generates symbolic machine names instructions and labels, the
assembly phase must
 Resolve label references.
 Generate binary machine instruction.
 Convert literals.
 Calculate addresses.
 Generate storage.
3. Explain intermediate form of compiler.
The process of generating the object code for each construction after
determining the syntactic construction is known as Intermediate form.
Advantages of Intermediate form:
 This facilitates optimization of object code.
 It allows logical separation between the machine-independent phase and
machine-dependent phase.
The Intermediate form depends on syntactic construct such as:
1. Arithmetic statements.
2. Non-Arithmetic statements.
3. Non-Executable statements.
Arithmetic statements: One intermediate form of the arithmetic statement is
a parse tree.
The rules for converting arithmetic statement into a parse tree are:
1. Any variable is a terminal node of a tree.
2. For every operand, construct a binary tree whose left branch is the tree for
operand1 and whose right branch is for opernad2.

41
Use algebra rules to construct the tree:
1. Highest priority is given to the expression in the ().
2. * and / are having second priority.
3. + and – are having third priority.
4. If the sequence of the operators is same, then start solving from left to right.
Consider the ex. A= B+C
Then parse tree is
Non-Arithmetic statements: The statements DO, IF, GOTO etc. can all
replaced by a sequentially ordering of individual matrix entries.
RETURN (COST);
END;
Non-Executable statements:
1. It declares the compiler information that clarifies the referencing or
allocation of variables and associated storage.
2. There is no intermediate form of the statement, but the information
contained in a non executable statement is entered into tables, to be used by
other parts of the compiler.
Ex: DECLARE (COST, RATE, START, FINISH) FIXED BINARY (31)
STATIC;
Fixed binary datatype, 31 bits length and static storage.
Matrix
Operator Operand 1 Operand 2
Return COST
END

42
4. Discuss briefly about lexical phase of compiler.
Lexical Analysis: Recognition of basic elements or tokens and uniform
symbols.
The three tasks of the lexical phase are:
1. To parse the source program into the basic elements of the language.
2. To build a literal table and an identifier table.
3. To build a uniform symbol table.
Databases: It manipulates 5 databases.
1. Source Program: Original form of the program; appears to the compiler as
a string of character.
2. Terminal Table: It is a permanent table. Each entry consists of Terminal
symbol (arithmetic operators, keywords etc.,) an Indication of its
classification (operator, break character) and Precedence (used in later
phases).
SYMBOL INDICATOR PRECEDENCE
3. Literal Table: It is created by lexical analysis to describe all literals used
in the source program.
There is one entry for each literal consisting of a value, a number of
attributes, an address denoting the location of the literal at execution time,
and other information.
4. Identifier Table: It is created by lexical analysis to describe all identifiers
used in the source program.
There is one entry for each identifier and places the name of the identifier
into that entry. Since in many languages identifiers may be from 1 to 31
symbols long. The pointer points to the name in a table for efficiency
storage.
5. Uniform Symbol Table: It is created by lexical analysis to represent the
program as a string of tokens rather than individual characters.
There is one uniform symbol for every token in the program.
Each uniform symbol contains the identification of the table of which the token
is a member and its index within that table.

43
5. Explain the structure of a compiler with a neat diagram.
Structure of Compiler:
In analysing the compilation process, there are 7 distinct logical problems as
follows:
1. Lexical Analysis: Recognition of basic elements or tokens and uniform
symbols.
2. Syntax Analysis: Recognition of basic Syntactic construct through
reductions.
3. Interpretation: Definition of exact meaning, creation of matrix and tables.
4. Machine Independent Optimization: Creation of more optimal matrix.
5. Storage Assignment: Modification of identifier and literal tables.
6. Code Generation: A macro processor is used to produce more optimal
assembly code.
7. Assembly and Output: Resolving symbolic addresses and generating the
machine language.
Phase 1 to 4 is machine-independent and language dependent.
Phase 5 to 7 is machine-dependent and language independent.

44
The following are the database used by the compiler.
 Source code: The user program is the source code.
 Uniform symbol table: It consists of full or partial list of the tokens.
 Terminal table: It lists all keywords and special symbols.
 Identifier table: It contains all variable in the program.
 Literal table: It consists of constants in the program.
 Reductions: It is a permanent table of decision rules in the form of pattern
for matching with the uniform symbol table.
 Matrix: It is created by the intermediate form of the program which is in
turn created by the action routines.
 Code productions: It is a permanent table of definitions. There is one entry
defining code for each matrix operator.
 Assembly code: Assembly language version of the program which is
created by the code generation phase and is input to the assembly phase.
 Relocatable object code: The final output of the assembly phase, ready to
be used as input to loader.
6. Explain the passes of compiler with neat diagram.
Pass1: Corresponds to the lexical analysis of a compiler. It scans the source
program and creates the identifiers, literals and uniform symbol tables.
Pass2: Corresponds to the syntactic and interpretation phases. Pass2 scans the
uniform symbol table produces the matrix and place the information about
identifiers into the identifier table.
Pass3 through N-3: They correspond to the optimization phase. Each separate
type of optimization may require passes over the matrix.
Pass N-2: It corresponds to the storage assignment phase. This is a pass over
the identifier and literal tables rather than the program itself.
Pass N-1: It corresponds to code generation phase. It scans the matrix.
Pass N: It corresponds to assembly phase. It resolves symbolic address and
creates information for the loader.

45
7. Explain Machine independent optimisation.
Removing or deleting the duplicate entries in the matrix and modifying all
references to the deleted entries is one type of machine independent
optimization.
Other machine independent optimization are:
1. Compile time computation of operations, both of whose operands are
constants.
2. Movement of computations involving non varying operands out of loops.
3. Use of the properties of Boolean expression to minimize their computation.
Ex: When a subexpression occurs in the same statement more than once, the
duplicate entries can be deleted in the matrix and can be modified as shown in
the table.

46
Matrix with common
subexpression
Matrix after elimination of
common subexpression
1 - START FINISH
2 * RATE M1
3 * 2 RATE
4 - START FINISH
5 - M4 100
6 - M3 M5
7 - M2 M6
8 = COST M7
1 - START FINISH
2 * RATE M1
3 * 2 RATE
4
5 - M4 100
6 - M3 M5
7 - M2 M6
8 = COST M7
Elimination of common subexpression
8. Explain syntax phase of a compiler.
Syntax Phase: The process of recognizing and separating the syntactic
construct and interpreting their meaning is known as Syntax Analysis.
 Each syntactic construct is associated with a defined meaning which is in
the form of actual code or intermediate form of the construction.
 Syntactic constructs are recognized using two general methods:
1. Reductions: They are rules (reductions) that specify the syntax form of the
source language.
2. Action Routines: After defining the syntactic construction by the
Reductions, appropriate compile routines or action routines are executed
when a construction is recognized.
Ex:
COST=RATE*(START-FINISH)+2*RATE*(SART-FINISH-100);
The reduction will specify that any identifier is followed by an equal sign, then
call the action routine “ARITHMETIC_STM”.
1. Reductions are tested consecutively for match between Old Top of Stack
field and the actual Top of Stack, until match is found.
2. When match is found, the action routines specified in the action field are
executed in order from left to right.
3. When control returns to the syntax analyser, it modifies the Top of Stack
field.
4. Step 1 is repeated starting with the reduction specified in the next reduction
field.
*****

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