System Software module 1

Chapter 1: SIC/SIC-XE Machine Architecture
Chapter 2: Assemblers
Prof. Shweta Nirmanik, Dept. CSE, REC HKT

Chapter 1: SIC/SIC-XE Machine
Architecture

Chapter 1: SIC/SIC-XE Machine
Architecture
 Hardware units
Input Devices
Output Devices
Hardware System Unit Ex: CPU, Memory,
N/W Card, Power Supply, Graphics
Card Etc
Storage Devices

System Software
 System Software is a set of programs that support the
operation of a computer.
 It consist of a variety of programs that support the
operations of a computer.
 The system software manages the computer resources
while processing the data and control various
hardware components

Difference Between System
Software And Application Software
SYSTEM SOFTWARE APPLICATION SOFTWARE
1 System software is machine
dependent
Application software is machine
independent
2 Focus is on the operation that with
computer as a took rather than
application
Focus is on the application not on the
computing system
3 It is middleware that runs the
infrastructure that supports the
application software
Application software that most of the
end users are familiar with and where
the work is done by average persons
4 Development of system software is
complex
Development of application software
is easier when compared to system
software
5 Ex: assemblers translate mnemonic
instructions into machine code
instruction formats etc
Ex: word processors, spreadsheets,
media players etc

SIC [Simplified Instruction Computer]
 Design to list the most commonly encountered habit
features and the concepts of while avoiding most of the
idiosyncrasies often that are found in real machines
 The SIC architecture depends upon on the following
features:
1. Memory
2. Register
3. Data Formats
4. Instruction Formats
5. Addressing Modes
6. Instruction Set
7. Input and output

1. MEMORY
 The number of address lines in standard model of SIC
is 15.
 So , the size of the memory that can be addressed = 215
=32768 bytes.
 Memory consists of 8 bits that makes one byte
 Any three consecutive bytes form a word
 All SIC addresses are a byte address and words are
addressed by the location of the lowest number byte

1 BIT 1 BIT 1 BIT 1 BIT 1 BIT 1 BIT 1 BIT 1 BIT
b7 b6 b5 b4 b3 b2 b1 b0
1 BYTE
1 BYTE = 8 BITS
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
1 BYTE 1 BYTE
1 BYTE
3 BYTE= 1 WORD

2. REGISTER
 There are five registers all of which have a specified
special use
MNEMONICS CODE SPECIAL USE
A 0
Accumulator Used For
Arithmetic Operations.
X 1 Index Register
L 2 Linkage Register
PC 8 Program Counter
SW 9
Status Word Includes A
Variety Of Information
Including Condition Codes

3. Data Formats
 Integers are stored has 24 bit binary number
 No floating points variables in SIC.
 Characters are stored using 8 bits ASCII codes
 All machine instructions on the standard versions of
SIC are 24 bits format.
 the flag bit ‘x’ is used to indexed addressing mode
OPCODE X ADDRESS

5. ADDRESSING MODES
 There are two types of addressing modes indicated by
setting the flag bit ‘X’ in the instruction
Mode Indication Target Address
Direct Mode X=0 TA= Address
Indexed Mode X=1 TA= Address+[X]

6. Instruction Set
 SIC provides basic set of instructions
 load and storage registers.
 Integer arithmetic operations.
 All automatic operations involve Register A and word in the
memory with the result being left in the register.
7. Input & Output
 Input and output are performed by transferring 1bite to or from
the rightmost 8 bit of register A.
 There are three input and output instructions each of which
specifies the device code has an operand
a) The test device: instructions does so whether the address
address to device is ready to send or receive data
b) read data in this sequence should be repeated for
each data to be read or written.
c) write data

SIC/XE [Simplified Instruction
Computer Extended]
 Design to list the most commonly encountered habit
features and the concepts of while avoiding most of the
idiosyncrasies often that are found in real machines
 The SIC/XE architecture depends upon on the following
features:
1. Memory
2. Register
3. Data Formats
5. Addressing Modes
6. Instruction Set
7. Input and output

1. MEMORY
 The size of the memory that can be addressed = 220
=1MB
 Memory consists of 8 bits that makes one byte
 Any three consecutive bytes form a word
 All SIC/XE addresses are a byte address and words are
addressed by the location of the lowest number byte.

2. REGISTER
 There are nine registers all of which have a specified special
use
MNEMONICS CODE SPECIAL USE
A 0 Accumulator Used For Arithmetic Operations.
X 1 Index Register
L 2 Linkage Register
PC 8 Program Counter
SW 9
Status Word Includes A Variety Of Information
Including Condition Codes
B 3 Base Register; used for addressing
S 4 General Purpose Register
T 5 General Purpose Register
F 6 Floating Point Accumulator (48 bits)

3. Data Formats
 The exponent has value ‘e’ and the fraction has value
‘f’, the absolute value of the number is represented as
f*2(e-1024)
 S=0-> positive
 S=1->negative
 Fraction->value between 0 to 1
 Exponent-> value between 0 to 2047
 Total data format is 48bits
S Exponent Fraction
1 bit 11 bit 36 bit

a. Format 1 (I byte)
b. Format 2 (2 byte)
c. Format 3 (3 bytes)
d. Format 4 (4 bytes)
Opcode
8 bits
Opcode Register1 Register2
8 bits 4 bits 4 bits
Opcode n i x b p e
displaceme
nt
6 bits 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 12 bits
Opcode n i x b p e Address
6 bits 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 20 bits

5. Addressing Mode
n i x b p e Addressing modes
0 1 Immediate Addressing Modes (#)
1 0 Indirect Addressing Modes (@)
1 1
Not immediate not indirect Addressing
Modes or simple addressing mode
1 Indexed Addressing Modes
0 Direct Addressing Modes
0 1
Program Counter Relative Addressing
Modes
1 0 Base relative Addressing Modes
0 0 Simple Addressing Modes
1 1 Invalid Addressing Modes
0 Format 3
1 Format 4

6. Instruction Set
 SIC/XE provides all set of instruction set similar to SIC
standard version
 In addition floating point arithmetic operations they
include register-to-register operations [ADDR, SUBR,
MULR, DIVR]
 SVC (Supervisor Call)- specially provided in SIC/XE to
generate an interrupt used to communicate with
operating system.
7. Input & Output
 There are I/O channels that can be used to perform I/O
while CPU is executing other instructions.
 It also provides same set of I/O that of SIC standard
version provides

Target Address Calculation
 If b=006000 H, p=003000 H and x=000090
H
1. 032600 H
0000 0011 0010 0110 0000 0000
TA = PC+ displacement
= 003000+600
=003600 H
0000 00 1 1 0 0 1 0 0110 0000 0000
opcode n i X B P E displacement

Target Address Calculation
 If b=006000 H, p=003000 H and x=000090
H
2. 03C300 H
0000 0011 1100 0011 0000 0000
TA = b+x+displacement
= 006000+000090+300
=006390 H
0000 00 1 1 1 1 0 0 0011 0000 0000
opcode n i x b p e displacement

HOME WORK
3. 022030 H
4. 010030 H
5. 003600 H
6. 0310C303 H

Examples program in SIC & SIC/XE
 Write SIC & SIC/XE program on Arithmetic operation involving APLHA+INCR-
1=BETA, GAMMA+INCR-1=DELTA
 SIC PROGRAM
LDA APLHA
ADD INCR
SUB ONE
LDA BETA
STA BETA
LDA GAMMA
ADD INCR
SUB ONE
LDA DELTA
STA DELTA
.
.
.
ONE WORD 1
ALPHA RESW 1
BETA RESW 1
GAMMARESW 1
DELTA RESW 1

 SIC/XE PROGRAM
LDS INCR
LDA ALPHA
ADD S,A
SUB #1
STA BETA
LDA GAMMA
ADDR S,A
SUB #1
STA DELTA
.
.
.
ALPHA RESW 1
BETA RESW 1
GAMMA RESW 1
DELTA RESW 1
INCR RESW 1

 HOME WORK
Write SIC & SIC/XE program on Arithmetic operation
involving
BETA=ALPHA-INCR+2
DELTA=GAMMA-INCR+2
INFRA=BETA+DELTA

 Write SIC & SIC/XE program on string operation involving
the string “system software”
SIC PROGRAM
LDX ZERO
MOVECH LDCH STR1,X
LDCH STR2,X
TIX FIFTEEN
JLT MOVECH
.
.
.
STR1 BYTE C ‘system software’
STR2 RESB 15
ZERO WORD 0
FIFTEEN WORD 15

SIC/XE PROGRAM
LDT #15
LDX #0
MOVECH LDCHSTR1,X
LDCHSTR2,X
TIXR T
JLT MOVECH
.
.
.
STR1 BYTE C ‘system software’
STR2 RESB 15

CHAPTER 2: ASSEMBLERS

Assemblers
 An assembler is a program that converts an assembly
language program to its equivalent object program.
The assembler accepts source program written in
assembly language as the input and produces object
program as the output.
 The object program consists of:
object code (also called machine code) which are
nothing but code for the instructions given to the
microprocessor
It also contains instructions to the loader.

Assembler Function
5 COPY START 1000
10 FIRST STL RETADDR
FIG. 2.1 PROGRAM EXAMPLE
 Figure 2.1 shows the assembly language for the basic
question of SIC.
 The line numbers are for reference only and are not the
part of the program

 In addition to the mnemonics machine instructions may
have assembler directives
1. Start : Specifies name and starting the address of the
program
2. End : indicates the end of the source program and specify
the first executable instruction in the program
3. BYTE : generate the character or hexadecimal constant,
occupying has many bytes has needed to represent
constant
4. Word: generator one word integer constant
5. RESB : reserve the indicated number of bytes for the data
area
6. RESW: reserved the indicates the number of words for data
area.
The main routine calls the subroutine to read the record or
write the record or execute the instructions.
Assembler Directives

SIC Assemblers
 The translation of the source program to object code
that requires us to accomplish the following functions
1. Convert the mnemonic operation codes to there
machine level language equivalent. Ex: STL to 14
2. Convert the symbol operand to their equivalent
machine address. Ex: writing location number
3. Build the machine instruction in the proper format.
4. Convert the data constant specified in their source
program into their internal machine representation
5. Write the object program and assembly listing
The details of the formats are arbitrary away work
information obtained in these records must be present in
the object program

Header Record
Col 1 H
Col 2-7 Program name
Col 8-13 Starting address of the object program
(hexadecimal)
Col 14-19 Length of the object program in bytes
(hexadecimal)
Text Record
Col 1 T
Col 2-7 Starting address for object code in this text record
(hexadecimal)
Col 8-9 Length of object code in this text record in bytes
Col 10-69 Object code represented in hexadecimal ( 2
columns per bytes of object code)
End Record
Col 1 E
Col 2-7 Address of the first executable instruction in
object program. (hexadecimal)

Data structures used in the design
of assemblers
1. OPTAB
2. SYMTAB
3. LOCCTR

OPTAB :-Operation Code Table
• It must contain at least the mnemonic operation code
and its machine language equivalent.
• During Pass 1: when a statement is read from the table
or file, the mnemonic is searched in OPTAB, if it is
present its length is obtained from OPTAB and
location counter (LOCCTR) is been updated by the
length of the instruction if not present then it is not
valued mnemonic.
• During pass2: when the mnemonic is searched in
OPTAB, its OPCODE and the corresponding format is
read. Using the OPCODE and the format the
assembler converts the language instructions to its
equivalent object code.

 SYMTAB: Symbol Table
• SYMTAB includes the name and the value for each
label in the source program together with flags to
indicate error condition
• During Pass1 of the assembler tables are entered into
the STMTAB as they are encountered in the assembler
instruction
• Since the entries are rarely deleted from their table
efficiency of deletion is not an important issue.
• For this reason pass1 usually writes an immediate file
that contains each source program statements
together with its assigned address errors indicating
indicators etc
• This file is used as input to the past 2.

 LOCCTR: Location Counter
• LOCCTR is initialized to the beginning address
specified in the start
• After each source statement is processed, the length of
the assembled instruction or the length of data area is
added to LOCCTR.
• Thus, LOCCTR Is used to assign address to each
instruction.

Pass1
algorithm.

Pass2
Algorithm

 Rules:
1. Find the length of each instruction
a) An instruction that has register as first operand = 2
bytes.
b) An instruction that has two operand = 2 bytes
c) An instruction that has symbol as operand = 3 bytes
d) An instruction that has RSUB has length of= 3 bytes
e) An instruction preceding with ‘+’ has length of = 4
bytes
2. Find the length of data area.
3. Initialize LOOCTR to 0000 and start updating
LOCCTR by adding the length of each instruction or
length of data area, when we encounter a label enter
the label and current LOCCTR into symbol table.

4. Label length calculation
a) WORD= add 3 to LOCCTR
b) RESW = add(3* value of operand) to LOCCTR
c) RESB= add value of operand to LOCCTR
d) BYTE= find the length of constant in bytes add into LOCCTR
5. Create SYMTAB for labels.
6. Calculate the object code of each instruction based on location,
length and addressing modes with specified instruction formats.
7. Generate the object program using all three records

LOC LENGTH LABEL MNEMONIC OPERAND OBJECT CODE
SUM START 0
FIRST LDX #0
LDA #0
+LDB #TABLE2
BASE TABLE2
LOOP ADD TABLE, X
ADD TABLE2, X
TIX COUNT
JLT LOOP
+STA TOTAL
RSUB
COUNT RESW 1
TABLE RESW 2000
TABL2 RESW 2000
TOTAL RESW 1
END SUM

LOC LENGT
H
LABEL MNEMON
IC
OPERAND OBJECT CODE
SUM START 0
3 FIRST LDX #0
3 LDA #0
4 +LDB #TABLE2
BASE TABLE2
3 LOOP ADD TABLE, X
3 ADD TABLE2, X
3 TIX COUNT
3 JLT LOOP
4 +STA TOTAL
3 RSUB
3 COUNT RESW 1
1770 TABLE RESW 2000
1770 TABL2 RESW 2000
3 TOTAL RESW 1
END SUM

0000 SUM START 0
0000 3 FIRST LDX #0
0003 3 LDA #0
0006 4 +LDB #TABLE2
BASE TABLE2
000A 3 LOOP ADD TABLE, X
000D 3 ADD TABLE2, X
0010 3 TIX COUNT
0013 3 JLT LOOP
0016 4 +STA TOTAL
001A 3 RSUB
001D 3 COUNT RESW 1
0020 1770 TABLE RESW 2000
1790 1770 TABL2 RESW 2000
2F00 3 TOTAL RESW 1
2F03 END SUM

 TOTAL ADDRESS =LOCCTR+STARTING ADDRES
=2F03+0000
=2F03 H
SYMTAB:
LABEL LOC
FIRST 0000
LOOP 000A
COUNT 001D
TABLE 0020
TABLE2 1790
TOTAL 2F00

0000 SUM START 0 0
0000 3 FIRST LDX #0 058000 H
0003 3 LDA #0 010000 H
0006 4 +LDB #TABLE2 69101790 H
BASE TABLE2
000A 3 LOOP ADD TABLE, X 1BA013 H
000D 3 ADD TABLE2, X 1BC000 H
0010 3 TIX COUNT 2F200A H
0013 3 JLT LOOP 3B2FF4 H
0016 4 +STA TOTAL 0F102F200 H
001A 3 RSUB 4F0000 H
001D 3 COUNT RESW 1
0020 1770 TABLE RESW 2000
1790 1770 TABL2 RESW 2000
2F00 3 TOTAL RESW 1
2F03 END SUM

 OBJECT PROGRAM
HSUM000000002F03
T0000001D058000010000691017901BA013
1BC0002F200A0F102F004F0000
E000000

LOC LENGTH LABEL MNEMO
NIC
OPERAND OBJECT CODE
COPY START 1000
CLOOP +JSUB RDREC
LDA LENGTH
COMP ZERO
JEQ EXIT
J CLOOP
EXIT STA BUFFER
LDA THREE
STA TOTAL_LENGTH
RSUB
BUFFER RESW 100
EOF BYTE C ‘EOF’
ZERO WORD 0
THREE WORD 3
LENGTH RESW 1
TOTAL_LENGTH RESW 1
RDREC LDX ZERO

Machine Independent Assembler
Function
 Literals
 Symbol defining statements
 Expression
 Program blocks

Literals
 A literals is a constant specified in a operand field of an
instruction.
 Literals should be enclosed within a pair of a single
quotes and it must be preceded with symbol =
 Ex:
 For decimal ex: LDA =4095
 For hexadecimal ex: TD =‘X’
 For string ex: LDA =C‘EOF’

Advantages of Literals
 Literals will receive the programmer from defining a
constant else where in the program within the help of
the assembler directive BYTE or WORD and make a
label for it.
 When the programmer uses a literal in the operand
field of the instruction has shown LDA =X‘FFFF’ the
assembler will define all the literals constant at the end
of the program i.e., once the END directive is been
detected
 Assembler gathers information of all the literals in the
first pass and insert them into the end of the program
and uses the addresses to assemble the instruction in
the second pass

LTORG
 It is possible to insert the literals in the middle of the
object code also with the help of the assembler
directive LTORG, is an assembler directive using which
we can tell the assembler to define and insert the
liberal values along with their addresses at the point
where LTORG is present.

 So to place the definition of literals nearer to the
instruction which references them we use assembler
directive
 By using LTORG we can avoid the format 4, which will
take you just storage area
Line no Label Mnemonic Operand Comment
5 COPY START 0
. . .
25 LDA =C ‘EOF’ EOF is defined at line no 25
. . .
50 LTORG
EOF will be inserted from the line no 50 after
reading LTORG
. . .
70 LDA =C ‘ABC’ ABC is defined at line no 75
90 END COPY
ABC will be inserted from the line no 90 after
reading END

Handling Literals by assemblers
 Literals are handled by assemblers using a basic data
structures called literal table “LITTAB”.
 During Pass1
 If literal is encountered during scanning, the assembler
searches LITTAB, if present no problem, If not the literal
should be added.
 When the assembler directive such as LTORG or END is
encountered, the assembler makes a scan of the literal
table. The first undefined address is filled with value
obtained from LOCCTR.
 As address are assigned the location counter is updated
to reflect the no of bytes occupied by each literal.
NAME VALUE LENGTH ADDRESS

 EX: LOC MNEMONIC OPERAND
001A LDA =C ‘EOF’ ----------------------------- 1
…
…
LTORG ------------------------------------------------------- 2
…
Assuming when LTORG is encountered,
Let LOCCTR=002D
1. When the literal =C ‘EOF’ is encountered, it is entered into the literal table
as shown
2. When LTORG is encountered assume LOCCTR value=002D. This value is
entered as the address of the literal in the literal table as
NAME VALUE LENGTH ADDRESS
=C ‘EOF’ 454F46 3 BYTES 002D

3. LOCCTR is updated as LOCCTR=LOCCTR+3=002D+3=0030
 During Pass2
 When a literal is encountered its address is obtained from the
literal table and instruction will be assigned using this address
and object code is generated as
DISPLACEMENT = ADDRESS OF LITERAL-PC
= 002D-001D (PC VALUE AFTER LDA =C ‘EOF’)
= 0010
OPCODE=00, N=1, I=1, X=0, B=0, P=1, E=0, FORMAT 3
0PCODE 6 BITS N I X B P E DISPLACEMENT 12 BITS
0000 00 1 1 0 0 1 0 0000 0001 0000

Symbol Defining Statements
 The programmer can define the symbols and specify
the values associated with the symbols using the
following two assembler directives
1. EQU Ex: MAX_LEN EQU 1024
1. ORG Ex: ORG 3000

1. EQU
 The assembler directive using which we can tell the
assembler to assign the values of the labels.
 Syntax Symbol EQU Value
Ex: MAX_LEN EQU 1024
 The symbol directive specifies the symbol entered into
the SYMTAB and assigns the values specified in the
operand to the symbol
 The value may be constant or an expression.
 Normally EQU is used to replace the constant in the
program with the symbol names and it is used for
improving the readability of the program.

 Ex: MAXLEN EQU 4096
+LDT #MAXLEN
 During assembly of the above program segment, when
EQU is encountered the pass 1 assembler enters the
MAXLEN into the symbol table SYMTAB with the
value 4096.
 During pass2 while assembling +LDT instructions,
assembler searches the SYMTAB for the symbol
MAXLEN and obtains its value. This value is used to
assemble the current instructions and generate the
object code

2. Org
 is a assembler directive using which we tell the assembler
was to indirectly assign the values to symbols by initiating
LOCCTR with the new value
 It allows the assemblers to reset the PC value to the
specified value
 When ORG is encountered the assembler initialises the
LOCCTR to the specified value.
 ORG will affect the value of all the labels defined until next
ORG is encountered.
 Syntax: ORG VALUE
 EX: ORG 3000
 About statement initially is the LOCCTR with the value of
3000
 Using ORG we can also tell the assemblers to assemble the
new LOCCTR.

COPY START 1000
FIRST STL RETADDR
. . .
. . .
ORG 3000
SECOND STA LENGTH
 Assembler assigns 1000 to LOCCTR and assigns address to
each instruction in the program based on length.
 When ORG 3000 is encountered the LOCCTR value is
initialized to 3000 and assigns from this point onwards.

Expression
 Expressions are allowed in operand field, assembler
evaluates the expression and produces a single
operand address or value.
 There are two types of expression
1. Relative expression
2. Absolute expression

Relative Expression
 The expression that are used in the operand field of the
instructions normally contain the symbols defined as
labels. As the current location changes the value of the
corresponding label changes.
 The expression whose value changes relative to the
current location value are called as relative expressions.

COPY START 0000
0000 . . . . . .
. . . . . . . . .
. . . . . . . . .
0050 BUFFER RESB 05
0055 BUFFEND EQU *
MAXLEN EQU BUFFEND
 Starting address in 0000, hence BUFFER is 0050, if starting
address was from 1000, then BUFFEND is 1050.
 The expression * indicates the current location value is used
Now, BUFFER is 0050, then BUFFEND is 0055
If, BUFFER is 1050, then BUFFEND is 1055
 Value of operand with BUFFEND depends on location of the
starting address, hence operand with BUFFEND becomes in
relative terms Prof. Shweta Nirmanik, Dept. CSE, REC HKT

Absolute Expression
 Expression that contains only absolute terms is an absolute
expression
 Ex: 1. Constant values:
MAXLEN EQU 1024
MAXLEN EQU 1024+5
2. Absolute expression may contain relative terms
MAXLEN EQU BUFFEND-BUFFER
 if starting address is 0000 then,
BUFFER=0050 and BUFFEND=0055, MAXLEN=0055-0050 = 05
 if starting address is 1000 then,
BUFFER=1050 and BUFFEND=1055, MAXLEN=1055-1050 = 05
In both the above conditions the value of MAXLEN is same i.e., 05. hence MAXLEN
is absolute expression.

Program Blocks
 Features resulting in the creation of the several independent parts of the object
program maintain their identity and handle the separated by the loader.
 Program blocks are used to refer to segments of the code that are rearranged within
the single object program unit and the control section to refer the segment that are
translated into independent object program units.
 The assembler directive USE indicates which portion of the source programmer
belongs to various blocks
 At the beginning of the program of statements are assumed to be the part of the
unnamed (default) block
 If no USE statement are included the entire single program belongs to this single
block.
 The Example provided in the text book (page number 83) USE statement on line
number 92 signals the beginning of the block name the CDATA
 Source statements are associated with its block until you statement on line number
103 which begins the block named CBLKS
 The USE statement may also indicate the continuation of the previously begin block
the statement online number 123 resume default block. Thus statement on line
number 183 resumes the block named the CDATA

 During pass 1
 Separate LOCCTR for each program block is created
LOCCTR is initialised to zero.
 Each level in the program is assigned as an address that is
relative to the start of the block that contains it
 At the end of the pass 1, the latest value of the location
counter for each block indicates the length of that block
 Assembler assigns to each block a starting address in the
object program
 During pass 2
 The assembler needs the address for the each symbol
relative to start of the object program
 This is easily formed from the information in the SYMTAB
 The column location / block shows the relative addresses
assigned to each source line and the block number
indicating which program blocks is involved
0= default block, 1=CDATA, 2=CBLKS

 At the end of the pass 2 the assembler will construct a
table that contains the starting address to length of all
the blocks
Block name Block no. Address length
Default 0 0000 0066
CDATA 1 0066 000B
CBLKS 2 0071 1000

 Consider the below statement
20 0006 0 LDA LENGTH 032060 --------1
 SYMTAB shows the value of the operands has a relative
location 0003 within the program block 1
 The starting address of CDATA is 0066
 The desired target address (LENGTH) for this instruction is
TA= 0003+ 0066=0069
 Thus the instruction to be assembled with the program
counter relative addressing mode
 Opcode =LDA=00, Format 3, n=1, i=1, x=0, b=0, p=1, e=0,
displacement= TA-PC= 0069-PC.
 The PC contains the address 0009 of the following
instruction in line number 25
displacement= TA-PC= 0069-PC=0069-0009=0060.

Assembler Design Options
 One Pass Assembler: these assemblers are used when
it is necessary or desirable to avoid a second pass over
the source program.
 Multi Pass Assembler: an extension to the two pass
logic that allows an assembler to handle forward during
symbol definition.

One Pass Assembler
 The assemblers that makes only one pass while converting
from assembly language to the source language or viva versa
is called single pass or one pass assembler
 Main problem in trying to assemble a program in one pass
involves forward referencing
 It easy to eliminate the forward reference to the data items
just by defining of the symbols in the source program before
they are referenced
 To reduce the size of the problem many one pass assembler
do probability for references of data items.
 There are mainly two types of one pass assembler
1. Producers of that caused directly in the memory for
immediate instruction
2. Produces object code in the memory for immediate
execution

Produces object code in the memory for immediate
execution
 No Object code is written out and no loader is needed this kind of
load and go assembler are useful in object oriented programming
development and testing
 A load-and-go assembler avoid the overheadof a writing the object
program out and reading it back in.
 Object program is produced in memory rather than being written
out on the secondary storage. Thus the handling of forward
reference is less difficult
 The assembler simply generates the object code instructions as it
scans the source program.
 The samples were used as an operand is entered into the symbol
table the entry is flagged to indicate that the symbol is been
defined
 Undefined symbol is added into a list of a forward reference is
associated with the symbol table entry.
 When the definition for the symbol is been encountered for the
forward reference list for that symbol is a scanned and the proper
address is inserted into any instruction previously generated

Procedures the usual kind of object program for later
execution
1. One pass assembler that produces object program has the output
of an used are often used on the system where the external
working storage devices are not available.
2. One pass assembler that produces object program follows a
strictly different procedure from that of the previously described
3. Forward References are entered into a list as before, however the
definition of the symbol is encountered instruction that made
the forward reference to that symbol or may no longer be
available in the memory for modification
4. In General, they will already have been written out as the part of
the text record in this object program
5. In this case the assembler must generate text record with the
correct operand address.
6. When the program is loaded this address will be inserted into the
instructions by the action of the loader

Multi - Pass Assembler
Ex: ALPHA EQU BETA
BETA EQU DELTA
DELTA EQU 1
 Assemblers that make only two sequential passes over the source
program cannot resolve such a sequence of definitions.
 Some assemblers are designed to eliminate the need for such
restrictions the general solution is a strong multi pass assemblies
that can make as many as passes as needed to produce the
definition of the symbols
 It is not necessary for such an assembler to make more than two
passes over the entire program instead the portion of the
program that in was for references in the simple definitions are
saved during pass 1
 Additional Passes through these stored definitions are made as
assembly progresses. This is process followed by a normal pass 2

EX:
1. HALFSZ EQU MAXLEN/2
2. MAXLEN EQU BUFEND-BUFFER
3. PREVBT EQU BUFFER-1
4. BUFFER RESB 4096
5. BUFFEND EQU *
 MAXLEN has to be defined the HALFSN cannot be
computed
 The defining expression is HALFSN is stored in the
symbol table

HALFSZ &1 MAXLEN/2
MAXLEN *
A
Fig: Example of multi pass assembler operations

BUFEND *
HALFSZ &1 MAXLEN/2
MAXLEN &2 BUFEND-BUFFER
BUFFER *
B
MAXLEN
MAXLEN
HALFSZ

BUFEND *
HALFSZ &1 MAXLEN/2
PREVBT &1 BUFFER-1
MAXLEN &2 BUFEND-BUFFER
BUFFER *
C
MAXLEN
MAXLEN
HALFSZ
PREVBT

BUFEND *
HALFSZ &1 MAXLEN/2
PREVBT 1033
MAXLEN &1 BUFEND-1034
BUFFER 1034
D
MAXLEN
HALFSZ

BUFEND 2034
HALFSZ 800
PREVBT 1033
MAXLEN 1000
BUFFER 1034
E

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