System software unit ii, Lecture notes for System Programming. Visvesvaraya Technological University

System software unit ii, Lecture notes for System Programming. Visvesvaraya Technological University

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KCG College of Technology,Chennai-96 Computer Science and Engineering


1) Why an Assembly Language is needed? Programming in machine code, by supplying the computer with the numbers of the operations it must perform, can be quite a burden, because for every operation the corresponding number must be looked up or remembered. Looking up all numbers takes a lot of time, and mis-remembering a number may introduce computer bugs. So Assembly Languages are evolved which contains mnemonic instructions corresponding to the Machine codes using which the program can be written easily.

Therefore a set of mnemonics was devised. Each number was represented by an alphabetic code. So instead of entering the number corresponding to addition to add two numbers one can enter "add".

Although mnemonics differ between different CPU designs some are common, for instance: "sub" (subtract), "div" (divide), "add" (add) and "mul" (multiply). 2) What is an Assembler? An assembler is a program that accepts an assembly language program as input and produces its machine language equivalent along with information for the loader (An Assembler translates a program written in an assembly language to it machine language equivalent)

3) Explain the terms a)Label,b)Opcode,c)Operand,and d)Comment (What is the format in which the assembly language program is written?).

Label field. • The label is a symbolic name that represents the memory address of an

executable statement or a variable. • Opcode/directive fields.

• The opcode (e.g. operation code) specifies the symbolic name for a machine instruction.

• The directive specifies commands to the assembler about the way to assemble the program.

Operand field. • The operand specifies the data that is needed by a statement.

Comment field. • The comment provides clear explanation for a statement.

4) What are the basic functions of an assembler? Functions of a Basic Assembler

• Convert mnemonic operation codes to their machine language equivalents

E.g. STL -> 14 (line 10) • Convert symbolic operands to their equivalent machine addresses E.g. RETADR -> 1033 (line 10) • Build the machine instructions in the proper format • Convert the data constants to internal machine representations E.g. EOF -> 454F46 (line 80) • Write the object program and the assembly listing

5) What are assembler Directives? Assembler directives are Pseudo-instructions that are not translated into machine instructions and they provide instructions to the assembler itself.

The SIC assembler directives. • START

• Specification of the name and start address of the program. • END

• Indication of the end of the program and optionally the address of the first executable instruction.

• BYTE • Declaration of character or string constants.

• WORD • Declaration of integer constants.

• RESB • Declaration of character variables or arrays.

• RESW • Declaration of integer variables or arrays.

6) What are the functions of two pass assembler?

Functions of Two Pass Assembler • Pass 1 - define symbols (assign addresses)

• Assign addresses to all statements in the program • Save the values assigned to all labels for use in Pass 2 • Process some assembler directives

Pass 2 - assemble instructions and generate object program Assemble instructions

• Generate data values defined by BYTE, WORD, etc. • Process the assembler directives not done in Pass 1 • Write the object program and the assembly listing

7) What is the format of the Object Program generated by the Assembler? Contains 3 types of records: Header record: Col. 1 H

Col. 2-7 Program name Col. 8-13 Starting address (hex) Col. 14-19 Length of object program in bytes (hex) Text record Col.1 T Col.2-7 Starting address in this record (hex) Col. 8-9 Length of object code in this record in bytes (hex) Col. 10-69 Object code (hex) (2 columns per byte) End record Col.1 E Col.2~7 Address of first executable instruction (hex) (END program_name)

8) Give an example of object program generated by an Assembler.

9) What is forward reference? Forward reference is a reference to a label that is defined later in the program. Example 10 STL RETADR o RETADRis not yet defined when we encounter STL instruction o So it is called forward reference

10) Give an example of Assembly language along with the objectcode generated.

Line Loc Source statement Object code

5 1000 COPY START 1000 10 1000 FIRST STL RETADR 141033 15 1003 CLOOP JSUB RDREC 482039 20 1006 LDA LENGTH 001036 25 1009 COMP ZERO 281030 30 100C JEQ ENDFIL 301015 35 100F JSUB WRREC 482061 40 1012 J CLOOP 3C1003 45 1015 ENDFIL LDA EOF 00102A 50 1018 STA BUFFER 0C1039 55 101B LDA THREE 00102D 60 101E STA LENGTH 0C1036 65 1021 JSUB WRREC 482061 70 1024 LDL RETADR 081033 75 1027 RSUB 4C0000 80 102A EOF BYTE C’EOF’ 454F46 85 102D THREE WORD 3 000003 90 1030 ZERO WORD 0 000000 95 1033 RETADR RESW 1 100 1036 LENGTH RESW 1 105 1039 BUFFER RESB 4096 110 .

115 . SUBROUTINE TO READ RECORD INTO BUFFER 120 . 125 2039 RDREC LDX ZERO 041030 130 203C LDA ZERO 001030 135 203F RLOOP TD INPUT E0205D 140 2042 JEQ RLOOP 30203D 145 2045 RD INPUT D8205D 150 2048 COMP ZERO 281030 155 204B JEQ EXIT 302057 160 204E STCH BUFFER,X 549039 165 2051 TIX MAXLEN 2C205E 170 2054 JLT RLOOP 38203F 175 2057 EXIT STX LENGTH 101036 180 205A RSUB 4C0000 185 205D INPUT BYTE X’F1’ F1 190 205E MAXLEN WORD 4096 001000 195 . 200 . SUBROUTINE TO WRITE RECORD FROM BUFFER 205 . 210 2061 WRREC LDX ZERO 041030 215 2064 WLOOP TD OUTPUT E02079 220 2067 JEQ WLOOP 302064 225 206A LDCHBUFFER,X 509039 230 206D WD OUTPUT DC2079 235 2070 TIX LENGTH 2C1036 240 2073 JLT WLOOP 382064 245 2076 RSUB 4C0000 250 2079 OUTPUT BYTE X’05’ 05 255 END FIRST

10) Write an Algorithm for pass 1 of SIC Assembler.

11) Write an algorithm for pass 2 of SIC assembler.

12) What are the Data Structures used in an Assembler? Data Structures:

Operation Code Table (OPTAB) Symbol Table (SYMTAB) Location Counter(LOCCTR)

13) Explain the features of a Symbol Table. SYMTAB (symbol table) Content Label name and its value (address) May also include flag (type, length) etc. • Usage Pass 1: labels are entered into SYMTAB with their address (from LOCCTR) as they are encountered in the source program Pass 2: symbols used as operands are looked up in SYMTAB to obtain the address to be inserted in the assembled instruction • Characteristic Dynamic table (insert, delete, search) • Implementation Hash table for efficiency of insertion and retrieval

COPY 1000 FIRST 1000 CLOOP 1003 ENDFIL 1015 EOF 1024 THREE 102D ZERO 1030

RETADR 1033 LENGTH 1036 BUFFER 1039 RDREC 2039


14) What is Location Counter? Location Counter

• A variable used to help in assignment of addresses • Initialized to the beginning address specified in the START statement • Counted in bytes

15) What are the machine dependant fetures of a SIC/XE Assembler? Machine-dependent features of assemblers

Features of the SIC/XE machine Programming features.

15)..a # symbol. 15)..b Indication of the immediate addressing mode. 15)..c Immediate addressing provides a faster access to an

operand reference. 15)..d @ symbol.

15)..d..i Indication of the indirect addressing mode. 15)..d..ii Indirect addressing reduces the number of instructions.

15)..e + symbol. 15)..e..i Explicit selection of the format 4 instruction with a direct

addressing mode. 15)..e..ii Format 4 is selected when the 12-bit displacement of

format 3 is too small. 15)..f BASE directive.

15)..f..i Indication that the base register B holds a base address used in a base addressing.

15)..f..ii NOBASE directive disables the base register. 15)..f..iii LDB instruction loads the base register with a base address.

15)..g Register-to-register addressing. 15)..g..i Register addressing reduces the size of a machine

instruction and speeds up a computation Assembling features.

15)..h Multiprogramming.

15)..h..i Larger memory allows us to load many programs. 15)..h..ii The object code is relative to zero because the load address

is variable. 15)..h..iii Program must be relocated when it is loaded in memory.

15)..i Register set mapping. 15)..i..i A separate register table can store the numeric values of the

registers. 15)..i..ii The numeric values of the registers can be preloaded with

the symbol table. 15)..j Relative (PC and base) addressing mode.

15)..j..i Operand value is subtracted from PC or base register value. 15)..j..ii PC relative addressing provides a displacement from –2048

to +2047. 15)..j..iii Base relative addressing provides a displacement from 0 to


16) What is Program Relocation? Program relocation

Principles. • The load address of an object program is unknown at assembly time if the

system implements the multiprogramming feature. • The assembler generates addresses relative to zero in the object program. • At load time, relocation is performed by adding the load address to the

relative addresses. • Operands of instructions that use direct addressing must be relocated, and

the assembler provides the relocation information in the object program. • Operands of instructions that use relative addressing do not need to be

relocated. • Relocation can be processed by the loader or by the CPU using relocation

registers. 17) What are the advantages of program relocation?

Program Relocation • The larger main memory of SIC/XE

• Several programs can be loaded and run at the same time. • This kind of sharing of the machine between programs • is called multiprogramming

• To take full advantage • Load programs into memory wherever there is room • Not specifying a fixed address at assembly time • Called program relocation

18) What are program blocks? Program Blocks • Refer to segments of code that are rearranged within a single object program unit • USE [blockname]

• At the beginning, statements are assumed to be part of the unnamed (default) block

• If no USE statements are included, the entire program belongs to this single block • Each program block may actually contain several separate segments of the source

program 19) How the program blocks are assembled? Program Blocks - Implementation • Pass 1

• Each program block has a separate location counter • Each label is assigned an address that is relative to the start of the block

that contains it • At the end of Pass 1, the latest value of the location counter for each block

indicates the length of that block • The assembler can then assign to each block a starting address in the

object program • Pass 2

• The address of each symbol can be computed by adding the assigned block starting address and the relative address of the symbol to that block

• Each source line is given a relative address assigned and a block number

20) What is one pass assembler? Explain the functioning of one-pass assembler. • One-pass assemblers are used when

• it is necessary or desirable to avoid a second pass over the source program • the external storage for the intermediate file between two passes is slow or

is inconvenient to use • Main problem: forward references to both data and instructions • One simple way to eliminate this problem: require that all areas be defined before

they are referenced. • It is possible, although inconvenient, to do so for data items. • Forward jump to instruction items cannot be easily eliminated.

Sample Program for a One-Pass Assembler

Load-and-Go Assembler • Load-and-go assembler generates their object code in memory for immediate

execution. • No object program is written out, no loader is needed. • It is useful in a system oriented toward program development and testing such

that the efficiency of the assembly process is an important consideration.

How to Handle Forward References • Load-and-go assembler

• Omits the operand address if the symbol has not yet been defined • Enters this undefined symbol into SYMTAB and indicates that it is

undefined • Adds the address of this operand address to a list of forward references

associated with the SYMTAB entry • Scans the reference list and inserts the address when the definition for the

symbol is encountered. • Reports the error if there are still SYMTAB entries indicated undefined

symbols at the end of the program • Search SYMTAB for the symbol named in the END statement and jumps

to this location to begin execution if there is no error

21) What is a multi-pass assembler? Explain with an example,the functioning of a multi-pass assembler.

Multi-Pass Assemblers • Prohibiting forward references in symbol definition:

• This restriction is not a serious inconvenience. • Forward references tend to create difficulty for a person reading the

program. • Allowing forward references

• To provide more flexibility • Solution:

• A multi-pass assembler that can make as many passes as are needed to process the definitions of symbols.

• Only the portions of the program that involve forward references in symbol definition are saved for multi-pass reading.

• For a two pass assembler, forward references in symbol definition are not allowed:


• Reason: symbol definition must be completed in pass 1. • Motivation for using a multi-pass assembler

• DELTA can be defined in pass 1 • BETA can be defined in pass 2 • ALPHA can be defined in pass 3

Implementation • A symbol table is used

• to store symbol definitions that involve forward references • to indicate which symbols are dependant on the values of others • to facilitate symbol evaluation

• For a forward reference in symbol definition, we store in the SYMTAB: • the symbol name

• the defining expression • the number of undefined symbols in the defining expression • the undefined symbol (marked with a flag *) associated with a list of

symbols depend on this undefined symbol. • When a symbol is defined, we can recursively evaluate the symbol expressions

depending on the newly defined symbol.

two undefined symbol in the defining expression


Forward Reference Example Object Program from One-Pass Assembler

After scanning line 160



appended to the list


depending list

depending list

undefined symbol

undefined symbol

defining expression




Source program

Object codes

Intermediate file

Pass 2

Pass 1

Object Code in Memory and SYMTAB

Object Code in Memory and SYMTAB After scanning line 40

Sample Program for a One-Pass Assembler

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