SYSTEMS PROGRAMMING
CHAPTER 2
PROGRAMMING IN ASSEMBLY LANGUAGE
Er. Bharadwaj Choudhury
HIGH LEVEL LANGUAGE CONSTRUCTS
Constants: Numerical, string or other quantities whose unchanging actual values are known when the program is written
Variables: Name given to a value. The value may change when the program is executed
Reserve words: if, then, else, return, float, real Procedures: Functions or subroutines, which
may or may not have arguments and may or may not return values
ASSEMBLY LEVEL LANGUAGE CONSTRUCTS Computer’s memory: A sequence of bytes,
words, blocks. Each computer have a fixed amount of working memory
CPU’s registers: Each computer have a set of working as well as special registers
Instructions: Each assembly language must have a set of instructions based on the microprocessor used in the computer, along with additional instructions available in the assembler
Location Counter: Its task is to keep track of the current location of the instruction being processed relative to the beginning of the next group of instructions
HOW TO RUN ASSEMBLY PROGRAMS ?
Step 1 : Enter the assembly language program through a text editor or an inbuilt editor of the assembler. The inbuilt editor is known as work bench program
Step 2 : The assembly program file or the source file is a ASCII file having the extension .ASM
Step 3 : The assembler accepts this source file and converts it into an object file .OBJ in machine language. The linker converts such object file(s) into some executable form, such as .EXE, .BIN, etc through different passes. Sometimes the extension of inclusion file is .INC
SOME COMMERCIAL ASSEMBLERS Borland turbo assembler (TASM) : This is a MS-
DOS based assembler. Suppose MYTASM.ASM is a source program in TASM syntax, then the command C:\TASM>TASM MYTASM assemblers the source program and produces MYTASM.OBJ, provided the assembly program is correct.
Microsoft assembler (MASM) : The name has traditionally been referred to the Microsoft Macro assembler. The software for MASM contains an executable program file ML.EXE. Here, ML assembles and links one or more assembly language source program files, producing a .OBJ file and an executable .EXE file
SOME COMMERCIAL ASSEMBLERS
MicroAsm : It is a windows-based Integrated Development Environment(IDE) for the assembly programmer. This includes a source editor and an Intel 8086 assembler with easy syntax
Netwide Assembler(NASM) : It is a 80x86 processor-based assembler. It supports a wide variety of object file formats such as a.out, ELF, COFF, Win32, etc. Simple binary files are also generated by this assembler. This assembler works under different operating environments such as MS-DOS, WINDOWS and LINUX
ASSEMBLER TYPES
One pass assemblers Two pass assemblers One and half pass assemblers Multi-pass assemblers
ASSEMBLY LANGUAGE FORMATS Different assembly language instruction formats
differ on details like pseudo operations used, length of identifiers, formats of a constant, etc
However, most assembly language instructions has 2 mandatory and 2 optional fields.
Label : a symbol of the assembly program. Can store maximum of 6-8 characters
Opcode : contains mnemonic code for machine or pseudo operations
Operand : specifies 0 or more operands, separated by commas
Comments : high-level description of the program
[Label] Opcode Operand [Comments]
HOW ASSEMBLY PROGRAMS ARE EXECUTED?
Consider the assembly program shown below [to be executed in Intel 8086 machine]:
Stm. No. Label Opcode Operand
1 0RG 100h
2 NUM1 DW 20
3 NUM2 DW 13
4 SUM DW ?
5 MOV AX, NUM1
6 ADD AX, NUM2
7 MOV SUM,AX
8 END
Red color indicates machine operations, Blue color indicates pseudo operations
For your understanding
MOTAB & POTAB
Mnemonic Opcode Length(byte)
MOV AX,memo B8 3
ADD AX,memo 02 3
MOV memo,AX A2 3
Pseudo operation
Task
ORG Start the program and initialize LC with the value of operand, else initialize it to 0
DW Define the constant and word storage requirement. Update LC value
END End indicates the end of program. Also, enables allocation of addresses for the unallocated literals in the literal table. Control send to the next pass
PASS 1 OF THE ASSEMBLY PROCESS
Stm. No. 1 : LC ← 100h Stm. No. 2 : NUM1 in symbol table with value 100h LC ← 100h + 2h = 102h Stm. No. 3 : NUM2 in symbol table with value 102h LC ← 102h + 2h = 104h Stm. No. 4 : SUM in symbol table with value 104h LC ← 104h + 2h = 106h Stm. No. 5: LC ← LC + 3h =109h Stm. No. 6 : LC ← LC + 3h =10Ch Stm. No. 7 : LC ← LC + 3h =10Fh Stm. No. 8: Opcode is END, indicating end of
program
PASS 2 OF THE ASSEMBLY PROCESS
Stm. No. 1 : Allocate the LC ← 100h Stm. No. 2-4 : Already processed in pass 1 and
symbols stored in symbol table
Stm. No. 5: Machine code= B8 + Address of NUM1 = B80100
Stm. No. 6 : Machine code= 02 + Address of NUM2 = 020102
Stm. No. 7 : Machine code= A2 + Address of SUM = A20104
Stm. No. 8: Opcode is END, indicating termination of program
SUMMARY : THE TWO PASSES
Pass I is to construct a symbol table and a literal table using the MOTab and POTab
Pass I keeps a copy of the user’s program for Pass II
An intermediate mnemonic program is output of Pass I
The task of Pass II is to generate the object code from the copy of the user’s program in Pass I
Pass II provides the linker & loader with the necessary information for linking object code with link libraries and ultimately loading the executable code in memory for execution
PRACTICE PROBLEM
Generate symbol table (Pass 1) and machine code(Pass 2) for the assembly program below running on a SIC machine [Note: SIC is a 24-bit machine]Stm.
No.Label Opcode Operan
d
1 SIC START 02
2 LDA NUM1
3 ADD NUM2
4 STA SUM
5 NUM1 WORD 20
6 NUM2 WORD 13
7 SUM WORD 0
8 ENDRed color indicates machine operations, Blue color indicates pseudo
operations
MOTAB & POTAB FOR PRACTICE PROBLEM
Mnemonic Opcode Length(byte)
LDA m 00 3
ADD m 18 3
STA m 0C 3
Pseudo operation
Task
START Start the program and initialize LC with the value of operand, else initialize it to 0
WORD Define the constant and storage requirement. Update LC value
END End indicates the end of program. Also, enables allocation of addresses for the unallocated literals in the literal table. Control send to the next pass
THANK YOU