Lab 01: Introduction to Assembly Language
OBJECTIVE
To learn the basic commands, CPU registers and assembly language program structure.
Introduction
Assemble language focuses on programming Intel microprocessors.
Machine language is a Boolean language consisting of numbers that is specifically understood by a computer’s processor (the CPU).
Of all languages, Assembly has the closest resemblance to the native machine language of the computer.
For this reason it is fast/speedy and it provides you direct access to the computer hardware.
The only disadvantage is that assembly language programs are large and they would take too much time to write and maintain.
An assembler is a program that converts source-code programs from assembly language into machine language.
A debugger provides a way for a programmer to trace the execution of a program and examine the contents of the memory.
One Assembly language instruction corresponds to one machine language insruction.
Theory
Central Processing Unit (CPU)
The Central Processing Unit (CPU) is the physical device that performs instructions. The instructions that CPUs perform are generally very simple. Instructions may require the data they act on to be in special storage locations in the CPU itself called registers. The CPU can access data in registers much faster than data in memory. However, the number of registers in a CPU is limited, so the programmer must take care to keep only currently used data in registers.
CPU Registers
Registers are special work areas inside the CPU designed to be accessed at high speed. The registers are 16-bit long but you have the option of accessing the upper or lower halves of the four registers:
• Data Registers 16-bit: AX, BX, CX, and DX
8-bit: AH, AL, BH, BL, CH, CL, DH, DL
• Segment Registers CS, DS, SS, ES (Code, Data, Stack, Extra)
• Index Registers SI, DI, BP (Source, Destination, Base Pointer)
• Special Registers IP, SP (Instruction Pointer, Stack Pointer)
• Flag Registers Overflow, Direction, Interrupt, Trap, Sign, Zero, Auxiliary,
Carry, Parity
Data Registers
Four Registers named data registers or general purpose registers are used for arithmetic and data movement. Each register may be either addressed as 16-bit or 8-bit value. Bit positions are always numbered from right to left, starting with 0:
Each general purpose register has special attributes:
AX (Accumulator Register): AX is called the accumulator register because it is favored by the CPU for arithmetic operations.
BX (Base Register): BX register can also perform arithmetic and data movement, and it has special addressing abilities. It can hold a memory address that points to another variable.
CX (Counter Register): It acts as a counter for repeating or looping instructions. These instructions automatically repeat and decrement CX and quit when it is equal to 0.
DX (Data Register): DX is used for output purpose/display. It has a special role in multiply and divide operations. When multiplying for example DX holds the high 16-bits of the product.
• Registers available in a microprocessor are Ax, Bx, Cx, Dx.
Ax and Bx are 16 bit data registers. They are mainly used for arithmetic operations and saving addresses.
They are further subdivided into AH/BH = High byte and AL/BL = Low Byte.
Cx is used for counter implementation (in loops)
Dx is used for output purpose (display)
• How to Start an Assembly Program
Essential code instruction for all programs start with the following commands:
.model small
.stack 100h
.data
.code
main proc
(your function is written inside this)
main endp
end main
.model determines the size of code and data a program can have e.g. small.
.stack 100h stores current address of ax, bx registers into memory block.
Push/Pop command e.g. Push ax in stack Pop ax out. (address)
.data corresponds to data segment, and it initializes global variables or strings. It is used for all the variable definitions.
.code corresponds to code segment and all instructions come in this.
main proc is the ‘procedure statement’ and all code is written below this.
main endp is the ‘end procedure’ statement.
end main is the last line of code.
NOTE: Any function can be defined between main proc and main endp.
• IN/OUT Library
func # 1 is for ‘input’ and func # 2 is for ‘output’.
Example: ah = func # (e.g. 1 or 2)
INPUT EXAMPLE: (function # 1 for single character input) [Read Operation]
e.g. mov ah,1
int 21h
int is for ‘interrupt’. ‘Int21h’ has the highest priority and it can invoke large no. of DOS functions.
al is used for single character input by the user (Default register byte).
NOTE: Before we call interrupt we assign ‘ah’ some value to read from.
OUTPUT EXAMPLE: (function # 2 for single character output/display)
e.g mov ah,2
mov dl, ‘A’
int 21h
NOTE: Default output byte is ‘dl’.
; is used to write comments to a line.
Example: mov ah,1; single character read in from ‘al’.
• To Generate ENTER/New Line following two commands are used:
0dh : (zero) carriage return (backspace).
0ah : new row (line generation)
4ch returns control to DOS, is (used to exit). Example: mov ah,4ch.
Processor performs two basic functions:
Data Transfer: that includes read operations e.g. load is a read operation. In a read operation memory is not changed, only the contents are fetched.
load R1, a
load R2, b
Data Transformation: that includes data manipulation and write operations e.g. store, add are write operations. In a write operation memory is over written.
store c, R3
add R3, R2, R1; R3 = R3 + R2 + R1
Assembly language
An assembly language program is stored as text (just as a higher level language program). Each assembly instruction represents exactly one machine instruction. For example, the addition instruction described by machine language
B0 05
would be represented in assembly language as
mov al, 05; al = 05
Here the meaning of the instruction is much clearer than in machine code. The word add is a mnemonic for the addition instruction. The general form of an assembly instruction is:
mnemonic operand(s)
The most basic instruction in assembly is the move instruction. It moves data from one location to another. It takes two operands:
mov destination, source
The data specified by source is copied to destination. Both operands may or may not be memory locations. Another important point to be kept in mind is that the source and destination operands must be same size.
dec- decrements the value stored in register by 1
inc- increments the value stored in register by 1
add- instruction is used to add integers
add destination, source
add ax, 05
add ax, bx
sub- instruction is used to subtract integers
sub destination, source
sub ax, 05
sub ax, bx
Assembly Language Compiler is called Emu8086
• Open ‘emu8086’ and select New > empty work space.
• Write your code in the provided white space.
• To run the program click on ‘emulate’ button.
• Then click on ‘run’.
Note: Assembly language compiler is NOT case sensitive.
Code: Display assigned char, input char at run time, and display in new line.
