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Assembly Language
CS 6303
Rashad Sierra
What is Assembly Language and
why do we need it?
The CPU only understands machine language.
Machine language is “a binary language, which
means that all machine language instructions are
in binary form” (Runnion pg2)
“Assembly Language is just a symbolic form of
machine language.” (Runnion pg2)
In order to run on the CPU, every program must
be converted into machine language.
In essence we are programming within the CPU.
The CPU Design
Graphic taken from Art of Assembly by Randall Hyde
Assembler
The Assembler performs the translation
from assembly language to machine
language.
It is essentially a compiler that lets you
address and access memory directly.
It has a defined Instruction set consisting
of opcodes and operands.
It gives you access to the processor
registers set.
The Registers Set
“The Register set consist of all registers in the
CPU that are accessible to the programmer.”
(Mano & Kime, pg 484)
A register is the fastest of all temporary storage
type because it is a logical circuit that only
depends on the clock and input
All the registers are 16 bits long in Intel 8088
processor.
There are eight general-purpose registers, four
Segment Registers, and two Control Registers
available to the programmer.
General Purpose Registers
The General Registers have three
categories
A.
B.
C.
Data Registers (AX,BX,CX,DX)
Pointer Registers (BP,SP)
Index Registers (DI,SI)
Data Registers
AX (Accumulator register)- used for arithmetic
operations.
BX (Base register)- used to access an item in
data structure. Ex. Element in array
CX (Count register) - used as a counter in loop
operations
DX (Data Register) – used as an extension of
AX for 32 bit arithmetic
Each 16 bit register can be separated into two 8
bit registers


AH-High order 8 Bits
AL-Low order 8 Bits
Pointer Registers
SP (Stack Pointer)- Its used together with
the Stack Segment to calculate the top of
the stack SS:SP
BP (Base Pointer)- useful as an auxiliary
stack segment pointer.
Index Registers
DI (Destination Index Registers) - used
with string instructions.
SI (source Index Register) – it two is used
for string instructions and manipulations
Segment Registers
CS (Code Segment)- contains the origin address
of the active code segment. Contains
Executable instructions.
SS (Stack Segment)- contains the origin address
of the active stack segment. Contains dynamic
data that is pushed or popped on the stack.
DS (Data Segment)-contains the origin address
of the active stack segment. Contains static data
ES (Extra Segment)- contains the origin address
of the active extra segment. Extra segment for
data segment.
Control Registers
IP (Instruction Pointer) – The offset in the code
segment where the next instruction is. CS:IP
Flag Register – a 16 bit register that indicates
the current state of the microprocessor.









Overflow Flag- allows overflow or not
Direction Flag- to add or subtract from index registers
Interrupt Flag- allows system interrupts
Trap Flag- allows to trace single-step execution
Sign Flag- signed or unsigned numbers
Zero Flag- tests if operation produces zero
Auxiliary Carry Flag- carry or borrow in arithmetic
operations
Parity Flag- parity bit is odd or even
Carry Flag- used to detect overflow
Flag Register
Graphic taken from “A guide to Debug”
http://mirror.href.com/thestarman/asm/debug/8086REGs.htm#REGS
Instruction categories
Data Transfer Instructions
Stack Instructions
Data Manipulation Instructions


Arithmetic
Logical
Program Control Instructions
Interrupt Instructions
Data Transfer Instructions
MOVE INSTRUCTION
MOV <destination>,<source>

MOV AX,0001
EXCHAGE INSTRUCTION
XCHG <destination>,<source>

XCHG AX,DX
Stack Instructions
We cannot directly move or exchange data to
the stack because it is not a register. The stack
is a block of contiguous memory addresses.
We must push the data on to the stack and pop
it off the stack.
PUSH <source>
POP <destination>
We can push the 16 bit Flag register on to the
stack by using the following
PUSHF
POPF
Data Manipulation Instructions
Arithmetic Instructions
ADDITION
ADD <Destination>,<source>

ADD AX,0001
SUBTRACTION
SUB <Destination>,<source>

SUB AX,0001
MULTIPLICATION
MUL <source>

MUL BL ; Multiply AL=AL*BL notice only low bits
IMUL <source>

IMUL BX ; Integer Multiply AX=AX*BX notice the whole 16 bits
DIVISION
DIV <source>

DIV BL ; Divide AL=AL/BL notice only low bits.
IDIV <source>

IDIV BX ; Integer Divide AX=AX/BX notice the whole 16 bits.
Data Manipulation Instructions #2
Logical Instruction
Logical AND
AND <destination>,<source>

AND AL,BL
Logical OR
OR <destination>,<source>

OR AL,BL
Exclusive OR
XOR <destination>,<source>

XOR AL,BL
NEGATE/ 2’S COMPLEMENT
NEG <destination>

NET AL
NOT/ 1’S COMPLEMENT
NOT <destination>

NOT AL
Program Control Instructions
JMP <destination> ; Unconditional Jump
CMP <address1>,<address2> ; Compare and
set flags
JA <destination> ; Jump if above
JB <destination>; Jump if bellow
JE <destination>; Jump if Equal
JNE <destination>; Jump if not equal
Instructions are used to make logical decisions
in a program
Interrupt Instructions
An interrupt is “an external request for service-a
request for the processor to stop executing a
procedure and to being executing a procedure
that has been previously designated to service
interrupts of the designated type” (Runnion, pg
374)
We will use Interrupts to get Input/Output (I/O)
in the assembly language.
We can use interrupts to write onto a hard disk,
read data from the keyboard, and display
information to the monitor.
Printing ! to the Screen
MOV AH,02 ; Interrupt instruction to print
MOV DL,24 ; 24 ASCII code for ‘!’
INT 21 ; Call the Interrupt 21
MOV AH,4C; Interrupt instruction to return
INT 21; Call INT 21
Reading Char from keyboard until ‘!’
Assume instruction start at memory address 100
JMP 108 ; Jump to memory location
MOV AH,02 ; Move 02 Print instruction
MOV DL,07 ; Move 07 into DL to beep
INT 21
; Call Interrupt 21
MOV AH,01 ; Move 01 Read Char from keyboard
INT 21
; Call Interrupt 21
CMP AL,21 ; Compare if Char is ‘!’
JNZ 102
; if not print char and read again
MOV AH,4C ; Move 4C to quit safely
INT 21
; Call Interrupt
NOP
; NO Operation
Debug –Assembly code in action
A small tool called “debug”, which is
included in every windows computer, will
show the registers and the assembly
language instructions.
Works Cited
Runnion, William C. “Structured Programming
in Assembly language for the IBM PC”
PWS-KENT –Massachusetts 1998
Mano & Kime, M.Morris & Charles. “Logic and
Computer Design Fundamentals”
Prentice Hall – New Jersey 2004
Hyde, Randall. “The Art of Assembly”
No Starch Press- California 2003