Microprocessors & Interfacing
Assembly
Programming
Fundamentals
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Introduction
Machine language: is a numeric language (‘0’s
and ‘1’s) specifically understood by a computer’s
processor (the CPU).
Assembly language: is a series of statements
which are either are Instructions or Directives.
Assembly language: has a one-to-one relationship
with machine language.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Directives
Directives:
are statements that give directions to the
assembler about how it should translate the assembly language
instructions into machine code.
Directives:
are used by the assembler to organize the
program as well as other output files. SEGMENT, DB, ENDS,
ASSUME, END, and ENDP are examples of directives.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Instructions
Instruction:
an assembly language instruction consists
of four fields:
[label:] mnemonic [operands] [;comments]
 Brackets indicate that the field is optional. Brackets are not
typed.
 The label field allows the program to refer to a line of code by
name. In a line of assembly language program there can be
mnemonic (instruction) and operand(s).
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Instruction format
Instructions:
 Examples:
MOV AL,BL
;
ADD AL,16H
;
MOV and ADD are mnemonic opcodes, and AL, BL, and
16H are the operands.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
MOV Instruction:
MOV destination, source; copy source operand to destination
Mnemonic
Dr. Abduljalil Radman
Comment
Operands
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
• These types of operands are supported:
MOV REG, Memory
MOV Memory, REG
MOV REG, REG
MOV Memory, constant
MOV REG, Constant
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH,
DL, DI, SI, BP, SP.
Memory: [BX], [BX+SI+7], variable, etc...
Constant: 5, -24, 3Fh, 10001101b, etc...
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
Examples:
MOV CL,16H
; register, constant
MOV DL,CL
; register, register
MOV BH,25-5
; register, constant-expression
MOV BL,[010A]
; register, memory
MOV
CX,468FH
;move 468FH into CX (CH =46 , CL=8F)
MOV
AX,CX
;move/copy the contents of CX into AX
MOV
BX,AX
;BX=AX=468FH
MOV
DX,BX
;DX=BX=468FH
MOV
DI,AX
;DI=AX=468FH
MOV
SI,DI
;SI=DI=468FH
MOV
DS,SI
;DS=SI=468FH
MOV
BP,DS
;BP=DS=468FH
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Rules Regarding MOV Instruction
• Data can be moved among all registers except the
flag register.
• Source and destination registers must be in the
same size.
• Data can be moved directly into non segment
registers only. You can’t move data to segment
registers directly.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
Example:
Dr. Abduljalil Radman
MOV
BX,A45DH
MOV
SI,ADCFH
MOV
CS,23AFH
MOV
DS,BX
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Adding and Subtracting Integers
Example:
Dr. Abduljalil Radman
MOV AL,5
; move 5 to the AL register.
ADD AL,6
; add 6 to the AL register.
SUB AL,2
; sub 2 from the AL register.
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
• For segment registers, only these types of
MOV are supported:
MOV
MOV
MOV
MOV
SREG, memory
memory, SREG
REG, SREG
SREG, REG
SREG: DS, ES, SS, and only as source operand: CS.
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
Memory: [BX], [BX+SI+7], variable, etc...
The MOV instruction cannot be used to set the value of the CS and IP registers.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
MOV Instruction
Example:
MOV
AX, 0B800H
; set AX to hexadecimal value of B800h.
MOV
DS, AX
; copy value of AX to DS.
MOV
CL, 'A’
; set CL to ASCII code of 'A', it is 41H.
MOV
CH, 01011111b ; set CH to binary value.
MOV
BX, 15Eh
; set BX to 15Eh.
MOV
[BX], CX
; copy contents of CX to memory at B800:015E
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Full Segment Definition
SEGMENT and ENDS directives:
indicate the
beginning and ending of a segment and have the following format:
label SEGMENT [options]
; place the statements belong to this segment here
label ENDS
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Full Segment Definition
• Stack Segment Definition
STSEG SEGMENT
DB 64 DUP (?)
;directive reserves 64 bytes of memory for the Stack
STSEG ENDS
• Data Segment Definition
There are three data items in this sample program:
DATA1, DATA2 and SUM. Each is defined as DB (Define Byte).
 The DB directive is used to allocate memory in byte sized chunks.
 DW (Define Word) allocates 2 bytes of memory. DATA1 and
DATA2 have initial values but SUM is reserved for later use.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Full Segment Definition
• Code segment definition
 The first line after the SEGMENT directive is the PROC
directive. A procedure is a group of instructions designed to
accomplish a specific function.
 The PROC and ENDP directives must have the same label. The
PROC directive may have the option FAR or NEAR .
 ASSUME directive associates segment registers with specific
segments by assuming that the segment register is equal to the
segment labels used in the program.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Simplified Segment Definition
• The directives .stack, .code, and .data are used to
identify the three different segments.
• With this definition, the .MODEL directive is
introduced to specify the memory model used.
 The common used memory model is SMALL (one
segment for Data, and one segment for code).
• The .MODEL directive should be before any segment
definition.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Simplified Segment Definition
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Full and Simplified Segment Definition
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Assembler Data Directives
• DB : Define Byte.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Assembler Data Directives
• DUP : Duplicate.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Assembler Data Directives
• DW : Define Word.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Assembler Data Directives
• DD : Define Double Word.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Stack
Why and how to be accessed
• The stack:
is a section of memory, used by the CPU to store
information temporarily:
 The SS (stack segment) register
 The SP (stack pointer) register
 These registers must be loaded before any instructions accessing
the stack are used.
• Storing a CPU register in the stack is called a PUSH.
• Loading the contents of the stack into the CPU register is called a
POP.
 Logical Address in Stack Segment is represented by using
segment address in SS register and Offset Address in SP register:
SS:SP.
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
PUSH and POP
Instruction format
PUSH
source
; Copy the content of source into the stack
Mnemonic
operand
comment
POP
destination
; Copy the content of destination into the
stack
Mnemonic
Dr. Abduljalil Radman
operand
comment
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
PUSH
Example: Assuming that
SP
=
1236 H ,
AX
=
24B6 H ,
DI
=
85C2 H ,
and
DX
=
5F93 H ,
show the contents of the stack as each of the following instructions is
executed:
PUSH
PUSH
PUSH
Dr. Abduljalil Radman
AX
DI
DX
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
PUSH
Solution:
*Note that in 80x86 the lower byte of the register is stored to the lower
address. (Little Endian Convention).
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
POP
Example: Assuming that
SP
=
18FA H ,
show the contents of the stack as each of the following instructions is
executed:
POP
POP
POP
CX
DX
BX
Last In First out (LIFO)
*Note that in 80x86 the lower
byte of the register is stored to
the lower address. (Little Endian
Convention).
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
Flag register
There are two instructions that can be used to
use/change the content of the flag register.
 PUSHF
 POPF
; Copy the flag register into stack
; Copy from the stack into the flag register
Example:
PUSHF
POPF
Dr. Abduljalil Radman
; copy the content of Flag register into the stack.
;
Microprocessors & Interfacing
Taiz University 2020
Assembly Programming
8086 EMULATOR
Dr. Abduljalil Radman
Microprocessors & Interfacing
Taiz University 2020