8086 Assembly Language
Programming I
Week 4 – Overview of the x86
assembly language prgoramming
Assembly language programming
• Learning assembly language programming will
help understanding the operations of the
microprocessor
• To learn:
– Need to know the functions of various registers
– Need to know how external memory is organized and
how it is addressed to obtain instructions and data
(different addressing modes)
– Need to know what operations (or the instruction set)
are supported by the CPU. For example, powerful CPUs
support floating-point operations but simple CPUs only
support integer operations
2
How to learn programming
•
•
•
•
C –Concept
L – Logic thinking
P – Practice
Concept – we must learn the basic syntax, such as
how a program statement is written
• Logic thinking – programming is problem solving
so we must think logically in order to derive a
solution
• Practice – write programs
3
Assembly Program
• The native language is machine language (using
0,1 to represent the operation)
• A single machine instruction can take up one or
more bytes of code
• Assembly language is used to write the program
using alphanumeric symbols (or mnemonic), eg
ADD, MOV, PUSH etc.
• The program will then be assembled (similar to
compiled) and linked into an executable program.
• The executable program could be .com, .exe,
or .bin files
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Flow of program development
Program
.asm
Object file
.obj
Assemble
link
Executable file
.exe
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Example
• Machine code for mov AL, 00H
• B4 00 (2 bytes)
• After assembled, the value B400 will be
stored in the memory
• When the program is executed, then the
value B400 is read from memory, decoded
and carry out the task
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Assembly Program
• Each instruction is represented by one assembly
language statement
• The statement must specify which operation (opcode)
is to be performed and the operands
• Eg ADD AX, BX
• ADD is the operation
• AX is called the destination operand
• BX is called the source operand
• The result is AX = AX + BX
• When writing assembly language program, you need
to think in the instruction level
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Example
• In c++, you can do A = (B+C)*100
• In assembly language, only one instruction
per statement
A=B
A = A+C
A = A*100
; only one instruction - MOVE
; only one instruction - ADD
; only one instruction - Multiply
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Format of Assembly language
• General format for an assembly language
statement
• Label
Instruction
Comment
• Start: Mov AX, BX
; copy BX into AX
Start is a user defined name and you only put in a
label in your statement when necessary!!!!
The symbol : is used to indicate that it is a label
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8086 Software Model
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Software model
• In 8086, memory is divided into segments
• Only 4 64K-byte segments are active and these are: code,
stack, data, and extra
• When you write your assembly language program for an
8086, theoretically you should define the different
segments!!!
• To access the active segments, it is via the segment
register: CS (code), SS (stack), DS (data), ES (extra)
• So when writing assembly language program, you must
make use of the proper segment register or index register
when you want to access the memory
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Registers
• In assembly programming, you cannot operate on
two memory locations in the same instruction
• So you usually need to store (move) value of one
location into a register and then perform your
operation
• After the operation, you then put the result back
to the memory location
• Therefore, one form of operation that you will use
very frequent is the store (move) operation!!!
• And using registers!!!!!
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Example
• In C++ A = B+C ; A, B, C are variables
• In assembly language A,B, C representing
memory locations so you cannot do A =
B+C
– MOV AL, B ; move value of B into AL register
– ADD, AL, C ; do the add AL = AL +C
– MOV A, AL ; put the result to A
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Data registers
• AX, BX, CX,and DX – these are the general purpose
registers but each of the registers also has special function
Example
– AX is called the accumulator – to store result in arithmetic
operations
• Registers are 16-bit but can be used as 2 8-bit storage
• Each of the 4 data registers can be used as the source or
destination of an operand during an arithmetic, logic, shift,
or rotate operation.
• In some operations, the use of the accumulator is
assumed, eg in I/O mapped input and output operations
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Data register
• In based addressing mode, base register BX is
used as a pointer to an operand in the current
data segment.
• CX is used as a counter in some instructions, eg.
CL contains the count of the number of bits by
which the contents of the operand must be
rotated or shifted by multiple-bit rotate
• DX, data register, is used in all multiplication and
division, it also contains an input/output port
address for some types of input/output
operations
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Pointer and index registers
• Stack – is used as a temporary storage
• Data can be stored by the PUSH instruction and
extracted by the POP instruction
• Stack is accessed via the SP (Stack Pointer) and BP
(Base Pointer)
• The BP contains an offset address in the current
stack segment. This offset address is employed
when using the based addressing mode and is
commonly used by instructions in a subroutine
that reference parameters that were passed by
using the stack
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Pointer and Index Register
• Source index register (SI) and Destination index
register (DI) are used to hold offset addresses for
use in indexed addressing of operands in memory
• When indexed type of addressing is used, then SI
refers to the current data segment and DI refers
to the current extra segment
• The index registers can also be used as source or
destination registers in arithmetic and logical
operations. But must be used in 16-bit mode
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Data types
• Data can be in three forms: 8-bit, 16-bit, or 32-bit
(double word)
• Integer could be signed or unsigned and in bytewide or word-wide
• For signed integer (2’s complement format), the
MSB is used as the sign-bit (0 for positive, 1 for
negative)
• Signed 8-bit integer 127 to –128,
• For signed word 32767 to –32768
• Latest microprocessors can also support 64-bit or
even 128-bit data
• In 8086, only integer operations are supported!!!
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A sample program
.code
.startup
mov
mov
mov
mov
mov
mov
; indicate start of code segment
; indicate start of program
AX, 0
BX, 0000H
CX, 0
SI, AX
DI, AX
BP, AX
END
; end of file
The flow of the program is usually top-down and
instructions are executed one by one!!!
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Assembly programming
In general, an assembly program must include the code segment!!
Other segments, such as stack segment, data segment are not
compulsory
There are key words to indicate the beginning of a segment as
well as the end of a segment. Just like using main(){} in C++
Programming
Example
DSEG segment ‘data’ ; define the start of a data segment
DSEG ENDS ; defines the end of a data segment
Segment is the keyword DSEG is the name of the segment
Similarly key words are used to define the beginning of a program,
as well as the end.
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Assembly language programming
Example
CSEG segment ‘code’
START PROC FAR
; define the start of a program (procedure)
RET
; return
START ENDP
; define the end of a procedure
CSEG
ends
End start ; end of everything
Different assembler may have different syntax for the definition
of the key words !!!!!
Start is just a name it could be my_prog, ABC etc
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More sample
Stacksg segment para ‘stack’
….
; define the stack segment
Stacksg ends
Datasg segment para
……
; declare data inside the data segment
Datasg ends
Codesg segment para ‘code’
Main proc far
;
assume ss:stacksg, ds: datasg, cs:codesg
mov ax, datasg
mov ds, ax
….
mov ax, 4c00H
int 21H
Main
endp
Codesg ends
end
main
End of everything
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Definitions
To declare a segment, the syntax is:
segment_name SEGMENT alignment class
Example Stacksg segment PARA (this statement is used in previous slide)
PARA – define the alignment of the segment base address, the segment with a starting
addressing that is evenly Divisible by 16. But the default value is also base address
divisible by 16 so the key word PARA can be ignored!
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Definition
• ‘data’, ‘code’ – class entry. Is used to group related
segments when linking. The linker automatically
groups segments of the same class in memory
• PROC – define procedures (similar to a function)
inside the code segment. Each procedure must be
identified by an unique name. At the end of the
procedure, you must include the ENDP
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Definitions
FAR – is related to program execution. When you request execution
of a program, the program loader uses this procedure as the entry
point for the first instruction to execute.
Assume – to associate, or to assign, the name of a segment with a segment register
In some assembler, you need to move the base address of a segment directly into the
segment register!!!
END – ends the entire program and appears as the last statement. Usually the name of the
first or only PROC designated as FAR is put after END
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Syntax of a simple assembly
language program
• If you are doing something simple then you do
not need to define the segment
• Everything will be stored in the code segment
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start:
mov DL, 0H
mov CL, op1
mov AL, data
; move 0H to DL
; move op1 to CL
; move data to AL
step:
cmp AL, op1
jc label1
sub AL, op1
inc DL
jmp step
; compare AL and op1
; if carry =1 jump to label1
; AL = AL –op1
; DL = DL+1
; jump to step
label1:
mov AH, DL
; move DL to AH
HLT
; Halt end of program
data db 45 ; define a variable called data
op1 db 6 ; define a variable called op1
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Assembler for 8086
Emu8086 (http:// www.emu8086.com) – there is a trial version but
it does not support all the features such as interrupt
The emu8086 consists of a tutorial and the reference for a
complete instruction set
Keil – www.keil.com
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Defining data in a program
Data is usually stored in the data segment
You can define constants, work areas (a chunk of memory )
Data can be defined in different length (8-bit, 16-bit)
8-bit then use DB
16-bit then use DW
The definition for data:
[name] Dn expression ; Dn is either DB or DW
Name – a program that references a data item by means of a name.
The name of an item is otherwise optional
Dn – this is called the directives. It defines length of the data
Expression – define the values (content) for the data
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Examples for data
FLDA
DB
? ; define an uninitialized item called FLDA 8-bit
FLDB
DB
25 ; initialize a data to 25
Define multiple data under the same name (like an array)
FLDC
DB
21, 22, 23, 34 ; the data are stored in adjacent bytes
FLDC stores the first value
FLDC + 1 stores the second value
You can do
mov AL, FLDC+3
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Example for data definition
DUP – duplicate
DUP can be used to define multiple storages
DB 10 DUP (?) ; defines 10 bytes not initialize
DB 5 DUP (12) ; 5 data all initialized to 12
String :
DB ‘this is a test’
EQU – this directive does not define a data item; instead, it defines
a value that the assembler can use to substitute in other instructions
(similar to defining a constant in C programming or using the #define )
factor
EQU 12
mov
CX, factor
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Assembly Program
• Assembly language should be more effective and
it will take up less memory space and run faster
• In real-time application, the use of assembly
program is required because program that is
written in a high-level language probably could
not respond quickly enough
• You can also put assembly codes into your C++
program in order to reduce the execution time!!!!
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Assembly language programming
• The syntax for different microprocessor may
be different but the concept is the same so
once you learn the assembly programming for
one microprocessor, you can easily program
other kinds of system
• For example, programming the 8051 series is
very similar to the 8086
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Addressing Modes
• Function of the addressing modes is to access the
operands
• Available modes (9 modes): register addressing,
immediate addressing, direct addressing, register
indirect addressing, based addressing, indexed
addressing, based indexed addressing, string
addressing, and port addressing
• Addressing modes provide different ways of
computing the address of an operand
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Why addressing mode is important?
• In c++, you can define an array, or a variable
–
–
–
–
–
int x[10], y, *z;
Then to access different elements, you can do
Z=x;
*(x+2);
x[0] = y
How this can be done using assembly language
programming? This is via different addressing
modes!!!!
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Register addressing mode
• The operand to be accessed is specified as
residing in an internal register of the 8086
• Eg MOV AX, BX
• Move (MOV) contents of BX (the source
operand), to AX (the destination operand)
• Both operands are in the internal registers
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37
Pay attention to the value of IP and content of AX, BX
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Immediate addressing mode
Source operand is part of the instruction
Usually immediate operands represent constant data
The operands can be either a byte or word
e.g MOV
AL, 15
15 is a byte wide immediate source operand
Or it could be
MOV AL, #15
The immediate operand is stored in program storage
memory (i.e the code segment)
This value is also fetched into the instruction queue
in the BIU
No external memory bus cycle is initiated!
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40
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Direct addressing mode
• Move a byte or word between a memory location
and a register
• the locations following the instruction opcode
hold an effective memory address (EA) instead of
data
• The address is a 16-bit offset of the storage
location of the operand from the current value in
the data segment register
• Physcial address = DS + offset
• The instruction set does not support a memoryto-memory transfer!
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Direct addressing
• Data is assumed to be stored in the data segment
so DS is used in calculating the physical address!!!
• External memory bus cycle is needed to do the
read
• Example of direct addressing: mov AL, var1
• Where Var1 can be regarded as a variable
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Register indirect addressing mode
• Transfer a byte or word between a register and a memory
location addressed by an index or base register
– Example MOV AL, [SI]
– SI – index register
– The symbol [] always refer to an indirect addressing
• The effective address (EA) is stored either in a pointer
register or an index register
• The pointer register can be either base register BX or base
pointer register BP
• The index register can be source index register SI, or
destination index register DI
• The default segment is either DS or ES
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Register indirect addressing
• Eg MOV AX, [SI]
• Value stored in the SI register is used as the offset
address
• The segment register is DS in this example
• Meaning of the above is to move the data stored
in the memory location : DS + SI to the AX
register
• In register indirect addressing mode, the EA
(effective address) is a variable and depends on
the index, or base register value
• Eg mov [BX], CL
• Which segment register will be used for the above
operation
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Address (in
HEX)
Content
01236
19
01235
18
01234
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According to the memory map
The result of the operation
Mov [BX], CL will result in what???
If CL = 88 and BX = 1233H and DS =0H
Physical address = DS + BX = 01233H
01233
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