1. 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
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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
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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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4. Flow of program development
.asm
Object file
.obj
Executable file
.exe
link
Assemble
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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 ; only one instruction - MOVE
A = A+C ; only one instruction - ADD
A = A*100 ; only one instruction - Multiply
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8. 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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15. 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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16. 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 byte-wide 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 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 ; indicate start of code segment
.startup ; indicate start of program
mov AX, 0
mov BX, 0000H
mov CX, 0
mov SI, AX
mov DI, AX
mov 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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20. 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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25. 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 ; move 0H to DL
mov CL, op1 ; move op1 to CL
mov AL, data ; move data to AL
step:
cmp AL, op1 ; compare AL and op1
jc label1 ; if carry =1 jump to label1
sub AL, op1 ; AL = AL –op1
inc DL ; DL = DL+1
jmp step ; jump to step
label1:
mov AH, DL ; move DL to AH
HLT ; Halt end of program
data db ; define a variable called data
op1 db 6 ; define a variable called op1
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Assembler for 8086
Emu8086 ( – 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 –
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28. 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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30. Example for data definition
DUP – duplicate
DUP can be used to define multiple storages
DB 10 DUP (?) ; defines 10 bytes not initialize
DB 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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