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Assembly Language
Introduction to Assembly
Assembly-Basic Concepts
Assembly Programming
Assembler language Instructions(1)
Assembler language Instructions(2)
Assembly Interruptions
Assembly Files
Assembly-Macros and procedures

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Assembler language Instructions(1)


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Assembler language Instructions

A D V E R T I S E M E N T
  • Transfer instructions

  • Loading instructions

  • Stack instructions

  • Logic instructions

  • Arithmetic instructions

Transfer instructions

They are used to move the contents of the operators. Each instruction can be used with different modes of addressing.

MOV
MOVS (MOVSB) (MOVSW)

MOV INSTRUCTION

Purpose: Data transfer between memory cells, registers and the accumulator.

Syntax:

MOV Destiny, Source

Where Destiny is the place where the data will be moved and Source is the place where the data is.

The different movements of data allowed for this instruction are:

*Destiny: memory. Source: accumulator
*Destiny: accumulator. Source: memory
*Destiny: segment register. Source: memory/register
*Destiny: memory/register. Source: segment register
*Destiny: register. Source: register
*Destiny: register. Source: memory
*Destiny: memory. Source: register
*Destiny: register. Source: immediate data
*Destiny: memory. Source: immediate data

Example:

MOV AX,0006h
MOV BX,AX
MOV AX,4C00h
INT 21H

This small program moves the value of 0006H to the AX register, then it moves the content of AX (0006h) to the BX register, and lastly it moves the 4C00h value to the AX register to end the execution with the 4C option of the 21h interruption.

MOVS (MOVSB) (MOVSW) Instruction

Purpose: To move byte or word chains from the source, addressed by SI, to the destiny addressed by DI.

Syntax:

MOVS

This command does not need parameters since it takes as source address the content of the SI register and as destination the content of DI. The following sequence of instructions illustrates this:

MOV SI, OFFSET VAR1
MOV DI, OFFSET VAR2
MOVS

First we initialize the values of SI and DI with the addresses of the VAR1 and VAR2 variables respectively, then after executing MOVS the content of VAR1 is copied onto VAR2.

The MOVSB and MOVSW are used in the same way as MOVS, the first one moves one byte and the second one moves a word.

Loading instructions

They are specific register instructions. They are used to load bytes or chains of bytes onto a register.

LODS (LODSB) (LODSW)
LAHF
LDS
LEA
LES
LODS (LODSB) (LODSW) INSTRUCTION

Purpose: To load chains of a byte or a word into the accumulator.

Syntax:

LODS

This instruction takes the chain found on the address specified by SI, loads it to the AL (or AX) register and adds or subtracts , depending on the state of DF, to SI if it is a bytes transfer or if it is a words transfer.

MOV SI, OFFSET VAR1
LODS

The first line loads the VAR1 address on SI and the second line takes the content of that locality to the AL register.

The LODSB and LODSW commands are used in the same way, the first one loads a byte and the second one a word (it uses the complete AX register).

LAHF INSTRUCTION

Purpose: It transfers the content of the flags to the AH register.

Syntax:

LAHF

This instruction is useful to verify the state of the flags during the execution of our program.

The flags are left in the following order inside the register:

SF ZF ?? AF ?? PF ?? CF

LDS INSTRUCTION

Purpose: To load the register of the data segment

Syntax:

LDS destiny, source

The source operator must be a double word in memory. The word associated with the largest address is transferred to DS, in other words it is taken as the segment address. The word associated with the smaller address is the displacement address and it is deposited in the register indicated as destiny.

LEA INSTRUCTION

Purpose: To load the address of the source operator

Syntax:

LEA destiny, source

The source operator must be located in memory, and its displacement is placed on the index register or specified pointer in destiny.

To illustrate one of the facilities we have with this command let us write an equivalence:

MOV SI,OFFSET VAR1

Is equivalent to:

LEA SI,VAR1

It is very probable that for the programmer it is much easier to create extensive programs by using this last format.

LES INSTRUCTION

Purpose: To load the register of the extra segment

Syntax:

LES destiny, source

The source operator must be a double word operator in memory. The content of the word with the larger address is interpreted as the segment address and it is placed in ES. The word with the smaller address is the displacement address and it is placed in the specified register on the destiny parameter.

Stack instructions

These instructions allow the use of the stack to store or retrieve data.

POP
POPF
PUSH
PUSHF


POP INSTRUCTION

Purpose: It recovers a piece of information from the stack

Syntax:

POP destiny

This instruction transfers the last value stored on the stack to the destiny operator, it then increases by 2 the SP register. This increase is due to the fact that the stack grows from the highest memory segment address to the lowest, and the stack only works with words, 2 bytes, so then by increasing by two the SP register, in reality two are being subtracted from the real size of the stack.

POPF INSTRUCTION

Purpose: It extracts the flags stored on the stack

Syntax:

POPF

This command transfers bits of the word stored on the higher part of the stack to the flag register.

The way of transference is as follows:

BIT FLAG

0 CF
2 PF
4 AF
6 ZF
7 SF
8 TF
9 IF
10 DF
11 OF

These localities are the same for the PUSHF command.

Once the transference is done the SP register is increased by 2,
diminishing the size of the stack.

PUSH INSTRUCTION

Purpose: It places a word on the stack.

Syntax:

PUSH source

The PUSH instruction decreases by two the value of SP and then transfers the content of the source operator to the new resulting address on the recently modified register.

The decrease on the address is due to the fact that when adding values to the stack, this one grows from the greater to the smaller segment address, therefore by subtracting 2 from the SP register what we do is to increase the size of the stack by two bytes, which is the only quantity of information the stack can handle on each input and output of information.

PUSHF INSTRUCTION

Purpose: It places the value of the flags on the stack.

Syntax:

PUSHF

This command decreases by 2 the value of the SP register and then the content of the flag register is transferred to the stack, on the address indicated by SP.

The flags are left stored in memory on the same bits indicated on the POPF command.

Logic instructions

They are used to perform logic operations on the operators.

AND
NEG
NOT
OR
TEST
XOR


AND INSTRUCTION

Purpose: It performs the conjunction of the operators bit by bit.

Syntax:

AND destiny, source

With this instruction the "y" logic operation for both operators is carried
out:

Source Destiny | Destiny
-----------------------------
1 1 | 1
1 0 | 0
0 1 | 0
0 0 | 0

The result of this operation is stored on the destiny operator.

NEG INSTRUCTION

Purpose: It generates the complement to 2.

Syntax:

NEG destiny

This instruction generates the complement to 2 of the destiny operator and stores it on the same operator.

For example, if AX stores the value of 1234H, then:

NEG AX

This would leave the EDCCH value stored on the AX register.

NOT INSTRUCTION

Purpose: It carries out the negation of the destiny operator bit by bit.

Syntax:

NOT destiny

The result is stored on the same destiny operator.

OR INSTRUCTION

Purpose: Logic inclusive OR

Syntax:

OR destiny, source

The OR instruction carries out, bit by bit, the logic inclusive disjunction
of the two operators:

Source Destiny | Destiny
-----------------------------------
1 1 | 1
1 0 | 1
0 1 | 1
0 0 | 0


TEST INSTRUCTION

Purpose: It logically compares the operators

Syntax:

TEST destiny, source

It performs a conjunction, bit by bit, of the operators, but differing from AND, this instruction does not place the result on the destiny operator, it only has effect on the state of the flags.

XOR INSTRUCTION

Purpose: OR exclusive

Syntax:

XOR destiny, source Its function is to perform the logic exclusive disjunction of the two operators bit by bit.

Source Destiny | Destiny
-----------------------------------
1 1 | 0
0 0 | 1
0 1 | 1
0 0 | 0

Arithmetic instructions

They are used to perform arithmetic operations on the operators.

ADC
ADD
DIV
IDIV
MUL
IMUL
SBB
SUB

ADC INSTRUCTION

Purpose: Cartage addition

Syntax:

ADC destiny, source

It carries out the addition of two operators and adds one to the result in case the CF flag is activated, this is in case there is carried.

The result is stored on the destiny operator.

ADD INSTRUCTION

Purpose: Addition of the operators.

Syntax:

ADD destiny, source

It adds the two operators and stores the result on the destiny operator.

DIV INSTRUCTION

Purpose: Division without sign.

Syntax:

DIV source

The divider can be a byte or a word and it is the operator which is given the instruction.

If the divider is 8 bits, the 16 bits AX register is taken as dividend and if the divider is 16 bits the even DX:AX register will be taken as dividend, taking the DX high word and AX as the low.

If the divider was a byte then the quotient will be stored on the AL register and the residue on AH, if it was a word then the quotient is stored on AX and the residue on DX.

IDIV INSTRUCTION

Purpose: Division with sign.

Syntax:

IDIV source

It basically consists on the same as the DIV instruction, and the only difference is that this one performs the operation with sign.For its results it used the same registers as the DIV instruction.

MUL INSTRUCTION

Purpose: Multiplication with sign.

Syntax:

MUL source

The assembler assumes that the multiplicand will be of the same size as the multiplier, therefore it multiplies the value stored on the register given as operator by the one found to be contained in AH if the multiplier is 8 bits or by AX if the multiplier is 16 bits. When a multiplication is done with 8 bit values, the result is stored on the AX register and when the multiplication is with 16 bit values the result is stored on the even DX:AX register.

IMUL INSTRUCTION

Purpose: Multiplication of two whole numbers with sign.

Syntax:

IMUL source

This command does the same as the one before, only that this one does take into account the signs of the numbers being multiplied.

The results are kept in the same registers that the MOV instruction uses.

SBB INSTRUCTION

Purpose: Subtraction with cartage.

Syntax:

SBB destiny, source

This instruction subtracts the operators and subtracts one to the result if CF is activated. The source operator is always subtracted from the destiny.

This kind of subtraction is used when one is working with 32 bits quantities.

SUB INSTRUCTION

Purpose: Subtraction.

Syntax:

SUB destiny, source

It subtracts the source operator from the destiny.



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Keywords: assembly language,assemly instructions,assembly macros,assembly procedures,assembly interruptions

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