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

The "??" means that there will be an undefined value in those bits.

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.

4.3 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.

Date: 2014-12-22; view: 804