4.1 DATA MOVEMENT IN ANY PROGRAM IT IS NECESSARY TO MOVE THE DATA IN...

3.4.1 Data movement

In any program it is necessary to move the data in the memory and in the CPU

registers; there are several ways to do this: it can copy data in the

memory to some register, from register to register, from a register to a

stack, from a stack to a register, to transmit data to external devices as

well as vice versa.

This movement of data is subject to rules and restrictions. The following

are some of them:

*It is not possible to move data from a memory locality to another

directly; it is necessary to first move the data of the origin locality to a

register and then from the register to the destiny locality.

*It is not possible to move a constant directly to a segment register; it

first must be moved to a register in the CPU.

It is possible to move data blocks by means of the movs instructions, which

copies a chain of bytes or words; movsb which copies n bytes from a

locality to another; and movsw copies n words from a locality to another.

The last two instructions take the values from the defined addresses by

DS:SI as a group of data to move and ES:DI as the new localization of the

data.

To move data there are also structures called batteries, where the data is

introduced with the push instruction and are extracted with the pop

instruction.

In a stack the first data to be introduced is the last one we can take,

this is, if in our program we use these instructions:

PUSH AX

PUSH BX

PUSH CX

To return the correct values to each register at the moment of taking them

from the stack it is necessary to do it in the following order:

POP CX

POP BX

POP AX

For the communication with external devices the out command is used to send

information to a port and the in command to read the information received

from a port.

The syntax of the out command is:

OUT DX,AX

Where DX contains the value of the port which will be used for the

communication and AX contains the information which will be sent.

The syntax of the in command is:

IN AX,DX

Where AX is the register where the incoming information will be kept and DX

contains the address of the port by which the information will arrive.