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.
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