Data movement
I am listing some assembly language Instructions for data operations here for your knowledge and convenience, when we’ll be doing programming with the help of interrupts and other BIOS operations we’ll need its basics.
Detailed introduction and thorough study of assembly language is beyond the limit of this book. Though the knowledge of basics of assembly is necessary to proceed in the further important programming chapters yet if you feel it difficult to understand all the Instructions, you need not to worry but it is recommended that you should gain the basic idea of Instructions.
In any program it is necessary to move the data in the memory and in the CPU registers and 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 one memory location to another directly. It is necessary to first move the data of the source location to a register and then from the register to the destination 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. MOCSB which copies n bytes from one location to another and MOVSW copies n words from one location 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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