Using Immediate and Memory Operands (120070)

SUMMARY

An immediate operand is a constant value or expression. A direct memory operand refers to data at a given address. An indirect memory operand also refers to the contents of a given address, rather than the address itself, but the address is calculated at run time by referring to the contents of the specified register.

Initially, it would appear that putting brackets ([]) around the operand makes it a memory operand. This article demonstrates how the assembler determines the addressing mode to use based on the operand type and the brackets, not just on the presence or absence of brackets.

MORE INFORMATION

Case 1

In the following code fragment, both mov instructions move dataitem into the ax register.

   .DATA
   dataitem DW 0

   .CODE
   mov ax, dataitem
   mov ax, [dataitem]

The results are the same because the assembler determines the addressing mode based on the way that dataitem is declared. In both cases, dataitem is a label, so the assembler will access the data located at the label.

If you really want to access dataitem as a constant value, use the following syntax:

   .CODE
   mov ax, OFFSET dataitem

In this case, OFFSET tells the assembler to treat dataitem as a constant value rather than as a data item that is located at a memory location.

Case 2

In the following code fragment, both mov instructions move 0 into the ax register.

   datavalue EQU 0

   .CODE
   mov ax, datavalue
   mov ax, [datavalue]

Again, the result is the same, because the assembler determines the addressing mode based on the way datavalue is declared. In both cases, datavalue is an equate, so the assembler will treat datavalue as a constant value.

If you want to access datavalue as a data item located at a memory location, use the following syntax:

   .CODE
   mov ax, ds:datavalue

In this case, the segment override tells the assembler to treat datavalue as a data item that is located at ds:datavalue, rather than treating datavalue as a constant value.

Case 3

On the other hand, with the following code fragment, the mov instructions have different results.

   .CODE
   mov ax, bx
   mov ax, [bx]

In the first case, we are moving what is in bx into ax. In the second case, we are using bx as an offset and moving what is at that offset into ax. The assembler determines what to do based on the brackets or lack of brackets around the register. This is the only case where the brackets have this effect.