Question

I have the next code:

mov  ax,@data
mov  ds,ax

Why I can not write just like this?

mov ds,@data

All source:

   .MODEL small
   .STACK 100h
   .DATA
   HelloMessage DB 'Hello, world',13,10,'$'
   .CODE 
   .startup
   mov  ax,@data
   mov  ds,ax
   mov  ah,9
   mov  dx,OFFSET HelloMessage
   int  21h
   mov  ah,4ch
   int  21h
   END

Thank you!

Answer 1

You can’t, because the instruction set doesn’t contain an instruction to do that. It is just one of the many idiosyncrasies of the x86.

These kind of restrictions are fairly normal for assembly languages. Most architectures contain some registers that are treated specially (for example the processor status word), though usually fewer than the x86 architecture.

The reason to not provide an instruction for all possible moves is to reduce the size of the instruction set, so that an instruction takes less memory. Overall it is more efficient to do moves that are rarely needed in two steps.