In the following tutorial:
http://www.friedspace.com/assembly/moving.php
, could you tell me the differences and the effects of the differences between the following statements?
MOV ,CH

MOV myvar1,CH
Posted on 2011-10-06 00:36:37 by bolzano_1989
First one is using a pointer, the second one is not!
Means that in:
case 1: ch is being copied to the location pointed to by var1
case 2: ch is being copied directly to var1

Having said that, although its essential to use with some compilers,
it can sometimes be ignored by others (masm for example) at least in this particular case
[] will be discarded.
Point being that I think its always good practice to test what you read.
Posted on 2011-10-06 06:38:00 by MACH4
In MASM-like assemblers it's the same. In other assemblers AFAIK it's the same (maybe some raise an error for case 2 about bracket requirement) - there is no double dereferencing in asm! The difference is (AFAIK) in "mov eax,var" where one would take offset of a var in some assemblers and contents of a var in others. Furthermore, some assemblers have type checking, so "mov eax,var" for dword var is ok but for byte var is not.
Posted on 2011-10-06 07:30:13 by drizz
The page says: "If we want to move the data at a byte memory location"

Thought I'd clarify what I said! To access memory we have to use pointers, No comment on other compilers but in masm this is an example of what would happen:
.data
myVar db "abc",0

.code
mov al, myVar
mov al, BYTE PTR myVar
mov al, BYTE PTR
Either of above will compile as: mov al, BYTE PTR ds:

For a dword just keep the sizes right

.data
myVar dd 10

.code
mov eax, myVar
mov eax, DWORD PTR  myVar
mov eax, DWORD PTR
either of above will compile as: mov eax, DWORD PTR ds:


I always add the sq brackets just to make the code more infomative...
Posted on 2011-10-06 10:31:33 by MACH4
Just out of curiosity I downloaded fasm and tried it:

mov  al, _message                ;no
mov  al,BYTE PTR [_message]      ;no
mov  al,BYTE PTR ds:[_message]  ;no
mov  al, [_message]              ;yes
mov  al,BYTE PTR _message        ;yes


With fasmW the sq brackets appear to hold the command of the appropriate size pointer! ie brackets must be used, or "BYTE PTR" etc
Posted on 2011-10-06 13:48:35 by MACH4
fasm has one more you skipped: mov al, byte [_message]. Also, notice that fasm requires "ds:" (or any other segment register) to be inside the square brackets: mov al, byte .
Posted on 2011-10-06 17:07:22 by LocoDelAssembly
FWIW, Nasm is more like Fasm. An unadorned "myvar" means "address of myvar"... like "offset myvar" in Masm-like assemblers. Nasm requires "" to mean "contents", while Masm (etc) would treat either "myvar" or "" the same (contents). Gas uses "$" like "offset".

Nasm syntax:

mov al, ;
mov eax, myvar ; address (offset) of myvar
mov al, myvar ; error - won't fit!
mov , al
mov myvar, al ; error!


Best,
Frank

Posted on 2011-10-06 18:14:02 by fbkotler