assembly language! please help me! thanks in advance!

topheraholic · 03-19-2011, 03:37 AM

what is the difference between this two instructions? thanks

Code:

array WORD 1,2,3,4,5
mov esi,OFFSET array+4
mov esi,array+4

business_kid · 03-19-2011, 04:08 AM

Get real, please.
You have multiple queries with the same subject and no details. What assembler? What CPU? Business or Pleasure?

Quote:

mov esi,OFFSET array+4
mov esi,array+4

In many assemblers, OFFSET would be taken as a variable. So your instruction would be seen as
mov esi $OFFSET
and array+4 on that line would produce a syntax error about an extra operand. I would expect every assembler I have used to barf on mov esi,array+4 as a syntax error.

topheraholic · 03-19-2011, 04:26 AM

Quote:

Originally Posted by business_kid

Get real, please.
You have multiple queries with the same subject and no details. What assembler? What CPU? Business or Pleasure?

In many assemblers, OFFSET would be taken as a variable. So your instruction would be seen as
mov esi $OFFSET
and array+4 on that line would produce a syntax error about an extra operand. I would expect every assembler I have used to barf on mov esi,array+4 as a syntax error.

oh sorry! my assembler is nasm(intel syntax).and yours is GAS(AT&T syntax),right? for pleaseure.thanks

theNbomr · 03-19-2011, 10:24 AM

This is one of those quirky x86-isms. Its been a while since I've done any assembler coding there, but here goes. The OFFSET assembler directive tells the assembler to use the specified address as an OFFSET (duh..) for the start of the default segment. This is distinct from the actual address, since the segment may get loaded such that its segment register is not zero-based. It is one of those things that seems to have turned a lot of people off of the whole x86 family.

--- rod.

topheraholic · 03-19-2011, 10:48 AM

Quote:

Originally Posted by theNbomr

This is one of those quirky x86-isms. Its been a while since I've done any assembler coding there, but here goes. The OFFSET assembler directive tells the assembler to use the specified address as an OFFSET (duh..) for the start of the default segment. This is distinct from the actual address, since the segment may get loaded such that its segment register is not zero-based. It is one of those things that seems to have turned a lot of people off of the whole x86 family.

--- rod.

thanks,but i am not quite understand what you are saying,could you say it more in detail?thanks very much!

theNbomr · 03-19-2011, 02:21 PM

I might be able to, but it sounds like you lack some of the background knowledge to understand it. To write enough to build all of that would be beyond the scope of this forum. Do you understand the relationship between the segment registers, the pointer registers, and the linker in building assembly language object code? With a thorough understanding of these concepts, you should be able to understand my original answer. Having said that, in practical terms, there are few cases where the use of the OFFSET directive adds anything.
The OFFSET directive is only used in the Microsoft Assembler to disambiguate between specification of an address and the content of an address. Since you are using NASM, this is already done by the assembler enforcing the use of the [square_bracket] notation to specify an address content. Example:

Code:

   mov   esi, myLabel      ; makes esi point to address 'myLabel'
                           ; (but, in MASM, it might do like below)
   mov   esi, [myLabel]    ; loads the word stored at ds:myLabel into register esi

See 2.2.2 NASM Requires Square Brackets For Memory References for more details.

--- rod.

smeezekitty · 03-19-2011, 08:15 PM

Quote:

Originally Posted by theNbomr

It is one of those things that seems to have turned a lot of people off of the whole x86 family.

--- rod.

But its still better then the backwards syntax and useless % of at&t syntax.
-
I could be wrong since ASM is not my specialty but I think that the second one copies the value and the first copies the offset of the segment

ffset pair.