flat assembler
Message board for the users of flat assembler.

Index > Compiler Internals > DS: prefix not generated for some 16-bit address modes

Author
Thread Post new topic Reply to topic
PopeInnocent



Joined: 01 Jan 2004
Posts: 18
Location: USA
PopeInnocent
The following instructions should generate identical machine code:

Code:
mov ax,[ds:si+bp]
mov ax,[ds:bp+si]
    


FASM 1.62 does not generate a DS: prefix for the first instruction. My understanding is that a 16-bit address with BP in it is always relative to the SS segment unless an override is present, so FASM should generate prefixes for those modes.

This appears to be an issue in any 16-bit addressing mode where BP is not the first register to appear in the address. For example,

Code:
mov ax,[ds:di+bp]
mov ax,[ds:10+si+bp]
    


-- Craig Putnam
Post 21 Jul 2005, 20:27
View user's profile Send private message Reply with quote
revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17327
Location: In your JS exploiting you and your system
revolution
You are correct. The Intel manual states that for 16 bit addressing:
Quote:
The default segment register is SS for the effective addresses containing a BP index, DS for other effective
addresses.
Post 22 Jul 2005, 02:10
View user's profile Send private message Visit poster's website Reply with quote
vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
Quote:
The default segment register is SS for the effective addresses containing a BP index, DS for other effective
addresses.

And what about SP then?
Post 22 Jul 2005, 07:31
View user's profile Send private message Visit poster's website AIM Address MSN Messenger ICQ Number Reply with quote
revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17327
Location: In your JS exploiting you and your system
revolution
Quote:
And what about SP then?
SP cannot be used for explicit addressing, remember in 16 bit we only have BX, SI, DI and BP. of course push, pop, call and ret all use SS by default but that is not using "effective addressing mode".
Post 22 Jul 2005, 07:45
View user's profile Send private message Visit poster's website Reply with quote
vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
oh yes, too much time spent in 32bits. sorry.
Post 22 Jul 2005, 07:50
View user's profile Send private message Visit poster's website AIM Address MSN Messenger ICQ Number Reply with quote
revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17327
Location: In your JS exploiting you and your system
revolution
Hehe, my boss doesn't know but I took a bit of time to look into this:

Here is a fix for you:
Code:
;find this label around line 5742 in X86_64.INC
store_segment_prefix_if_necessary:
        mov     al,[segment_register]
        or      al,al
        jz      segment_prefix_ok
;insert the following 6 lines 
        cmp     [code_type],16
        jnz     segment_prefix_not_16bit
        cmp     bl,25h
        jnz     segment_prefix_not_16bit
        xchg    bh,bl
      segment_prefix_not_16bit:
;and the rest of the code follows on from here
        cmp     al,3
        je      ss_prefix
        cmp     al,4
;....
    
test it with this:
Code:
use16
mov ax,[ds:si+bp]
mov ax,[ds:bp+si]
mov ax,[es:si+bp]
mov ax,[es:bp+si]
mov ax,[ss:si+bp]
mov ax,[ss:bp+si]
mov ax,[cs:si+bp]
mov ax,[cs:bp+si]    

BTW: this is not a fix for 32 bit mode with 16 bit address override (67h). I will leave the real fix to the author.
Post 22 Jul 2005, 09:18
View user's profile Send private message Visit poster's website Reply with quote
Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7734
Location: Kraków, Poland
Tomasz Grysztar
Thanks, I will release a fix soon.
Post 26 Jul 2005, 08:57
View user's profile Send private message Visit poster's website Reply with quote
Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7734
Location: Kraków, Poland
Tomasz Grysztar
Fixed in 1.63.2.
Post 28 Jul 2005, 15:17
View user's profile Send private message Visit poster's website Reply with quote
Display posts from previous:
Post new topic Reply to topic

Jump to:  


< Last Thread | Next Thread >
Forum Rules:
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Copyright © 1999-2020, Tomasz Grysztar. Also on YouTube, Twitter.

Website powered by rwasa.