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Index > Main > is ds:dx disallowed in 16 bit?

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lazer1



Joined: 24 Jan 2006
Posts: 185
lazer1
the following code fragment doesnt compile:

Code:
use16
          cmp [ds:dx],byte 0
    


with error message:

error: reserved word used as symbol

whereas this one does compile:

Code:
use16  
         cmp [ds:bx], byte 0
    


also [ds:ax] and [ds:cx] dont compile Crying or Very sad

however [ds:di] and [ds:si] both do compile, Razz

does more recent 16 bit real x86 not recognise [ds:dx] , [ds:ax], [ds:cx]?
Post 19 Jun 2006, 15:45
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Quote:

does more recent 16 bit real x86 not recognise [ds:dx] , [ds:ax], [ds:cx]?


Actually there is no addressing mode that supports [dx], [ax], [cx] even if you use use32 (but you can use [edx], [eax], [ecx]). Anyway it's a little strange the error that FASM reports, I think it should be "Invalid addressing mode" or something similar
Post 19 Jun 2006, 16:20
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lazer1



Joined: 24 Jan 2006
Posts: 185
lazer1
locodelassembly wrote:
Quote:

does more recent 16 bit real x86 not recognise [ds:dx] , [ds:ax], [ds:cx]?


Actually there is no addressing mode that supports [dx], [ax], [cx] even if you use use32 (but you can use [edx], [eax], [ecx]). Anyway it's a little strange the error that FASM reports, I think it should be "Invalid addressing mode" or something similar


you are right, I tried it just now and use16 accepts [ds:eax] and
[ds:ecx] and [ds:edx],

however as all my code runs beyond the first 64K of memory
16 bit real will freeze up if any of the upper 16 bits of eax, ecx, edx, ebx
are nonzero.

at the moment I often do things like:

Code:
    org 20000h
use16
    ....
    mov ebx,xyz
    call f

xyz    dd  0

f:
    mov [ds:bx],10
    ret
    


here mov [ds:ebx] will freeze up the machine
if ds is 0 and eg ebx==21234h

so instead I have ds==2000h and ebx would need to
be 1234h,

simply because it can only deal with 16 bit offsets even though
it is a long way from even 1MB,
Post 19 Jun 2006, 17:49
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
yes, unless you enable flat-real mode, all offsets must be below 10000h. Search board for "flat real mode" and "unreal mode"
Post 20 Jun 2006, 08:38
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