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Artlav



Joined: 23 Dec 2004
Posts: 188
Location: Moscow, Russia
Artlav
In short - what is the meaning of the piece of code below, commonly present at start of many circa-1992 DOS programs?

The story is - i'm making a 80186&DOS emulator (for fun), and trying to run some period correct exe programs in it. Most, if not all, of them start with the piece of code provided below, that does something i can't understand.

None of the programs in asm i seen have this, and i don't have a compiler old enough to get an original disassembly. IDA says something about possibly packed file, and fails to do anything beyond the retf.

I checked every command involved, and i think i loaded the exe properly, but not sure about the latter due to a noticeable lack of googleable information about it.
Basically, the file is loaded at 1000:0, the registers are set, relocations applied, and emulation started.

What is it, and what could be wrong?
It looks as if the program expects to find something in the middle of nowhere, where i suppose it's my job to put it?

Code:
AX:0000h  BX:0000h  CX:8000h  DX:0000h
SP:0080h  BP:0000h  SI:0000h  DI:4000h
CS:490Bh  DS:1000h  ES:1000h  SS:4D11h
IP:0010h  FLAGS:0206h
490B:0010:SP=0080 [8Bh]:mov    bp,ax
490B:0012:SP=0080 [8Ch]:mov    ax,es
490B:0014:SP=0080 [05h]:add    ax,0010h
490B:0017:SP=0080 [0Eh]:push   cs
490B:0018:SP=007E [1Fh]:pop    ds
490B:0019:SP=0080 [A3h]:mov    [490B4h],ax ;0
490B:001C:SP=0080 [03h]:add    ax,[490BCh] ;3AF8h
490B:0020:SP=0080 [8Eh]:mov    es,ax
490B:0022:SP=0080 [8Bh]:mov    cx,[490B6h] ;1F82h
490B:0026:SP=0080 [8Bh]:mov    di,cx
490B:0028:SP=0080 [4Fh]:dec    di
490B:0029:SP=0080 [8Bh]:mov    si,di
490B:002B:SP=0080 [FDh]:std
490B:002D:SP=0080 [A4h]:movsb
490B:002E:SP=0080 [50h]:push   ax
490B:002F:SP=007E [B8h]:mov    ax,0034h
490B:0032:SP=007E [50h]:push   ax
490B:0033:SP=007C [CBh]:ret    far

AX:0034h  BX:0000h  CX:0000h  DX:0000h
SP:0080h  BP:0000h  SI:3F03h  DI:3F03h
CS:4B08h  DS:490Bh  ES:4B08h  SS:4D11h
IP:0034h  FLAGS:0606h

4B08:0034:SP=0080 [00h]:add    [bx+si],al
4B08:0036:SP=0080 [00h]:add    [bx+si],al
4B08:0038:SP=0080 [00h]:add    [bx+si],al
4B08:003A:SP=0080 [00h]:add    [bx+si],al
...etc...
    
Post 19 Dec 2009, 10:02
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17450
Location: In your JS exploiting you and your system
revolution
It is probably moving the code to a higher segment. Perhaps to the end of memory?
Post 19 Dec 2009, 10:09
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Artlav



Joined: 23 Dec 2004
Posts: 188
Location: Moscow, Russia
Artlav
Sorry, problem solved - movsb didn't account for the very rare (from my point of view) direction flag the std sets.
Post 19 Dec 2009, 10:29
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