OpenEuphoria

ASM Help

• Posted by Matthew Lewis <matthewwalkerlewis at YAHOO.COM> Mar 21, 2001
• 467 views

I'm hoping someone here can help me out with some ASM.  I'm trying to push
some parameters on the stack, first of all, and then call a function to
which I have a pointer.  I've managed to move the params from memory to the
stack, but I can't figure out how to call the other function.

I've been using asm.e from Pete Eberlein, and he only has support for call
near, which I don't think I can use here.  So I've been poking around, and
figured out how to put together opcodes for call far.

Has anyone done anything like this?  I've looked at a lot of stuff all over
the web about ASM, and most of it really doesn't help, since it's usually
geared toward MASM or the like, which is at a somewhat higher level than I'm
working.  Pete's docs doesn't seem to shed any light on this, either,
although I found the encoding schemes for mnemonics at 
http://www.imada.ou.dk/~jews/PInfo/intel.html


Here's my code (it's been modified somewhat at 1A from the original output).
The function I want to call is located at comfunc (which I poke in after
putting the routine into memory).  I suspect part of my problem may be not
pushing something (like the return address), but I've gotten the impression
that call should do this automatically.

com_asm = {
    #60,                    --    0: pusha
    #BB,#00,#00,#00,#00,    --    1: mov ebx, paramcount (2)
    #8B,#0D,#00,#00,#00,#00,--    6: mov ecx, [params] (8)
    #8B,#01,                --    C: start: mov eax, [ecx]
    #50,                    --    E: push eax
    #83,#C1,#04,            --    F: add ecx, 4
    #4B,                    --   12: dec ebx
    #75,#F7,                --   13: jnz start
    #B8,#00,#00,#00,#00,    --   15: mov edx, comfunc (22)
    #FF,#D8,                --   1A: call far eax
    #58,                    --   1C: pop eax
    #A3,#00,#00,#00,#00,    --   1D: mov [retpointer], eax (30)
    #58,
    #58,
    #58,
    #61,                    --   22: popa
    #C3},                   --   23: ret

When I call the routine, I get a fault.  Here's a dissasembly (the entry
point is 00595610):

---------- Code Disassembly ----------
function: <nosymbols>+0000
        0059560f 0060bb           add     [eax-0x45],ah
ds:00f30b76=??
        00595612 0300             add     eax,[eax]
ds:00591f78=b7b85bc3
        00595614 0000             add     [eax],al
ds:00591f78=c3
        00595616 8b0d087e5900     mov     ecx,[00597e08]
ds:00597e08=00597df8
        0059561c 8b01             mov     eax,[ecx]
ds:00597e04=00000011
        0059561e 50               push    eax
        0059561f 83c104           add     ecx,0x4
        00595622 4b               dec     ebx
        00595623 75f7             jnz     0059561c
        00595625 b8781f5900       mov     eax,0x591f78
FAULT ->0059562a ffd8             call    eax
        0059562c 58               pop     eax
        0059562d a300505659       mov     [59565000],eax
ds:59565000=????????
        00595632 005858           add     [eax+0x58],bl
ds:00f30b76=??
        00595635 61               popad
        00595636 c3               ret
        00595637 0920             or      [eax],esp
ds:00591f78=b7b85bc3
        00595639 2020             and     [eax],ah
ds:00591f78=c3
        0059563b 2011             and     [ecx],dl
ds:00597e04=11
        0059563d 0000             add     [eax],al
ds:00591f78=c3
        0059563f 0020             add     [eax],ah
ds:00591f78=c3
        00595641 005900           add     [ecx],bl
ds:00f36a02=??

Registers:
EAX: 00591F78 EBX: 00000000 ECX: 00597E04 EDX: 00000000 ESI: 00595610 EDI:
00000001
EIP: 0059562A ESP: 0056FC0C EBP: 0056FC46 IOPL: 0         nv up ei pl zr na
po nc
CS: 015F  SS: 0167  DS: 0167  ES: 0167  FS: 59CF  GS: 0000             EFL:
00010246

Stack:
---------- Top of the Stack ----------
0056FC0C  00580030 00594E78 005F4250 00000001  |0.X.xNY.PB_.....|
0056FC1C  00595610 0056FC46 0056FC38 00000028  |.VY.F.V.8.V.(...|
0056FC2C  00000000 005A2644 005A2644 0041454C  |....D&Z.D&Z.LEA.|
0056FC3C  005E0880 00000001 00423618 0056FC64  |..^......6B.d.V.|

---------- Stack Back Trace ----------
Stopped at 0059562A (0001:0059462A in (UNKNOWN))  Base=0x00000000
RVA=0x00001000

Matt Lewis