I have the following disassembly of winload!CmpFindNlsData function and I need to check the output parameters of this function. P.S.: CmpFindNLSData(int a1<eax>, int a2, PUNICODE_STRING pAnsiFileName, PUNICODE_STRING pOemFileName, PUNICODE_STRING pustrDefaultLanguage, PUNICODE_STRING pustrOemHalFontName) could be the declaration of this function. This is my first attempt at reverse engineering, could someone please guide through the steps I could take up to define the algorithm of this function.

00893c10 8bff            mov     edi,edi
00893c12 55              push    ebp
00893c13 8bec            mov     ebp,esp
00893c15 83ec40          sub     esp,40h
00893c18 53              push    ebx
00893c19 56              push    esi
00893c1a 57              push    edi
00893c1b 8d45e0          lea     eax,[ebp-20h]
00893c1e 8bf1            mov     esi,ecx
00893c20 50              push    eax
00893c21 33db            xor     ebx,ebx
00893c23 8975ec          mov     dword ptr [ebp-14h],esi
00893c26 83cfff          or      edi,0FFFFFFFFh
00893c29 895dcc          mov     dword ptr [ebp-34h],ebx
00893c2c 52              push    edx
00893c2d 56              push    esi
00893c2e 897dc8          mov     dword ptr [ebp-38h],edi
00893c31 897dd0          mov     dword ptr [ebp-30h],edi
00893c34 895dd4          mov     dword ptr [ebp-2Ch],ebx
00893c37 897de0          mov     dword ptr [ebp-20h],edi
00893c3a 895de4          mov     dword ptr [ebp-1Ch],ebx
00893c3d 897dc0          mov     dword ptr [ebp-40h],edi
00893c40 895dc4          mov     dword ptr [ebp-3Ch],ebx
00893c43 895de8          mov     dword ptr [ebp-18h],ebx
00893c46 ff5604          call    dword ptr [esi+4]
00893c49 85c0            test    eax,eax
00893c4b 0f845d010000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893c51 8d4df4          lea     ecx,[ebp-0Ch]
00893c54 8bd0            mov     edx,eax
00893c56 51              push    ecx
00893c57 68b8208e00      push    offset winload!CmpControlString (008e20b8)
00893c5c 8bce            mov     ecx,esi
00893c5e e869510000      call    winload!CmpFindSubKeyByNameWithStatus (00898dcc)
00893c63 8d45e0          lea     eax,[ebp-20h]
00893c66 50              push    eax
00893c67 56              push    esi
00893c68 ff5608          call    dword ptr [esi+8]
00893c6b 397df4          cmp     dword ptr [ebp-0Ch],edi
00893c6e 0f843a010000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893c74 8d45e0          lea     eax,[ebp-20h]
00893c77 50              push    eax
00893c78 ff75f4          push    dword ptr [ebp-0Ch]
00893c7b 56              push    esi
00893c7c ff5604          call    dword ptr [esi+4]
00893c7f 85c0            test    eax,eax
00893c81 0f8427010000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893c87 8d4df4          lea     ecx,[ebp-0Ch]
00893c8a 8bd0            mov     edx,eax
00893c8c 51              push    ecx
00893c8d 68f8208e00      push    offset winload!CmpNlsString (008e20f8)
00893c92 8bce            mov     ecx,esi
00893c94 e833510000      call    winload!CmpFindSubKeyByNameWithStatus (00898dcc)
00893c99 8d45e0          lea     eax,[ebp-20h]
00893c9c 50              push    eax
00893c9d 56              push    esi
00893c9e ff5608          call    dword ptr [esi+8]
00893ca1 397df4          cmp     dword ptr [ebp-0Ch],edi
00893ca4 0f8404010000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893caa 8d45e0          lea     eax,[ebp-20h]
00893cad 50              push    eax
00893cae ff75f4          push    dword ptr [ebp-0Ch]
00893cb1 56              push    esi
00893cb2 ff5604          call    dword ptr [esi+4]
00893cb5 85c0            test    eax,eax
00893cb7 0f84f1000000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893cbd 8d4df4          lea     ecx,[ebp-0Ch]
00893cc0 8bd0            mov     edx,eax
00893cc2 51              push    ecx
00893cc3 6808218e00      push    offset winload!CmpCodePageString (008e2108)
00893cc8 8bce            mov     ecx,esi
00893cca e8fd500000      call    winload!CmpFindSubKeyByNameWithStatus (00898dcc)
00893ccf 8d45e0          lea     eax,[ebp-20h]
00893cd2 50              push    eax
00893cd3 56              push    esi
00893cd4 ff5608          call    dword ptr [esi+8]
00893cd7 397df4          cmp     dword ptr [ebp-0Ch],edi
00893cda 0f84ce000000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893ce0 8d45e0          lea     eax,[ebp-20h]
00893ce3 50              push    eax
00893ce4 ff75f4          push    dword ptr [ebp-0Ch]
00893ce7 56              push    esi
00893ce8 ff5604          call    dword ptr [esi+4]
00893ceb 85c0            test    eax,eax
00893ced 0f84bb000000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893cf3 6870228e00      push    offset winload!CmpAcpString (008e2270)
00893cf8 8bd0            mov     edx,eax
00893cfa 8bce            mov     ecx,esi
00893cfc e885330000      call    winload!CmpFindValueByName (00897086)
00893d01 8bf8            mov     edi,eax
00893d03 8d45e0          lea     eax,[ebp-20h]
00893d06 50              push    eax
00893d07 56              push    esi
00893d08 ff5608          call    dword ptr [esi+8]
00893d0b 83ffff          cmp     edi,0FFFFFFFFh
00893d0e 0f849a000000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893d14 8d45d0          lea     eax,[ebp-30h]
00893d17 50              push    eax
00893d18 57              push    edi
00893d19 56              push    esi
00893d1a ff5604          call    dword ptr [esi+4]
00893d1d 85c0            test    eax,eax
00893d1f 0f8489000000    je      winload!CmpFindNLSData+0x19e (00893dae)
00893d25 8d4dc8          lea     ecx,[ebp-38h]
00893d28 8bd7            mov     edx,edi
00893d2a 51              push    ecx
00893d2b 8d4df8          lea     ecx,[ebp-8]
00893d2e 51              push    ecx
00893d2f 50              push    eax
00893d30 8bce            mov     ecx,esi
00893d32 e8c7340000      call    winload!CmpValueToData (008971fe)
00893d37 8bf8            mov     edi,eax
00893d39 8d45d0          lea     eax,[ebp-30h]
00893d3c 50              push    eax
00893d3d 56              push    esi
00893d3e 897ddc          mov     dword ptr [ebp-24h],edi
00893d41 ff5608          call    dword ptr [esi+8]
00893d44 85ff            test    edi,edi
00893d46 7466            je      winload!CmpFindNLSData+0x19e (00893dae)
00893d48 8b55f8          mov     edx,dword ptr [ebp-8]
00893d4b 33c9            xor     ecx,ecx
00893d4d 33c0            xor     eax,eax
00893d4f 668955da        mov     word ptr [ebp-26h],dx
00893d53 66894dd8        mov     word ptr [ebp-28h],cx
00893d57 c745f002000000  mov     dword ptr [ebp-10h],2
00893d5e 663bc2          cmp     ax,dx
00893d61 731d            jae     winload!CmpFindNLSData+0x170 (00893d80)
00893d63 8b75f0          mov     esi,dword ptr [ebp-10h]
00893d66 0fb7c1          movzx   eax,cx
00893d69 d1e8            shr     eax,1
00893d6b 66391c47        cmp     word ptr [edi+eax*2],bx
00893d6f 740c            je      winload!CmpFindNLSData+0x16d (00893d7d)
00893d71 6603ce          add     cx,si
00893d74 66894dd8        mov     word ptr [ebp-28h],cx
00893d78 663bca          cmp     cx,dx
00893d7b 72e9            jb      winload!CmpFindNLSData+0x156 (00893d66)
00893d7d 8b75ec          mov     esi,dword ptr [ebp-14h]
00893d80 8b5d08          mov     ebx,dword ptr [ebp+8]
00893d83 6a36            push    36h
00893d85 58              pop     eax
00893d86 663907          cmp     word ptr [edi],ax
00893d89 750c            jne     winload!CmpFindNLSData+0x187 (00893d97)
00893d8b c745e801000000  mov     dword ptr [ebp-18h],1
00893d92 e992000000      jmp     winload!CmpFindNLSData+0x219 (00893e29)
00893d97 8d45e0          lea     eax,[ebp-20h]
00893d9a 50              push    eax
00893d9b ff75f4          push    dword ptr [ebp-0Ch]
00893d9e 56              push    esi
00893d9f ff5604          call    dword ptr [esi+4]
00893da2 85c0            test    eax,eax
00893da4 7513            jne     winload!CmpFindNLSData+0x1a9 (00893db9)
00893da6 8d45c8          lea     eax,[ebp-38h]
00893da9 50              push    eax
00893daa 56              push    esi
00893dab ff5608          call    dword ptr [esi+8]
00893dae 32c0            xor     al,al
00893db0 5f              pop     edi
00893db1 5e              pop     esi
00893db2 5b              pop     ebx
00893db3 8be5            mov     esp,ebp
00893db5 5d              pop     ebp
00893db6 c20c00          ret     0Ch
00893db9 8d4dd8          lea     ecx,[ebp-28h]
00893dbc 8bd0            mov     edx,eax
00893dbe 51              push    ecx
00893dbf 8bce            mov     ecx,esi
00893dc1 e8c0320000      call    winload!CmpFindValueByName (00897086)
00893dc6 8bf8            mov     edi,eax
00893dc8 8d45c8          lea     eax,[ebp-38h]
00893dcb 50              push    eax
00893dcc 56              push    esi
00893dcd ff5608          call    dword ptr [esi+8]
00893dd0 33c0            xor     eax,eax
00893dd2 8945dc          mov     dword ptr [ebp-24h],eax
00893dd5 8d45e0          lea     eax,[ebp-20h]
00893dd8 50              push    eax
00893dd9 56              push    esi
00893dda ff5608          call    dword ptr [esi+8]
00893ddd 83ffff          cmp     edi,0FFFFFFFFh
00893de0 74cc            je      winload!CmpFindNLSData+0x19e (00893dae)
00893de2 8d45d0          lea     eax,[ebp-30h]
00893de5 50              push    eax
00893de6 57              push    edi
00893de7 56              push    esi
00893de8 ff5604          call    dword ptr [esi+4]
00893deb 85c0            test    eax,eax
00893ded 74bf            je      winload!CmpFindNLSData+0x19e (00893dae)
00893def 8d4dc0          lea     ecx,[ebp-40h]
00893df2 8bd7            mov     edx,edi
00893df4 51              push    ecx
00893df5 8d4df8          lea     ecx,[ebp-8]
00893df8 51              push    ecx
00893df9 50              push    eax
00893dfa 8bce            mov     ecx,esi
00893dfc e8fd330000      call    winload!CmpValueToData (008971fe)
00893e01 894304          mov     dword ptr [ebx+4],eax
00893e04 85c0            test    eax,eax
00893e06 7408            je      winload!CmpFindNLSData+0x200 (00893e10)
00893e08 8d45c0          lea     eax,[ebp-40h]
00893e0b 50              push    eax
00893e0c 56              push    esi
00893e0d ff5608          call    dword ptr [esi+8]
00893e10 8d45d0          lea     eax,[ebp-30h]
00893e13 50              push    eax
00893e14 56              push    esi
00893e15 ff5608          call    dword ptr [esi+8]
00893e18 33c0            xor     eax,eax
00893e1a 394304          cmp     dword ptr [ebx+4],eax
00893e1d 748f            je      winload!CmpFindNLSData+0x19e (00893dae)
00893e1f 8b45f8          mov     eax,dword ptr [ebp-8]
00893e22 66894302        mov     word ptr [ebx+2],ax
00893e26 668903          mov     word ptr [ebx],ax
00893e29 8d45e0          lea     eax,[ebp-20h]
00893e2c 50              push    eax
00893e2d ff75f4          push    dword ptr [ebp-0Ch]
00893e30 56              push    esi
00893e31 ff5604          call    dword ptr [esi+4]
00893e34 85c0            test    eax,eax
00893e36 0f8472ffffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893e3c 6848228e00      push    offset winload!CmpOemCpString (008e2248)
00893e41 8bd0            mov     edx,eax
00893e43 8bce            mov     ecx,esi
00893e45 e83c320000      call    winload!CmpFindValueByName (00897086)
00893e4a 8bf8            mov     edi,eax
00893e4c 8d45e0          lea     eax,[ebp-20h]
00893e4f 50              push    eax
00893e50 56              push    esi
00893e51 ff5608          call    dword ptr [esi+8]
00893e54 83ffff          cmp     edi,0FFFFFFFFh
00893e57 0f8451ffffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893e5d 8d45d0          lea     eax,[ebp-30h]
00893e60 50              push    eax
00893e61 57              push    edi
00893e62 56              push    esi
00893e63 ff5604          call    dword ptr [esi+4]
00893e66 85c0            test    eax,eax
00893e68 0f8440ffffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893e6e 8d4dc8          lea     ecx,[ebp-38h]
00893e71 8bd7            mov     edx,edi
00893e73 51              push    ecx
00893e74 8d4df8          lea     ecx,[ebp-8]
00893e77 51              push    ecx
00893e78 50              push    eax
00893e79 8bce            mov     ecx,esi
00893e7b e87e330000      call    winload!CmpValueToData (008971fe)
00893e80 8bf8            mov     edi,eax
00893e82 8d45d0          lea     eax,[ebp-30h]
00893e85 50              push    eax
00893e86 56              push    esi
00893e87 897ddc          mov     dword ptr [ebp-24h],edi
00893e8a ff5608          call    dword ptr [esi+8]
00893e8d 85ff            test    edi,edi
00893e8f 0f8419ffffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893e95 8b55f8          mov     edx,dword ptr [ebp-8]
00893e98 33c9            xor     ecx,ecx
00893e9a 33c0            xor     eax,eax
00893e9c 668955da        mov     word ptr [ebp-26h],dx
00893ea0 66894dd8        mov     word ptr [ebp-28h],cx
00893ea4 663bc2          cmp     ax,dx
00893ea7 7322            jae     winload!CmpFindNLSData+0x2bb (00893ecb)
00893ea9 8b75f0          mov     esi,dword ptr [ebp-10h]
00893eac 33db            xor     ebx,ebx
00893eae 0fb7c1          movzx   eax,cx
00893eb1 d1e8            shr     eax,1
00893eb3 66391c47        cmp     word ptr [edi+eax*2],bx
00893eb7 740c            je      winload!CmpFindNLSData+0x2b5 (00893ec5)
00893eb9 6603ce          add     cx,si
00893ebc 66894dd8        mov     word ptr [ebp-28h],cx
00893ec0 663bca          cmp     cx,dx
00893ec3 72e9            jb      winload!CmpFindNLSData+0x29e (00893eae)
00893ec5 8b75ec          mov     esi,dword ptr [ebp-14h]
00893ec8 8b5d08          mov     ebx,dword ptr [ebp+8]
00893ecb 6a36            push    36h
00893ecd 58              pop     eax
00893ece 663907          cmp     word ptr [edi],ax
00893ed1 8b7d0c          mov     edi,dword ptr [ebp+0Ch]
00893ed4 0f8496000000    je      winload!CmpFindNLSData+0x360 (00893f70)
00893eda 8d45e0          lea     eax,[ebp-20h]
00893edd 50              push    eax
00893ede ff75f4          push    dword ptr [ebp-0Ch]
00893ee1 56              push    esi
00893ee2 ff5604          call    dword ptr [esi+4]
00893ee5 85c0            test    eax,eax
00893ee7 0f84b9feffff    je      winload!CmpFindNLSData+0x196 (00893da6)
00893eed 8d4dd8          lea     ecx,[ebp-28h]
00893ef0 8bd0            mov     edx,eax
00893ef2 51              push    ecx
00893ef3 8bce            mov     ecx,esi
00893ef5 e88c310000      call    winload!CmpFindValueByName (00897086)
00893efa 894508          mov     dword ptr [ebp+8],eax
00893efd 8d45c8          lea     eax,[ebp-38h]
00893f00 50              push    eax
00893f01 56              push    esi
00893f02 ff5608          call    dword ptr [esi+8]
00893f05 8d45e0          lea     eax,[ebp-20h]
00893f08 50              push    eax
00893f09 56              push    esi
00893f0a ff5608          call    dword ptr [esi+8]
00893f0d 8b4508          mov     eax,dword ptr [ebp+8]
00893f10 83f8ff          cmp     eax,0FFFFFFFFh
00893f13 0f8495feffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893f19 8d4dd0          lea     ecx,[ebp-30h]
00893f1c 51              push    ecx
00893f1d 50              push    eax
00893f1e 56              push    esi
00893f1f ff5604          call    dword ptr [esi+4]
00893f22 85c0            test    eax,eax
00893f24 0f8484feffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893f2a 8b5508          mov     edx,dword ptr [ebp+8]
00893f2d 8d4dc0          lea     ecx,[ebp-40h]
00893f30 51              push    ecx
00893f31 8d4df8          lea     ecx,[ebp-8]
00893f34 51              push    ecx
00893f35 50              push    eax
00893f36 8bce            mov     ecx,esi
00893f38 e8c1320000      call    winload!CmpValueToData (008971fe)
00893f3d 894704          mov     dword ptr [edi+4],eax
00893f40 85c0            test    eax,eax
00893f42 7408            je      winload!CmpFindNLSData+0x33c (00893f4c)
00893f44 8d45c0          lea     eax,[ebp-40h]
00893f47 50              push    eax
00893f48 56              push    esi
00893f49 ff5608          call    dword ptr [esi+8]
00893f4c 8d45d0          lea     eax,[ebp-30h]
00893f4f 50              push    eax
00893f50 56              push    esi
00893f51 ff5608          call    dword ptr [esi+8]
00893f54 33c9            xor     ecx,ecx
00893f56 394f04          cmp     dword ptr [edi+4],ecx
00893f59 0f844ffeffff    je      winload!CmpFindNLSData+0x19e (00893dae)
00893f5f 8b45f8          mov     eax,dword ptr [ebp-8]
00893f62 66894702        mov     word ptr [edi+2],ax
00893f66 668907          mov     word ptr [edi],ax
00893f69 394de8          cmp     dword ptr [ebp-18h],ecx
00893f6c 7412            je      winload!CmpFindNLSData+0x370 (00893f80)
00893f6e eb02            jmp     winload!CmpFindNLSData+0x362 (00893f72)
00893f70 33c9            xor     ecx,ecx
00893f72 33c0            xor     eax,eax
00893f74 894b04          mov     dword ptr [ebx+4],ecx
00893f77 668903          mov     word ptr [ebx],ax
00893f7a 894f04          mov     dword ptr [edi+4],ecx
00893f7d 668907          mov     word ptr [edi],ax
00893f80 8b4d10          mov     ecx,dword ptr [ebp+10h]
00893f83 b001            mov     al,1
00893f85 6a14            push    14h
00893f87 5a              pop     edx
00893f88 c741044c1a8e00  mov     dword ptr [ecx+4],offset winload!`string' (008e1a4c)
00893f8f 66895102        mov     word ptr [ecx+2],dx
00893f93 668911          mov     word ptr [ecx],dx
00893f96 e915feffff      jmp     winload!CmpFindNLSData+0x1a0 (00893db0)
00893f9b cc              int     3
  • Breakpoint at the start or end of the function and look at the values of parameters, to check where the parameters are (stack/registers) you need to determine the call convention of this function. – morsisko Aug 28 at 12:17

The function's Disassembly Does Not Match the prototype
This function takes only 3 arguments which can be ascertained with the first/multiple failure Exits

if(!foo) { return false;}
00893c49 85c0            test    eax,eax  <<<< if(!foo)
00893c4b 0f845d010000    je      winload!CmpFindNLSData+0x19e (00893dae)  
00893dae 32c0            xor     al,al <<<<<<<    bool False
00893db0 5f              pop     edi  
00893db1 5e              pop     esi  
00893db2 5b              pop     ebx  
00893db3 8be5            mov     esp,ebp  
00893db5 5d              pop     ebp  
00893db6 c20c00          ret     0Ch  <<<<<<<<<< pops 3 arguments

we can also confirm the arguments Access in x86 by looking for ebp+ patterns (beware FPO )

copy pasting the disassembly from your query and grepping for ebp+8
yields only access to (ebp + [0x8,0xc,0x10]) so this function definitely takes only 3 arguments or 5 if this is _fastcall (ecx,edx)

:\>wc -l nlsdatadis.txt
350 nlsdatadis.txt

:\>grep ebp+ nlsdatadis.txt
00893d80 8b5d08          mov     ebx,dword ptr [ebp+8] <<<
00893ec8 8b5d08          mov     ebx,dword ptr [ebp+8]
00893ed1 8b7d0c          mov     edi,dword ptr [ebp+0Ch] <<<
00893efa 894508          mov     dword ptr [ebp+8],eax
00893f0d 8b4508          mov     eax,dword ptr [ebp+8]
00893f2a 8b5508          mov     edx,dword ptr [ebp+8]
00893f80 8b4d10          mov     ecx,dword ptr [ebp+10h] <<<<

we can infer that the last/3rd argument is a PUNICODE_STRING by looking at the Disassembly that Accesses [ebp+10]

00893f80 8b4d10          mov     ecx,dword ptr [ebp+10h]
00893f83 b001            mov     al,1   <<<<<<< bool TRUE
00893f85 6a14            push    14h    <<<< str length
00893f87 5a              pop     edx
00893f88 c741044c1a8e00  mov     dword ptr [ecx+4],offset winload!`string' (008e1a4c)   
<<< pointer to Buffer
00893f8f 66895102        mov     word ptr [ecx+2],dx  << (max len)
00893f93 668911          mov     word ptr [ecx],dx    << (len)     
00893f96 e915feffff      jmp     winload!CmpFindNLSData+0x1a0 (00893db0) << again    
jumps to Exit that is described above with True as Return)

2nd Argument is also PUNICODE_STRING (ebp+0c,edi) ist Argument is Some function pointer or the this call convention's this

as ebp+8 is modified several times by mov

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