Disassembled instructions of the decoding algorithm of Half-Life
################################################################

(source: SWDS.DLL of the retail game version 1.1.1.0)

NOTE: when you run the dedicated server and want to debug SWDS.DLL,
      remember to add 0x5690000 to the offsets that you see in SWDS.DLL
      For example 0x00D2251C in runtime will be at offset 0x63b251c





* Referenced by a CALL at Addresses:
|:00D46228   , :00D7A946   , :00D911C7   
|
:00D224F0 55                      push ebp
:00D224F1 8BEC                    mov ebp, esp
:00D224F3 51                      push ecx
:00D224F4 8B450C                  mov eax, dword ptr [ebp+0C]
:00D224F7 24FC                    and al, FC
:00D224F9 99                      cdq
:00D224FA 83E203                  and edx, 00000003
:00D224FD 03C2                    add eax, edx
:00D224FF C1F802                  sar eax, 02
:00D22502 85C0                    test eax, eax
:00D22504 7E6C                    jle 00D22572
:00D22506 8B4D10                  mov ecx, dword ptr [ebp+10]
:00D22509 53                      push ebx
:00D2250A 8B5D08                  mov ebx, dword ptr [ebp+08]
:00D2250D 56                      push esi
:00D2250E 57                      push edi
:00D2250F 8D7D0C                  lea edi, dword ptr [ebp+0C]
:00D22512 F7D1                    not ecx
:00D22514 894DFC                  mov dword ptr [ebp-04], ecx
:00D22517 894508                  mov dword ptr [ebp+08], eax
:00D2251A F7DF                    neg edi

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00D2256D(C)
|
:00D2251C 8B03                    mov eax, dword ptr [ebx]
:00D2251E 8B4D10                  mov ecx, dword ptr [ebp+10]
:00D22521 33C1                    xor eax, ecx
:00D22523 33C9                    xor ecx, ecx
:00D22525 89450C                  mov dword ptr [ebp+0C], eax

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00D2254D(C)
|
:00D22528 8D740D0C                lea esi, dword ptr [ebp+ecx+0C]
:00D2252C 8AC1                    mov al, cl
:00D2252E D2E0                    shl al, cl
:00D22530 8D1437                  lea edx, dword ptr [edi+esi]
:00D22533 83E20F                  and edx, 0000000F
:00D22536 8A92886CDE00            mov dl, byte ptr [edx+00DE6C88]
:00D2253C 0AD0                    or dl, al
:00D2253E 8A06                    mov al, byte ptr [esi]
:00D22540 0AD1                    or dl, cl
:00D22542 80CAA5                  or dl, A5
:00D22545 32C2                    xor al, dl
:00D22547 41                      inc ecx
:00D22548 83F904                  cmp ecx, 00000004
:00D2254B 8806                    mov byte ptr [esi], al
:00D2254D 7CD9                    jl 00D22528
:00D2254F 8B450C                  mov eax, dword ptr [ebp+0C]
:00D22552 50                      push eax
:00D22553 E838CAFFFF              call 00D1EF90
:00D22558 8B75FC                  mov esi, dword ptr [ebp-04]
:00D2255B 83C404                  add esp, 00000004
:00D2255E 33C6                    xor eax, esi
:00D22560 8903                    mov dword ptr [ebx], eax
:00D22562 8B4508                  mov eax, dword ptr [ebp+08]
:00D22565 83C304                  add ebx, 00000004
:00D22568 47                      inc edi
:00D22569 48                      dec eax
:00D2256A 894508                  mov dword ptr [ebp+08], eax
:00D2256D 75AD                    jne 00D2251C
:00D2256F 5F                      pop edi
:00D22570 5E                      pop esi
:00D22571 5B                      pop ebx

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00D22504(C)
|
:00D22572 8BE5                    mov esp, ebp
:00D22574 5D                      pop ebp
:00D22575 C3                      ret








The following is the function called at offset 0x00D22553
It is a simple byteswapping function:


:00D1EF90 55                      push ebp
:00D1EF91 8BEC                    mov ebp, esp
:00D1EF93 8B4D08                  mov ecx, dword ptr [ebp+08]
:00D1EF96 8BC1                    mov eax, ecx
:00D1EF98 8BD1                    mov edx, ecx
:00D1EF9A C1F808                  sar eax, 08
:00D1EF9D 81E2FF000000            and edx, 000000FF
:00D1EFA3 25FF000000              and eax, 000000FF
:00D1EFA8 C1E208                  shl edx, 08
:00D1EFAB 03C2                    add eax, edx
:00D1EFAD 8BD1                    mov edx, ecx
:00D1EFAF C1FA10                  sar edx, 10
:00D1EFB2 C1E008                  shl eax, 08
:00D1EFB5 81E2FF000000            and edx, 000000FF
:00D1EFBB 03C2                    add eax, edx
:00D1EFBD C1F918                  sar ecx, 18
:00D1EFC0 C1E008                  shl eax, 08
:00D1EFC3 81E1FF000000            and ecx, 000000FF
:00D1EFC9 03C1                    add eax, ecx
:00D1EFCB 5D                      pop ebp
:00D1EFCC C3                      ret