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;; asm_decoder_translate_e8();; Assembly implementation of this from decxlat.c;; 26-Jul-96 jforbes Initial version; TITLE XLATASM.ASM .386P.model FLAT
PUBLIC _asm_decoder_translate_e8
OFFSET_INSTR_POS equ 28OFFSET_FILE_SIZE equ 32OFFSET_MEM equ 36OFFSET_BYTES equ 40
;; ulong asm_decoder_translate_e8(instr_pos, file_size, mem *, bytes);; returns new instr_pos in EAX;_TEXT SEGMENT_asm_decoder_translate_e8 PROC NEAR
push ebx push ecx push edx push esi push edi push ebp
; edx = bytes mov edx, DWORD PTR [esp + OFFSET_BYTES]
; if (bytes >= 6) cmp edx, 6 jge greater_than_6_bytes
;; less than 6 bytes to translate, so don't translate;
; instr_pos += bytes add edx, [esp + OFFSET_INSTR_POS]
; return new instr_pos in eax mov eax, edx
pop ebp pop edi pop esi pop edx pop ecx pop ebx ret 0
greater_than_6_bytes:
; ebp = current_file_size mov ebp, DWORD PTR [esp + OFFSET_FILE_SIZE]
; ebx = 0xE8, our magic number mov ebx, 0E8h ; 232
; esi = instr_pos mov esi, DWORD PTR [esp + OFFSET_INSTR_POS]
; ecx = mem mov ecx, DWORD PTR [esp + OFFSET_MEM]
; edi = instr_pos + bytes - 6 mov edi, esi add edi, edx sub edi, 6
; backup the last 6 bytes in the buffer sub esp, 6
; eax = &mem[bytes-6] mov eax, ecx add eax, edx sub eax, 6
mov edx, [eax] mov [esp], edx
mov dx, WORD PTR [eax+4] mov WORD PTR [esp+4], dx
; now store 0xE8's in there mov DWORD PTR [eax], 0E8E8E8E8h mov WORD PTR [eax+4], 0E8E8h
; save &mem[bytes-6] push eax
;; main loop;; eax = temporary; ebx = 0xE8; ecx = source ptr; edx = temporary; esi = instr pos; edi = end_instr_pos; ebp = current_file_size;
loop_top:
;; while (*mem++ != 0xE8); ;
; eax = mem before mov eax, ecx ; u1 nop ; v1
main_subloop: cmp bl, [ecx] ; u2 je SHORT equals_e8 ; v1
cmp bl, [ecx+1] ; u2 je SHORT pre1 ; v1
cmp bl, [ecx+2] ; u2 je SHORT pre2 ; v1
cmp bl, [ecx+3] ; u2 je SHORT pre3 ; v1
add ecx, 4 ; u1 jmp SHORT main_subloop ; v1
pre3: add ecx, 3 ; u1 jmp SHORT equals_e8 ; v1
pre2: add ecx, 2 ; u1 jmp SHORT equals_e8 ; v1
pre1: inc ecx
equals_e8:
; instr_pos += bytes visited in above loop; esi := esi + (ecx - eax) sub esi, eax ; u1 add esi, ecx ; v1
;; Here is the only place we check for the end.;; We can do this because we force an 0xE8 at the end; of the buffer.;; We cannot overlap the MOV below in between the; cmp/jge, because ecx+1 may point to invalid memory.; cmp esi, edi ; u1 jge SHORT bottom ; v1
; eax = absolute = *(long *) mem mov eax, [ecx+1] ; u1 add ecx, 5 ; v1 memptr += 5
;; if (absolute < current_file_size && absolute >= 0);; use unsigned comparison here so that if absolute < 0; then it seems like it's some huge number;; this way we only do one comparison, abs >= file_size; cmp eax, ebp ; u1 jae SHORT second_check ; v1
;; instead of doing "offset = absolute - instr_pos" and; then storing it, we just say *mem -= instr_pos;; instead of:;; sub eax, esi; mov [ecx-4], eax;; we do:
sub [ecx-4], esi ; u3 add esi, 5 ; v1 instr_pos += 5
mov eax, ecx ; u1 (copied from loop_top) jmp SHORT main_subloop ; v1
;; we want (absolute < 0) && (absolute >= -instr_pos);; which can be rewritten as:;; (-absolute > 0) && (-absolute - instr_pos <= 0);; then:;; (-absolute > 0) && (-absolute <= instr_pos);; we can do both of these checks by checking with; unsigned arithmetic, since if absolute < 0 then it; will seem like some huge number:;; if ((ulong) (-(long) absolute) <= instr_pos);; note: absolute==0 is taken care of in the first case;second_check:
; edx = instr_pos + absolute
neg eax ; u
cmp eax, esi ; u1 ja SHORT no_conversion ; v1
;; instead of storing "offset = absolute + current_file_size"; we can do *mem += file_size.;; instead of doing:;; neg eax; add eax, ebp; mov DWORD PTR [ecx-4], eax;; we do:
add [ecx-4], ebp ; u3
no_conversion:
; instr_pos += 5 add esi, 5 ; v1 u1 jmp SHORT loop_top ; v1
bottom:
; instr_pos = end_instr_pos + 6 add edi, 6
; restore the 6 bytes
; get &mem[bytes-6] pop eax
mov edx, DWORD PTR [esp] mov DWORD PTR [eax], edx
mov dx, WORD PTR [esp+4] mov WORD PTR [eax+4], dx
add esp, 6
; return new instr_pos in eax mov eax, edi
pop ebp pop edi pop esi pop edx pop ecx pop ebx ret 0_asm_decoder_translate_e8 ENDP_TEXT ENDSEND
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