29A Issue 03 03 08

eZine lover (@eZine)

Published in 

29A

 · 28 Dec 2019

  

  
;=====================================; 
;            S  T  R  C               ; 
;-------------------------------------; 
;     Super Tiny Reloc Compressor     ; 
;=====================================; 
;          Made by Super/29A          ; 
;=====================================; 

Pushd      equ 4 
Pushad_edi equ 00h 
Pushad_esi equ 04h 
Pushad_ebp equ 08h 
Pushad_esp equ 0ch 
Pushad_ebx equ 10h 
Pushad_edx equ 14h 
Pushad_ecx equ 18h 
Pushad_eax equ 1ch 

 
;-----------------------------------------------------------; 
; Routine "STRC_max_compress" 
; 
; Tries different methods and compresses with the best ratio. 
; 
; INPUT: 
;   ESI = offset of start of reloc section 
;   EDI = buffer to store the compressed reloc data 
;   ECX = number of *bytes* of reloc section (not aligned) 
; 
; OUTPUT: 
;   BL  = selected method of compression 
;         (number of bits to encode relative offset) 
;   ECX = number of *bits* of compressed reloc data 
;-----------------------------------------------------------; 

STRC_max_compress proc near 
 push eax 
 stc 
 sbb eax,eax ;eax=-1 
 mov bl,0eh ;try with 14 bits 
@@1: 
 inc ebx 
 push ecx 
 call STRC_compress ;compress it 
 cmp ecx,eax ;this method better? 
 jnb @@2 ;jump if not 
 xchg ecx,eax 
 dec ebx 
 dec ebx ;go for next method 
@@2: 
 pop ecx 
 jnz @@1 ;try again if we havent found the perfect method 
 xchg ecx,eax 
 pop eax 
 ret 
STRC_max_compress endp 

 
;-----------------------------------------------------------; 
; Routine "STRC_compress" 
; 
; Compresses the reloc section. 
; 
; INPUT: 
;   BL  = method of compression 
;         (number of bits to encode relative offset) 
;   ESI = offset of start of reloc section 
;   EDI = buffer to store the compressed reloc data 
;   ECX = number of *bytes* of reloc section (not aligned) 
; 
; OUTPUT: 
;   ECX = number of *bits* of compressed reloc data 
;-----------------------------------------------------------; 

STRC_compress proc near 
 add ecx,esi ;ecx=end of input buffer 
 pushad 
 xor ebp,ebp 
 xor ebx,ebx ;bit count 
@@1: 
 lodsd 
 xchg edx,eax ;edx=virtual address 
 lodsd 
 lea ecx,[eax-8] ;ecx=size of this block 
@@2: 
 push ecx 
 push edx 
 lodsw 
 sub ah,40h 
 jnb @@6 ;jump if invalid relocation 
 add ah,10h 
 jnb @@6 ;jump if invalid relocation 
 cwde 
 add eax,edx ;add the virtual address of actual page 
 sub eax,ebp ;calculate relative address 
 add ebp,eax ;update last virtual address 
 push ebx 
 db 0fh,0bdh,0d8h ; bsr ebx,eax ;calculate number of bits taken 
 movzx ecx,byte ptr [esp+(Pushd*3)+Pushad_ebx] ;cl=bits chosen for this method 
 cmp bl,cl ;we need more bits to store it? 
 jb @@3 ;nope 
 shld edx,eax,cl           ; 
 shl eax,cl                ; 
 shr ebx,3                 ; 
 or al,bl                  ; insert number of bytes needed to store it 
 lea ecx,[ecx+(ebx*8)+8]   ; 
@@3: 
 pop ebx 
@@4: 
 btr [edi],ebx  ; 
 shr edx,1      ; 
 rcr eax,1      ; 
 jnb @@5        ; 
 bts [edi],ebx  ; store information bit by bit 
@@5:            ; 
 inc ebx        ; 
 loop @@4       ; 
@@6: 
 pop edx 
 pop ecx 
 dec ecx 
 loop @@2 ;next word of actual block 
 cmp esi,[esp+Pushad_ecx] ;have we processed all blocks? 
 jb @@1 ;nope 
STRC_exit: 
 mov [esp+Pushad_ecx],ebx ;return number of bits of compressed data 
 popad 
 ret 
STRC_compress endp 

 
;-----------------------------------------------------------; 
; Routine "STRC_decompress" 
; 
; Decompresses the reloc section. 
; 
; INPUT: 
;   BL  = method of compression 
;         (number of bits to encode relative offset) 
;   ESI = offset of compressed reloc data 
;   EDI = buffer to store the reloc section 
;   ECX = number of *bits* of compressed reloc data 
; 
; OUTPUT: 
;   ECX = number of *bytes* of decompressed reloc section 
;-----------------------------------------------------------; 

STRC_decompress proc near 
 pushad 
 xor ebp,ebp 
 xor ebx,ebx 
@@1: 
 cmp ebx,ecx ;have we finished? 
 jnb @@7 ;nope 
 push ecx 
 movzx ecx,byte ptr [esp+(Pushd*1)+Pushad_ebx] ;cl=bits chosen for this method 
@@2: 
 xor eax,eax 
 push ecx 
@@3: 
 bt [esi],ebx ; 
 rcr eax,1    ; 
 inc ebx      ; get relative offset 
 loop @@3     ; 
 pop ecx      ; 
 rol eax,cl   ; 
 lea ecx,[(eax*8)+8] ;ax=0--> 8 bits 
                     ;ax=1--> 16 bits 
                     ;ax=2--> 24 bits 
                     ;ax=3--> 32 bits 
 cmp eax,4 ;do we need to read more bits to get the whole relative offset 
 jb @@2 ;yes 
 pop ecx 
 add eax,ebp 
 dec ebp 
 mov ebp,eax ;update last virtual address 
 jns @@5 ;jump if this is not the first time 
@@4: 
 mov eax,ebp 
 and eax,-1000h 
 stosd ;store start virtual address of this page 
 xor eax,eax 
 mov al,8 
 mov edx,edi 
 stosd ;store size of this block 
@@5: 
 mov eax,ebp 
 sub eax,[edx-4] 
 cmp eax,1000h 
 jnb @@7 ;jump if we are in next page 
 sub ah,-30h ;make it of type 3       :-) 
@@6: 
 stosw ;store relocation 
 inc dword ptr [edx] ; 
 inc dword ptr [edx] ; increment block size by two 
 jb @@1 
@@7: 
 xor eax,eax 
 test byte ptr [edx],2 
 jnz @@6 ;align this block if necesary 
 cmp ebx,ecx ;finished? 
 jb @@4 ;nope 
 mov ebx,edi 
 sub ebx,[esp] ;calculate size of decompressed data 
 jmp STRC_exit 
STRC_decompress endp 

;------------------------------------------------------------------- 

 
ends 
end start