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; aplib_8088_small.S - size-optimized aPLib decompressor for 8088 - 145 bytes
;
; Copyright (C) 2019 Emmanuel Marty
;
; This software is provided 'as-is', without any express or implied
; warranty. In no event will the authors be held liable for any damages
; arising from the use of this software.
;
; Permission is granted to anyone to use this software for any purpose,
; including commercial applications, and to alter it and redistribute it
; freely, subject to the following restrictions:
;
; 1. The origin of this software must not be misrepresented; you must not
; claim that you wrote the original software. If you use this software
; in a product, an acknowledgment in the product documentation would be
; appreciated but is not required.
; 2. Altered source versions must be plainly marked as such, and must not be
; misrepresented as being the original software.
; 3. This notice may not be removed or altered from any source distribution.
segment .text
bits 16
; ---------------------------------------------------------------------------
; Decompress aPLib data
; inputs:
; * ds:si: compressed aPLib data
; * es:di: output buffer
; output:
; * ax: decompressed size
; ---------------------------------------------------------------------------
apl_decompress:
push di ; remember decompression offset
cld ; make string operations go forward
; === register map ===
; al: bit queue
; ah: unused, but value is trashed
; bx: follows_literal
; cx: scratch register for reading gamma2 codes and storing copy length
; dx: match offset (and rep-offset)
; si: input (compressed data) pointer
; di: output (decompressed data) pointer
; bp: offset of .get_bit
mov al,080H ; clear bit queue(al) and set high bit to move into carry
xor dx,dx ; invalidate rep offset
mov bp,.get_bit ; load offset of .get_bit, to be used with call bp
.literal:
movsb ; read and write literal byte
.next_command_after_literal:
mov bx,03H ; set follows_literal(bx) to 3
.next_command:
call bp ; read 'literal or match' bit
jnc .literal ; if 0: literal
; 1x: match
call bp ; read '8+n bits or other type' bit
jc .other ; 11x: other type of match
; 10: 8+n bits match
call .get_gamma2 ; read gamma2-coded high offset bits
sub cx,bx ; high offset bits == 2 when follows_literal == 3 ?
; (a gamma2 value is always >= 2, so substracting follows_literal when it
; is == 2 will never result in a negative value)
jae .not_repmatch ; if not, not a rep-match
call .get_gamma2 ; read match length
jmp short .got_len ; go copy
.not_repmatch:
mov dh,cl ; transfer high offset bits to dh
mov dl,[si] ; read low offset byte in dl
inc si
call .get_gamma2 ; read match length
cmp dh,07DH ; offset >= 32000 ?
jae .increase_len_by2 ; if so, increase match len by 2
cmp dh,05H ; offset >= 1280 ?
jae .increase_len_by1 ; if so, increase match len by 1
cmp dx,0080H ; offset < 128 ?
jae .got_len ; if so, increase match len by 2, otherwise it would be a 7+1 copy
.increase_len_by2:
inc cx ; increase length
.increase_len_by1:
inc cx ; increase length
; copy cx bytes from match offset dx
.got_len:
push ds ; save ds:si (current pointer to compressed data)
push si
push es
pop ds
mov si,di ; point to destination in es:di - offset in dx
sub si,dx
rep movsb ; copy matched bytes
pop si ; restore ds:si
pop ds
mov bl,02H ; set follows_literal to 2 (bx is unmodified by match commands)
jmp short .next_command
; read gamma2-coded value into cx
.get_gamma2:
xor cx,cx ; initialize to 1 so that value will start at 2
inc cx ; when shifted left in the adc below
.gamma2_loop:
call .get_dibits ; read data bit, shift into cx, read continuation bit
jc .gamma2_loop ; loop until a zero continuation bit is read
ret
; handle 7 bits offset + 1 bit len or 4 bits offset / 1 byte copy
.other:
xor cx,cx
call bp ; read '7+1 match or short literal' bit
jc .short_literal ; 111: 4 bit offset for 1-byte copy
; 110: 7 bits offset + 1 bit length
mov dl,[si] ; read offset + length in dl
inc si
inc cx ; prepare cx for length below
shr dl,1 ; shift len bit into carry, and offset in place
je .done ; if zero offset: EOD
adc cx,cx ; len in cx: 1*2 + carry bit = 2 or 3
xor dh,dh ; clear high bits of offset
jmp short .got_len
; 4 bits offset / 1 byte copy
.short_literal:
call .get_dibits ; read 2 offset bits
adc cx,cx
call .get_dibits ; read 2 offset bits
adc cx,cx
xchg ax,cx ; preserve bit queue in cx, put offset in ax
jz .write_zero ; if offset is 0, write a zero byte
; short offset 1-15
mov bx,di ; point to destination in es:di - offset in ax
sub bx,ax ; we trash bx, it will be reset to 3 when we loop
mov al,[es:bx] ; read byte from short offset
.write_zero:
stosb ; copy matched byte
xchg ax,cx ; restore bit queue in al
jmp .next_command_after_literal
.done:
pop ax ; retrieve the original decompression offset
xchg di,ax ; compute decompressed size
sub ax,di
ret
.get_dibits:
call bp ; read data bit
adc cx,cx ; shift into cx
.get_bit:
add al,al ; shift bit queue, and high bit into carry
jnz .got_bit ; queue not empty, bits remain
lodsb ; read 8 new bits
adc al,al ; shift bit queue, and high bit into carry
.got_bit:
ret
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