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https://github.com/exoticorn/upkr.git
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some clean up - move dos unpacker, fix arm32 unpacker formatting
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152
dos_unpacker/unpack_x86_16_DOS_no_relocation.asm
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152
dos_unpacker/unpack_x86_16_DOS_no_relocation.asm
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; Contributions from pestis, TomCat and exoticorn
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;
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; This is the 16-bit DOS x86 decompression stub for upkr, which decompresses the
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; code starting at address 0x3FFE (or whatever is defined by the entrypoint
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; below). Thus, the packed code needs to be assembled with org 0x3FFE to work.
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;
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; How to use:
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; 1) Put POPA as the first instruction of your compiled code and use org
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; 0x3FFE
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; 2) Pack your intro using upkr into data.bin with the --x86 command line
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; argument:
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;
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; $ upkr --x86 intro.com data.bin
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;
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; 2) Compile this .asm file using nasm (or any compatible assembler) e.g.
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;
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; $ nasm unpack_x86_16_DOS_no_relocation.asm -fbin -o intropck.com
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;
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; In specific cases, the unpacker stub can be further optimized to save a byte
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; or two:
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; 1) If your stub+compressed code is 2k or smaller, you can save 1 byte by
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; putting probs at 0x900 and initializing DI with SALC; XCHG AX, DI instead
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; of MOV DI, probs
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; 2) If you remove the PUSHA (and POPA in the compressed code), then you can
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; assume the registers as follows: AX = 0x00XX, BX = probs + 0x1XX, CX = 0
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; DX = (trash), SI = DI = right after your program, SP = as it was when the
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; program started, flags = carry set
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;
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; Note that even with the PUSHA / POPA, carry will be set (!) unlike normal dos
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; program.
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entry equ 0x3FFE
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probs equ entry - 0x1FE ; must be aligned to 256
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org 0x100
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; This is will be loaded at 0x100, but relocates the code and data to prog_start
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upkr_unpack:
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pusha
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xchg ax, bp ; position in bitstream = 0
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cwd ; upkr_state = 0;
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mov di, probs
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mov ax, 0x8080 ; for(int i = 0; i < sizeof(upkr_probs); ++i) upkr_probs[i] = 128;
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rep stosw
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push di
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.mainloop:
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mov bx, probs
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call upkr_decode_bit
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jc .else ; if(upkr_decode_bit(0)) {
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mov bh, (probs+256)/256
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jcxz .skip_call ; if(prev_was_match || upkr_decode_bit(257)) {
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call upkr_decode_bit
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jc .skipoffset
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.skip_call:
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stc
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call upkr_decode_number ; offset = upkr_decode_number(258) - 1;
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mov si, di
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loop .sub ; if(offset == 0)
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ret
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.sub:
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dec si
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loop .sub
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.skipoffset:
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mov bl, 128 ; int length = upkr_decode_number(384);
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call upkr_decode_number
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rep movsb ; *write_ptr = write_ptr[-offset];
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jmp .mainloop
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.byteloop:
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call upkr_decode_bit ; int bit = upkr_decode_bit(byte);
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.else:
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adc bl, bl ; byte = (byte << 1) + bit;
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jnc .byteloop
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xchg ax, bx
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stosb
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inc si
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mov cl, 1
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jmp .mainloop ; prev_was_match = 0;
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; upkr_decode_bit decodes one bit from the rANS entropy encoded bit stream.
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; parameters:
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; bx = memory address of the context probability
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; dx = decoder state
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; bp = bit position in input stream
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; returns:
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; dx = new decoder state
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; bp = new bit position in input stream
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; carry = bit
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; trashes ax
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upkr_load_bit:
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bt [compressed_data], bp
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inc bp
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adc dx, dx
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upkr_decode_bit:
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inc dx
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dec dx ; inc dx, dec dx is used to test the top (sign) bit of dx
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jns upkr_load_bit
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movzx ax, byte [bx] ; int prob = upkr_probs[context_index]
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push ax ; save prob
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cmp dl, al ; int bit = (upkr_state & 255) < prob ? 1 : 0; (carry = bit)
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pushf ; save bit flags
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jc .bit ; (skip if bit)
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neg al ; tmp = 256 - tmp;
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.bit:
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mov [bx], al ; tmp_new = tmp + (256 - tmp + 8) >> 4;
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neg byte [bx]
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shr byte [bx], 4
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adc [bx], al
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mul dh ; upkr_state = tmp * (upkr_state >> 8) + (upkr_state & 255);
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mov dh, 0
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add dx, ax
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popf
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pop ax
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jc .bit2 ; (skip if bit)
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neg byte [bx] ; tmp = 256 - tmp;
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sub dx, ax ; upkr_state -= prob; note that this will also leave carry always unset, which is what we want
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.bit2:
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ret ; flags = bit
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; upkr_decode_number loads a variable length encoded number (up to 16 bits) from
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; the compressed stream. Only numbers 1..65535 can be encoded. If the encoded
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; number has 4 bits and is 1ABC, it is encoded using a kind of an "interleaved
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; elias code": 0A0B0C1. The 1 in the end implies that no more bits are coming.
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; parameters:
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; cx = must be 0
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; bx = memory address of the context probability
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; dx = decoder state
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; bp = bit position in input stream
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; carry = must be 1
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; returns:
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; cx = length
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; dx = new decoder state
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; bp = new bit position in input stream
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; carry = 1
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; trashes bl, ax
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upkr_decode_number_loop:
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inc bx
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call upkr_decode_bit
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upkr_decode_number:
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rcr cx, 1
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inc bx
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call upkr_decode_bit
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jnc upkr_decode_number_loop ; while(upkr_decode_bit(context_index)) {
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.loop2:
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rcr cx, 1
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jnc .loop2
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ret
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compressed_data:
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incbin "data.bin"
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