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lzsa2.s
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lzsa2.s
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; ------------------------------------------------------------------------------
; LZSA2 BLOCK DECOMPRESSION FOR STM8
; ------------------------------------------------------------------------------
;
; lzsa2.s - Main LZSA2 decompression routine
;
; Copyright (c) 2021 Basil Hussain
;
; Permission is hereby granted, free of charge, to any person obtaining a copy
; of this software and associated documentation files (the "Software"), to deal
; in the Software without restriction, including without limitation the rights
; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
; copies of the Software, and to permit persons to whom the Software is
; furnished to do so, subject to the following conditions:
;
; The above copyright notice and this permission notice shall be included in all
; copies or substantial portions of the Software.
;
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
; SOFTWARE.
;
; ------------------------------------------------------------------------------
;
; Function declaration:
; void * lzsa2_decompress_block(void *dst, const void *src)
; Arguments:
; dst = pointer to destination decompression buffer
; src = pointer to source compressed data
; Returns:
; Pointer to a position in the given destination buffer after the last byte
; of decompressed data.
;
; NOTE: this function is not re-entrant, due to use of static variables.
;
; Inspiration for algorithm and structure taken from decompression routine for
; 6809 microprocessor by Emmanuel Marty.
; https://github.com/emmanuel-marty/lzsa
;
; LZSA2 block format documentation:
; https://github.com/emmanuel-marty/lzsa/blob/master/BlockFormat_LZSA2.md
.module lzsa2
.globl _lzsa2_decompress_block
; ------------------------------------------------------------------------------
; Static global variables (plus MSB/LSB aliases for convenience)
; ------------------------------------------------------------------------------
.area DATA
lit_len: .blkw 1
lit_len_msb .equ (lit_len+0)
lit_len_lsb .equ (lit_len+1)
match_off: .blkw 1
match_off_msb .equ (match_off+0)
match_off_lsb .equ (match_off+1)
match_len: .blkw 1
match_len_msb .equ (match_len+0)
match_len_lsb .equ (match_len+1)
nibbles: .blkb 1
nibbles_rdy: .blkb 1
; ------------------------------------------------------------------------------
; Function code
; ------------------------------------------------------------------------------
.area CODE
_lzsa2_decompress_block:
; Load source pointer to X reg and destination pointer to Y reg.
ldw x, (ARGS_SP_OFFSET+2, sp)
ldw y, (ARGS_SP_OFFSET+0, sp)
mov nibbles_rdy, #0x01
lzsa2_token:
; Token format: XYZ|LL|MMM
; Load next token into A. Also save it on the stack for later.
ld a, (x)
incw x
push a
; Mask off LL literal length from token in A. Branch if no literals (length
; is zero). Check if there is optional extra literal length byte (i.e.
; length is 3). If not, we have final count, so go ahead and copy literals.
and a, #0x18
jreq lzsa2_no_lit
cp a, #0x18
jrne lzsa2_decode_lit_len
; Fetch a nibble in to A reg. Add the existing literal length (3) to it and
; if it's now 18, an optional extra literal length byte follows. Otherwise,
; we have final length.
call_abs lzsa2_fetch_nibble
add a, #3
cp a, #18
jrne lzsa2_small_lit_len
; Load extra literal length byte and add to existing value. If there was no
; carry (i.e. byte read was 0-237), we have final length. Otherwise, value
; was 239, signifying two more bytes.
add a, (x)
incw x
jrnc lzsa2_small_lit_len
; Load two more bytes and set as length word var, converting from little- to
; big-endian as we go. Then go ahead and copy literals.
ld a, (x)
incw x
ld lit_len_lsb, a
ld a, (x)
incw x
ld lit_len_msb, a
jra lzsa2_got_lit_len
lzsa2_decode_lit_len:
; Shift literal length over 3 places.
srl a
srl a
srl a
lzsa2_small_lit_len:
; Clear MSB of literal length word variable, set current value of A to LSB.
clr lit_len_msb
ld lit_len_lsb, a
lzsa2_got_lit_len:
lzsa2_copy_lit_loop:
; Test if literal length variable value is zero. If so, proceed to handling
; match offset. Otherwise, continue to copy next literal byte.
tnz lit_len_msb
jrne lzsa2_copy_lit
tnz lit_len_lsb
jrne lzsa2_copy_lit
jra lzsa2_no_lit
lzsa2_copy_lit:
; Decrement literal length word variable in-place (without using X/Y
; registers and DECW instruction).
ld a, lit_len_lsb
sub a, #1
ld lit_len_lsb, a
ld a, lit_len_msb
sbc a, #0
ld lit_len_msb, a
; Copy a single byte from source to destination.
ld a, (x)
incw x
ld (y), a
incw y
; Loop around to next byte.
jra lzsa2_copy_lit_loop
lzsa2_no_lit:
; Retrieve token from stack (without popping it). Shift off the match offset
; mode X bit into carry. If set, we have 13- or 16-bit match offset. If not,
; then shift off Y bit into carry. If set, we have 9-bit match offset.
ld a, (1, sp)
sll a
jrc lzsa2_match_off_13b_16b
sll a
jrc lzsa2_match_off_9b
; Otherwise, we have a 5-bit match offset. Shift off Z bit of mode to carry.
; Read a nibble (into A) and rotate the value of that to offset bits 1-4 and
; Z bit from mode (in carry) to bit 0. Then XOR with a mask to set bits 5-7
; of the offset to 1 and flip the Z bit. Also set MSB of offset to all 1s.
sll a
call_abs lzsa2_fetch_nibble
rlc a
xor a, #0xE1
ld match_off_lsb, a
mov match_off_msb, #0xFF
jra lzsa2_got_match_off
lzsa2_match_off_9b:
; We have a 9-bit match offset. Shift off Z bit of mode to carry and invert.
; Set MSB of offset to all 1s, then rotate Z bit in to bit 8. Load another
; byte and set as LSB (bits 0-7) of offset.
sll a
ccf
mov match_off_msb, #0xFF
rlc match_off_msb
ld a, (x)
incw x
ld match_off_lsb, a
jra lzsa2_got_match_off
lzsa2_match_off_13b_16b:
; Shift off Y bit into carry. If set, we have a 16-bit match offset.
sll a
jrc lzsa2_match_off_16b
; Otherwise, we have a 13-bit offset. Shift off Z bit of mode to carry. Read
; a nibble (into A) and rotate the value of that to offset bits 9-12 and Z
; bit from mode (in carry) to bit 8. Then XOR with a mask to set bits 13-15
; of the offset to 1 and flip the Z bit. Subtract 512 from final offset by
; subtracting 2 from MSB. Finally, read a new byte and set as LSB (bits 0-7)
; of offset.
sll a
call_abs lzsa2_fetch_nibble
rlc a
xor a, #0xE1
sub a, #2
ld match_off_msb, a
ld a, (x)
incw x
ld match_off_lsb, a
jra lzsa2_got_match_off
lzsa2_match_off_16b:
; If Z bit of mode is set, we repeat the previous offset value.
jrmi lzsa2_got_match_off
; Otherwise, we have a 16-bit offset. Read two bytes containing the final
; match offset value, already in big-endian format.
ld a, (x)
incw x
ld match_off_msb, a
ld a, (x)
incw x
ld match_off_lsb, a
lzsa2_got_match_off:
; Retrieve token from stack (popping this time), mask off MMM match length
; bits, add the minimum match length (2) to the value.
pop a
and a, #0x07
add a, #2
; Check if we have optional extra match length bytes (i.e. match length was
; 7 before addition). Otherwise, we have final length, so proceed to copy
; matched bytes.
cp a, #9
jrne lzsa2_small_match_len
; Read a nibble (into A) and add the current match length (9) to it. If the
; nibble value was 0-14 (before addition), we have final match length, so
; proceed to copy matched bytes.
call_abs lzsa2_fetch_nibble
add a, #9
cp a, #24
jrne lzsa2_small_match_len
; Read another byte from source and add to current match length. If there is
; no carry, value was 0-231 and we have final length. If carry, but length
; is zero, value was 232, signifying end-of-data (EOD), so quit. Otherwise,
; value was 233, meaning two more bytes.
add a, (x)
incw x
jrnc lzsa2_small_match_len
tnz a
jreq lzsa2_end
; Load two more bytes and set as match length word variable, converting from
; little- to big-endian as we go. Then proceed to copy matched bytes.
ld a, (x)
incw x
ld match_len_lsb, a
ld a, (x)
incw x
ld match_len_msb, a
jra lzsa2_got_match_len
lzsa2_small_match_len:
; Place match length value in LSB of length word variable and clear MSB.
ld match_len_lsb, a
clr match_len_msb
lzsa2_got_match_len:
; Save current source pointer on stack. Copy current destination pointer to
; X reg and add match offset to it.
pushw x
ldw x, y
addw x, match_off
lzsa2_copy_match_loop:
; Test if match length variable value is zero. If not, continue to copy next
; matched byte. Otherwise, exit loop.
tnz match_len_msb
jrne lzsa2_copy_match
tnz match_len_lsb
jrne lzsa2_copy_match
jra lzsa2_no_match
lzsa2_copy_match:
; Decrement match length word variable in-place (without using X/Y registers
; and DECW instruction).
ld a, match_len_lsb
sub a, #1
ld match_len_lsb, a
ld a, match_len_msb
sbc a, #0
ld match_len_msb, a
; Copy a single byte from source to destination.
ld a, (x)
incw x
ld (y), a
incw y
; Loop around to next byte.
jra lzsa2_copy_match_loop
lzsa2_no_match:
; Restore source pointer from stack. Proceed to next token.
popw x
jump_abs lzsa2_token
lzsa2_end:
; Return current destination pointer in X reg.
ldw x, y
return
; ------------------------------------------------------------------------------
; NOTE: we must be careful in this function not to alter the carry flag! Calling
; code relies on the value of the carry flag being maintained.
lzsa2_fetch_nibble:
; Toggle the ready flag.
bcpl nibbles_rdy, #0
tnz nibbles_rdy ; }
jreq lzsa2_nib_not_rdy ; } Can't use btjf here as it changes carry.
; We have nibbles ready. Mask off the low nibble and return in A reg.
ld a, nibbles
and a, #0x0F
return
lzsa2_nib_not_rdy:
; Load a new pair of nibbles (i.e. a byte) from input and store. Mask off
; the high nibble, shift over and return the value in A reg.
ld a, (x)
incw x
ld nibbles, a
and a, #0xF0
swap a
return