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| author | Juan J. Martinez <jjm@usebox.net> | 2020-12-30 19:07:31 +0000 |
|---|---|---|
| committer | Juan J. Martinez <jjm@usebox.net> | 2020-12-30 19:23:41 +0000 |
| commit | 2682bc5d1d864341aaeb42a449db73c3ecd16d70 (patch) | |
| tree | 9116764364b4ee0ce7f6037305077807b57776de /tools/rasm/decrunch/dzx7_turbo.asm | |
| download | ubox-msx-lib-2682bc5d1d864341aaeb42a449db73c3ecd16d70.tar.gz ubox-msx-lib-2682bc5d1d864341aaeb42a449db73c3ecd16d70.zip | |
Initial import1.0
Diffstat (limited to 'tools/rasm/decrunch/dzx7_turbo.asm')
| -rw-r--r-- | tools/rasm/decrunch/dzx7_turbo.asm | 80 |
1 files changed, 80 insertions, 0 deletions
diff --git a/tools/rasm/decrunch/dzx7_turbo.asm b/tools/rasm/decrunch/dzx7_turbo.asm new file mode 100644 index 0000000..779ced5 --- /dev/null +++ b/tools/rasm/decrunch/dzx7_turbo.asm @@ -0,0 +1,80 @@ +; -----------------------------------------------------------------------------
+; ZX7 decoder by Einar Saukas & Urusergi
+; "Turbo" version (88 bytes, 25% faster)
+; -----------------------------------------------------------------------------
+; Parameters:
+; HL: source address (compressed data)
+; DE: destination address (decompressing)
+; -----------------------------------------------------------------------------
+
+dzx7_turbo:
+ ld a, $80
+dzx7t_copy_byte_loop:
+ ldi ; copy literal byte
+dzx7t_main_loop:
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ jr nc, dzx7t_copy_byte_loop ; next bit indicates either literal or sequence
+
+; determine number of bits used for length (Elias gamma coding)
+ push de
+ ld bc, 1
+ ld d, b
+dzx7t_len_size_loop:
+ inc d
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ jr nc, dzx7t_len_size_loop
+ jp dzx7t_len_value_start
+
+; determine length
+dzx7t_len_value_loop:
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ rl c
+ rl b
+ jr c, dzx7t_exit ; check end marker
+dzx7t_len_value_start:
+ dec d
+ jr nz, dzx7t_len_value_loop
+ inc bc ; adjust length
+
+; determine offset
+ ld e, (hl) ; load offset flag (1 bit) + offset value (7 bits)
+ inc hl
+ defb $cb, $33 ; opcode for undocumented instruction "SLL E" aka "SLS E"
+ jr nc, dzx7t_offset_end ; if offset flag is set, load 4 extra bits
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ rl d ; insert first bit into D
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ rl d ; insert second bit into D
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ rl d ; insert third bit into D
+ add a, a ; check next bit
+ call z, dzx7t_load_bits ; no more bits left?
+ ccf
+ jr c, dzx7t_offset_end
+ inc d ; equivalent to adding 128 to DE
+dzx7t_offset_end:
+ rr e ; insert inverted fourth bit into E
+
+; copy previous sequence
+ ex (sp), hl ; store source, restore destination
+ push hl ; store destination
+ sbc hl, de ; HL = destination - offset - 1
+ pop de ; DE = destination
+ ldir
+dzx7t_exit:
+ pop hl ; restore source address (compressed data)
+ jp nc, dzx7t_main_loop
+
+dzx7t_load_bits:
+ ld a, (hl) ; load another group of 8 bits
+ inc hl
+ rla
+ ret
+
+; -----------------------------------------------------------------------------
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