diff options
Diffstat (limited to 'src/engine/home.asm')
| -rw-r--r-- | src/engine/home.asm | 121 |
1 files changed, 65 insertions, 56 deletions
diff --git a/src/engine/home.asm b/src/engine/home.asm index c23b544..5a8c822 100644 --- a/src/engine/home.asm +++ b/src/engine/home.asm @@ -1534,33 +1534,31 @@ UpdateRNGSources: ; 089b (0:089b) pop hl ret -; initilizes variables used to decompress -; data in DecompressData -; de points to the source of compressed data -; b is used as the HIGH byte of the -; WRAM address to write to ($100 bytes of buffer space) +; initilizes variables used to decompress data in DecompressData +; de = source of compressed data +; b = HIGH byte of secondary buffer ($100 bytes of buffer space) ; also clears this $100 byte space InitDataDecompression: ; 08bf (0:08bf) - ld hl, wcad6 + ld hl, wDecompSourcePosPtr ld [hl], e inc hl ld [hl], d - ld hl, wcad8 - ld [hl], $1 + ld hl, wDecompNumCommandBitsLeft + ld [hl], 1 inc hl xor a - ld [hli], a ; wcad9 - ld [hli], a ; wcada - ld [hli], a ; wcadb - ld [hli], a ; wcadc - ld [hl], b ; wcadd + ld [hli], a ; wDecompCommandByte + ld [hli], a ; wDecompRepeatModeToggle + ld [hli], a ; wDecompRepeatLengths + ld [hli], a ; wDecompNumBytesToRepeat + ld [hl], b ; wDecompSecondaryBufferPtrHigh inc hl - ld [hli], a ; wcade - ld [hl], $ef ; wcadf + ld [hli], a ; wDecompRepeatSeqOffset + ld [hl], LOW(wDecompressionSecondaryBufferStart) ; wDecompSecondaryBufferPtrLow ; clear buffer ld h, b - ld l, LOW(wc000) + ld l, LOW(wDecompressionSecondaryBuffer) xor a .loop ld [hl], a @@ -1570,7 +1568,7 @@ InitDataDecompression: ; 08bf (0:08bf) ; decompresses data ; uses values initialized by InitDataDecompression -; wcad6 holds the pointer for compressed source +; wDecompSourcePosPtr holds the pointer for compressed source ; input: ; bc = row width ; de = buffer to place decompressed data @@ -1591,99 +1589,110 @@ DecompressData: ; 08de (0:08de) pop hl ret -; instructions start with a byte stored in wcad9 -; its bits are read from higher to lower bit -; wcad8 stores the current bit being read -; bit set: -; - 1 byte read and copied literally -; bit not set: -; - 2 bytes read WW XY ZZ, byte in pos WW -; copied (X + 1) times, then in pos ZZ -; copied (Y + 1) times +; decompression works as follows: +; first a command byte is read that will dictate how the +; following bytes will be copied +; the position will then move to the next byte (0xXY), and +; the command byte's bits are read from higher to lower bit +; - if command bit is set, then copy 0xXY to buffer; +; - if command bit is not set, then decompression enters "repeat mode," +; which means it stores 0xXY in memory as number of bytes to repeat +; from a given offset. This offset is in the next byte in the data, +; 0xZZ, which tells the offset to start repeating. A toggle is switched +; each time the algorithm hits "repeat mode": +; - if off -> on it reads 0xXY and stores it, +; then repeats (0x0X + 2) bytes from the offset starting at 0xZZ; +; - if on -> off, then the data only provides the offset, +; and the previous byte read for number of bytes to repeat, 0xXY, is reused +; in which case (0x0Y + 2) bytes are repeated starting from the offset. .Decompress: ; 08ef (0:08ef) - ld hl, wcadc + ld hl, wDecompNumBytesToRepeat ld a, [hl] or a - jr z, .read_instruction + jr z, .read_command -; still repeating byte +; still repeating sequence dec [hl] inc hl .repeat_byte - ld b, [hl] ; wcadd + ld b, [hl] ; wDecompSecondaryBufferPtrHigh inc hl - ld c, [hl] ; wcade + ld c, [hl] ; wDecompRepeatSeqOffset inc [hl] inc hl ld a, [bc] - ld c, [hl] ; wcadf + ld c, [hl] ; wDecompSecondaryBufferPtrLow inc [hl] ld [bc], a ret -.read_instruction - ld hl, wcad6 +.read_command + ld hl, wDecompSourcePosPtr ld c, [hl] inc hl ld b, [hl] - inc hl ; wcad8 + inc hl ; wDecompNumCommandBitsLeft dec [hl] - inc hl ; wcad9 - jr nz, .asm_914 - dec hl ; wcad8 - ld [hl], $8 ; number of bits - inc hl ; wcad9 + inc hl ; wDecompCommandByte + jr nz, .read_command_bit + dec hl ; wDecompNumCommandBitsLeft + ld [hl], 8 ; number of bits + inc hl ; wDecompCommandByte ld a, [bc] inc bc ld [hl], a -.asm_914 +.read_command_bit rl [hl] ld a, [bc] inc bc - jr nc, .asm_92a + jr nc, .repeat_command ; copy 1 byte literally - ld hl, wcad6 + ld hl, wDecompSourcePosPtr ld [hl], c inc hl ld [hl], b - ld hl, wcadd + ld hl, wDecompSecondaryBufferPtrHigh ld b, [hl] inc hl inc hl - ld c, [hl] ; wcadf + ld c, [hl] ; wDecompSecondaryBufferPtrLow inc [hl] ld [bc], a ret -.asm_92a - ld [wcade], a - ld hl, wcada +.repeat_command + ld [wDecompRepeatSeqOffset], a ; save the offset to repeat from + ld hl, wDecompRepeatModeToggle bit 0, [hl] - jr nz, .asm_94a + jr nz, .repeat_mode_toggle_on set 0, [hl] inc hl +; read byte for num of bytes to read +; and use its higher nybble ld a, [bc] inc bc - ld [hli], a ; wcadb + ld [hli], a ; wDecompRepeatLengths swap a -.asm_93c +.get_sequence_len and $f inc a ; number of times to repeat - ld [hli], a ; wcadc + ld [hli], a ; wDecompNumBytesToRepeat push hl - ld hl, wcad6 + ld hl, wDecompSourcePosPtr ld [hl], c inc hl ld [hl], b pop hl jr .repeat_byte -.asm_94a +.repeat_mode_toggle_on +; get the previous byte (num of bytes to repeat) +; and use its lower nybble res 0, [hl] inc hl - ld a, [hli] ; wcadb - jr .asm_93c + ld a, [hli] ; wDecompRepeatLengths + jr .get_sequence_len ; set attributes for [hl] sprites starting from wOAM + [wOAMOffset] / 4 ; return carry if reached end of wOAM before finishing |