SNES Development
The Legend of Zelda - A Link to the Past
Zelda - A Link to the Past Decompression Code
Commented by Peekin - Feburary 22nd, 2001
; 65816 SNES Disassembler v2.0a (C)opyright 1994 by John Corey
; Begin: $00e7a3 End: $00e852
; Hirom: No Quiet: No Comments: 2 DCB: No Symbols: No 65816: No
00e7a3 20 43 e8 JSR $e843 ; get next byte
00e7a6 c9 ff CMP #$ff ; end of compressed stream if code = 0FFh
00e7a8 d0 03 BNE $e7ad ; continue if any other code
00e7aa e2 10 SEP #$10 ; Index (8 bit)
00e7ac 60 RTS ; end of compression routine
; get code and length (code is upper 3 bits, length is lower 5)
00e7ad 85 cd STA $cd ; save byte
00e7af 29 e0 AND #$e0 ; get code (upper 3 bits)
00e7b1 c9 e0 CMP #$e0 ; special code for longer run count
00e7b3 f0 0a BEQ $e7bf
00e7b5 48 PHA ; save code
00e7b6 a5 cd LDA $cd ; retrieve byte
00e7b8 c2 20 REP #$20 ; Accum (16 bit)
00e7ba 29 1f 00 AND #$001f ; mask byte to get length (lower 5 bits)
00e7bd 80 12 BRA $e7d1
; long run count
00e7bf a5 cd LDA $cd ; retrieve byte
00e7c1 0a ASL ; shift byte left 3 times for new code
00e7c2 0a ASL
00e7c3 0a ASL
00e7c4 29 e0 AND #$e0 ; get new code (upper 3 bits)
00e7c6 48 PHA ; save code
00e7c7 a5 cd LDA $cd ; retrieve original byte
00e7c9 29 03 AND #$03 ; select lowest two bits for count
00e7cb eb XBA ; save count's two msb in AH (*256)
00e7cc 20 43 e8 JSR $e843 ; read next byte for a total 10bit count
00e7cf c2 20 REP #$20 ; Accum (16 bit)
; depending on if the code was E0, the count will either be 0-63 or 0-1023
00e7d1 1a INC ; count++
00e7d2 85 cb STA $cb ; store count
00e7d4 e2 20 SEP #$20 ; Accum (8 bit)
00e7d6 68 PLA ; retrieve code
00e7d7 f0 16 BEQ $e7ef ; transfer bytes from source
00e7d9 30 4a BMI $e825 ; transfer bytes from output buffer
00e7db 0a ASL
00e7dc 10 20 BPL $e7fe ; repeat single byte
00e7de 0a ASL
00e7df 10 2a BPL $e80b ; repeat two alternating bytes
; repeat single incrementing byte??
00e7e1 20 43 e8 JSR $e843 ; read single byte
00e7e4 a6 cb LDX $cb ; load count
00e7e6 97 00 STA [$00],Y ; write byte
00e7e8 1a INC ; increment byte value??
00e7e9 c8 INY ; destination ptr++
00e7ea ca DEX ; count--
00e7eb d0 f9 BNE $e7e6 ; loop while count <> 0
00e7ed 80 b4 BRA $e7a3 ; go to top of loop for next code
; transfer bytes directly
00e7ef 20 43 e8 JSR $e843 ; read next byte to transfer
00e7f2 97 00 STA [$00],Y ; write byte
00e7f4 c8 INY ; destination ptr++
00e7f5 a6 cb LDX $cb ; reload count (since ReadByte changed it)
00e7f7 ca DEX ; count--
00e7f8 86 cb STX $cb ; store count
00e7fa d0 f3 BNE $e7ef ; loop while count <> 0
00e7fc 80 a5 BRA $e7a3 ; go to top of loop for next code
; repeat single byte
00e7fe 20 43 e8 JSR $e843 ; read single byte to repeat
00e801 a6 cb LDX $cb ; load count
00e803 97 00 STA [$00],Y ; write byte
00e805 c8 INY ; destination ptr++
00e806 ca DEX ; count--
00e807 d0 fa BNE $e803 ; loop while count <> 0
00e809 80 98 BRA $e7a3 ; go to top of loop for next code
; repeat run of alternating even/odd bytes
00e80b 20 43 e8 JSR $e843 ; read first byte
00e80e eb XBA ; save first byte into AH
00e80f 20 43 e8 JSR $e843 ; read second byte
00e812 a6 cb LDX $cb ; load count
00e814 eb XBA ; swap first byte with second
00e815 97 00 STA [$00],Y ; write first byte
00e817 c8 INY ; destination ptr++
00e818 ca DEX ; count--
00e819 f0 07 BEQ $e822 ; exit loop if count = 0
00e81b eb XBA ; swap first byte with second
00e81c 97 00 STA [$00],Y ; write second byte
00e81e c8 INY ; destination ptr++
00e81f ca DEX ; count--
00e820 d0 f2 BNE $e814 ; loop while count <> 0
00e822 4c a3 e7 JMP $e7a3 ; go to top of loop for next code
; copy run of bytes already in output buffer to end
00e825 20 43 e8 JSR $e843 ; read low byte ptr
00e828 eb XBA
00e829 20 43 e8 JSR $e843 ; read high byte ptr
00e82c eb XBA
00e82d aa TAX ; copy buffer source to X
00e82e 5a PHY ; save destination ptr
00e82f 9b TXY ; move buffer source to Y for indexing
00e830 b7 00 LDA [$00],Y ; read existing buffer byte
00e832 bb TYX ; copy back to X, why??
00e833 7a PLY ; retrieve destination ptr
00e834 97 00 STA [$00],Y ; write byte
00e836 c8 INY ; destination ptr++
00e837 e8 INX ; buffer source++
00e838 c2 20 REP #$20 ; Accum (16 bit)
00e83a c6 cb DEC $cb ; count--
00e83c e2 20 SEP #$20 ; Accum (8 bit)
00e83e d0 ee BNE $e82e ; loop while count <> 0
00e840 4c a3 e7 JMP $e7a3 ; go to top of loop for next code
; read next byte
00e843 a7 c8 LDA [$c8] ; read single byte from ROM
00e845 a6 c8 LDX $c8 ; load source ptr
00e847 e8 INX ; source ptr++
00e848 d0 05 BNE $e84f ; if not beyond end of bank
00e84a a2 00 80 LDX #$8000 ; wrap source to beginning of next bank
00e84d e6 ca INC $ca ; increment to next source bank
00e84f 86 c8 STX $c8 ; store source ptr
00e851 60 RTS ; end of read byte
00e852 ff ff ff ff SBC $ffffff,X ; those familiar separating FF'sCompression Codes
| Code | Address | Description |
|---|---|---|
000..... | 00e7ef | transfer bytes from source transfer Count bytes after code to buffer |
001..... | 00e7fe | repeat single byte read next byte and repeat Count times |
010..... | 00e80b | repeat two alternating bytes read next two bytes and alternately repeat |
011..... | 00e7e1 | repeat single incrementing byte read next byte and repeat incrementing each time |
111..... | 00e7bf | long count bits 2-4 become 5-7 for the new code. bottom 2 bits become top two bits of count. read next byte for lower 8 bits of count +1. |
1xx..... | 00e825 | transfer bytes from output buffer read next two bytes for buffer source pointer (low/high) |
11111111 | 00e7aa | end of compressed stream |
Examples
Transfer bytes from source
03 12 34 56 -> 12 34 56Repeat single byte
23 12 -> 12 12 12Repeat two alternating bytes
45 12 34 -> 12 34 12 34 12Repeat single incrementing byte
63 12 -> 12 13 14Long count (followed by source transfer of 302h bytes)
E3 01 12 34 56 .. -> 12 34 56 78 90 12 34 ...Transfer byte from output buffer (starting at offset 4)
83 00 04 -> 90 12 34