192 lines
5.0 KiB
NASM
192 lines
5.0 KiB
NASM
// included by decode.asm
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// cycles on extra-padded "Elizabeth Mary Patricia James Robert":
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// without any interleaving of instructions and LUT:
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// cycles: 7305-55=7250
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// instrs: 2665-17=2648
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// program size: 0x96 (150, 22 of which are the end-of-string comparisons)
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// with JSRs branched-over instead of branched-to-and-back:
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// cycles: 7293-55=7238
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// instrs: 2649-17=2632
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// program size: 0x92 (146, 22 of which are the end-of-string comparisons)
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// with BPL instead of CLC+BCC:
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// cycles: 7151-55=7096
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// instrs: 2578-17=2561
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// program size: 0x90 (144, 22 of which are the end-of-string comparisons)
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// with interleaved instructions and LUT:
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// cycles: 7151-55=7096
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// instrs: 2578-17=2561
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// program size: 0xC3 (note that this will *always* be 0xC3 with this method)
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// so like, same as v3, except
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// instead of branching based on the high bits (through asl or ror),
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// we mask out the two bits being used, and ORA them in through Absolute,X mode.
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// so that means, at these memory positions (possibly offset by X, on another page),
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// we need to occupy a byte:
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// actually, wait, in v3, the bits we branch on are always the top two.
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// so what i could do is use X, both to offset to the end of the page,
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// and also for each decoding case.
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// X = A & %11000000
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// A = %00111100[X]
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align(0x100)
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decode_ilut: // internal look-up table
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db $00,$00,$00
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decode_advance:
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inc $02 // advance input
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beq die // never branch (unless page boundary)
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when_to_stop()
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ldy #0
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lda ($02),y // load from input
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tax // stash for after branch
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lda #4 // pairs remaining
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sta $04 // write pairs remaining
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rts
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decode_exit:
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pla
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pla
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rts
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nops(decode_ilut + 0x40)
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db $04,$01,$04
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decode_begin_next:
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pla
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pla
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decode:
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// NOTE: output/input pointers cannot cross page boundaries.
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// that means the effective longest lengths of output/input are 256/192 bytes.
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lda #4 // pairs remaining
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sta $04 // write pairs remaining
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ldy #0
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lda ($02),y // load from input
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tax // stash for after branch
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decode_xx: // decode from offset 0, unknown code length
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// two things need to be done here:
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// 1. zp[0x05] |= {%0, %100, %1000, %10000}[A >> 6]
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txa // TODO: unnecessary?
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and #$C0
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tay
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lda decode_ilut+0,y // would be ORA, but this is our first data point
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sta $05
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// 2. branch to decode_read4 is both bits were set, decode_read2 otherwise.
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tya
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eor #$C0
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beq decode_read4
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bne decode_read2 // always branch
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die:
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db $F2
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decode_nextbytew:
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jsr decode_advance
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bpl decode_xx // always branch
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decode_write:
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lda $05 // TODO: unnecessary?
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// decode_common stuff:
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tay
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lda decode_lut0xxx,y
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ldy #0
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sta ($00),y // write to output
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inc $00 // advance output
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beq die // never branch (unless page boundary)
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txa
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asl
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asl
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tax
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dec $04 // decrement pairs remaining
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bne decode_xx // branch if we're good, otherwise...
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beq decode_nextbytew // (always) branch if we need more pairs
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nops(decode_ilut + 0x80)
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db $08,$02,$08
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decode_read2:
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txa
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asl
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asl
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tax
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dec $04 // decrement pairs remaining
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bne +
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jsr decode_advance
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+;
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decode_read2_again:
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// we have at least one pair left to read from X
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// zp[0x05] |= {%0, %1, %10, %11}[A >> 6]
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txa // TODO: unnecessary?
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and #$C0
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tay
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lda $05
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ora decode_ilut+1,y
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sta $05
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bpl decode_write // always branch
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decode_read4:
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txa
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asl
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asl
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tax
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dec $04 // decrement pairs remaining
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bne +
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jsr decode_advance
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+;
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// we have at least one pair left to read from X
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// zp[0x05] |= {%0, %1, %10, %11}[A >> 6]
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txa // TODO: unnecessary?
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and #$C0
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tay
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lda $05
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ora decode_ilut+2,y
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sta $05
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bpl decode_read2 // always branch
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nops(decode_ilut + 0xC0)
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db $10,$03,$0C
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done:
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db $F2
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if 0 {
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align(0x100)
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decode_ilut: // internal look-up table
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $04,$01, $04,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $08,$02, $08,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $10,$03, $0C,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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db $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00, $00,$00
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}
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// decode_ilut[0b00000000] = 0b00000000
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// decode_ilut[0b01000000] = 0b00000100
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// decode_ilut[0b10000000] = 0b00001000
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// decode_ilut[0b11000000] = 0b00010000
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// vim:ft=snes_bass
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