148 lines
2.7 KiB
NASM
148 lines
2.7 KiB
NASM
// included by decode.asm
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// stats on extra-padded "Elizabeth Mary Patricia James Robert":
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// exiting after 2456 instructions and 6714 cycles. 0x84
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// with a bunch of optimization: (yeah i forgot)
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// exiting after 2483 instructions and 6599 cycles. 0x74
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// cycles: 6599-55=6544
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// instrs: 2483-17=2466
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// program size: 0x74 (116, 22 of which are the end-of-string comparisons)
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// with decode_read4_after moved further down:
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// exiting after 2459 instructions and 6527 cycles. 0x74
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// cycles: 6527-55=6472
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// instrs: 2459-17=2442
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// program size: 0x74 (116, 22 of which are the end-of-string comparisons)
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constant decode_internalize(1)
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if decode_internalize {
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decode_ilut:
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db "ETAOINSHRDLU????CMFPGWYBVKXJQZ. "
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}
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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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decode_xx:
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// NOTE: Y is always 0 here, if that helps at all.
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tax // stash for after branch
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and #%11000000
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lsr
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lsr
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lsr
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cmp #%00011000
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ror
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//bcc decode_read_either // always branch
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decode_read_either:
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sta $05
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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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bpl decode_read_again // always branch
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// we have at least one pair left to read from X
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+;
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txa
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asl
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asl
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decode_read_again:
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tax
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rol
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rol
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rol
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and #%00000011
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ora $05
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bmi decode_read4_after
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//cmp #%00011100
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//bcs decode_read4_after
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// read2, nothing left to do, so just write it
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//bpl decode_write // always branch
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decode_write:
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// decode_common stuff:
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tay
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if decode_internalize {
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lda decode_ilut,y
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} else {
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lda decode_lut0xxx,y
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}
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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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dec $04 // decrement pairs remaining
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beq + // branch if we need more pairs
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txa
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asl
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asl
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bvc decode_xx // always branch (FIXME: might depend on output address)
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+;
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if 1 {
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jsr decode_advance
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bpl decode_xx // always branch
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} else {
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// 1 cycle(?) slower and 1 byte larger
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lda #(decode_xx - 1) >> 8
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pha
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lda #(decode_xx - 1) & 0xFF
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pha
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// fallthru to decode_advance
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}
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decode_read4_after:
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and #%00000011
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asl
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asl
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ora #%00010000
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bpl decode_read_either // always branch
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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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ldx #4 // pairs remaining
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stx $04 // write pairs remaining
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rts // A = *input; X = 4; Y = 0
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decode_exit:
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if 1 {
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// 4+4=8 cycles, 2 bytes:
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pla
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pla
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} else {
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// 3+2=5 cycles, 3 bytes, also penalty for having to set up $06 in the first place:
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ldx $06
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txs
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}
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rts
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die:
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db $F2
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done:
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db $D2
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// vim:ft=snes_bass
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