backyard/6502_name_codec/decode_v1.asm

// included by decode.asm
// cycles on extra-padded "Elizabeth Mary Patricia James Robert":
// 3485-55=3430 (includes jsr and rts)
// program size: 0xFB
// instructions: 1248-17=1231 (includes jsr and rts)

decode:
// NOTE: output/input pointers cannot cross page boundaries.
// that means the effective longest lengths of output/input are 256/192 bytes.

decode00: // decode from offset 0, unknown code length       (READS A BYTE)
ldy #$00
lda ($02),y // load from input

tax // stash for after branch
eor #$C0 // TODO: just use a cmp instruction?
and #$C0
beq decode06 // branch when mask is fully set
// fallthru decode04

decode04: // decode from offset 0, 4-bit code (then 4 under)
txa
lsr
lsr
lsr
lsr
tay
lda decode_lut0xxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode40 // always branch (unless page boundary)
db $02 // jam

decode06: // decode from offset 0, 6-bit code (then 2 under)
txa
lsr
lsr
and #$0F
tay
lda decode_lut11xxxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode60 // always branch (unless page boundary)
db $02 // jam

decode20: // decode from offset 2, unknown code length
txa
eor #$30
and #$30
beq decode26 // branch when mask is fully set

decode24: // decode from offset 2, 4-bit code (then 2 under)
txa
lsr
lsr
and #$0F
tay
lda decode_lut0xxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode60 // always branch (unless page boundary)
db $02 // jam

decode26: // decode from offset 2, 6-bit code (then aligned)
txa
and #$0F
tay
lda decode_lut11xxxx,y
ldy #$00
sta ($00),y // write to output

//inc $02 // advance input
//beq die // never branch (unless page boundary)
jsr decode_advance_input

inc $00 // advance output
//jsr decode_advance_output
bne decode00 // always branch (unless page boundary)
db $02 // jam

decode40: // decode from offset 4, unknown code length
//inc $02 // advance input
//beq die // never branch (unless page boundary)
jsr decode_advance_input
txa
eor #$0C // TODO: just use a cmp instruction?
and #$0C
beq decode46 // branch when mask is fully set
// fallthru decode04

decode44: // decode from offset 4, 4-bit code (then aligned)
txa
and #$0F
tay
lda decode_lut0xxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode00 // always branch (unless page boundary)
db $02 // jam

decode46: // decode from offset 4, 6-bit code (then 2 over)  (READS A BYTE)
txa
and #$03
asl
asl
sta $04
ldy #$00
lda ($02),y // load from input
tax
rol
rol
rol
and #$03
ora $04
tay
lda decode_lut11xxxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode20 // always branch (unless page boundary)
db $02 // jam

decode60: // decode from offset 6, unknown code length       (READS A BYTE)
//inc $02 // advance input
//beq die // never branch (unless page boundary)
jsr decode_advance_input
ldy #$00
lda ($02),y // load from input
tay
txa
eor #$03
and #$03
beq decode66 // branch when mask is fully set
// fallthru decode64

decode64: // decode from offset 6, 4-bit code (then 2 over)
txa
and #$03
asl
asl
sta $04
tya // load in (restore) the new input
tax // and put it in X like the rest of the code expects
rol
rol
rol
and #$03
ora $04
tay
lda decode_lut0xxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode20_shim // always branch (unless page boundary)
db $02 // jam

die:
db $F2 // um lammer jammy

decode20_shim:
jmp decode20

decode66: // decode from offset 6, 6-bit code (then 4 over)
tya // load in (restore) the new input
tax // and put it in X like the rest of the code expects
lsr
lsr
lsr
lsr
tay
lda decode_lut11xxxx,y
ldy #$00
sta ($00),y // write to output

inc $00 // advance output
//jsr decode_advance_output
bne decode40 // always branch (unless page boundary)
db $02 // jam

decode_advance_input:
// do not modify X or Y here
inc $02
beq die // never branch (unless page boundary)
when_to_stop()
rts

if 0 {
    decode_advance_output:
    // do not modify X or Y here
    lda $02
    cmp #(name1 & 0xFF)
    beq decode_begin_next
    cmp #(name2 & 0xFF)
    beq decode_begin_next
    cmp #(name3 & 0xFF)
    beq decode_begin_next
    cmp #(name4 & 0xFF)
    beq decode_begin_next
    cmp #(name5 & 0xFF)
    beq decode_exit
    inc $00 // advance output
    //beq die // never branch (unless page boundary)
    rts
}

decode_begin_next:
//inc $00 // advance output
//beq die // never branch (unless page boundary)
pla
pla
jmp decode00

decode_exit:
pla
pla
rts
//db $D2 // jam (D is for Done, i guess)

// //
done:
db $02 // jam
// //

// vim:ft=snes_bass