124 lines
2.3 KiB
PHP
124 lines
2.3 KiB
PHP
ClockLFSR: ; uses af, bc and clocks one bit of the LFSR.
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; at return time, a holds the bottom eight of lfsr state.
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ld a, [rLFSR] ;
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ld b, a ; b has shifted value ; 1 cycle
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srl b ; second tap is two shifts ; 2 cycles
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srl b ; 2 cycles
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xor a, b ; 1 cycle
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srl b ; third tap iis one more shift ; 2 cycles
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xor a, b ; 1 cycle
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srl b ; fourth tap is two more shifts ; 2 cycles
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srl b ; 2 cycles
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xor a, b ; 1 cycle
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and a, $1 ; now the zero flag is set iff the tap bit is 0 ; 1 cycle
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; set carry flag from zero flag
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scf ; 1 cycle
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jr nz, .noComplementCarry ; 2 or 3 cycles
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ccf ; 1 cycle if prev was 2 cycles :)
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.noComplementCarry
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ld a, [rLFSR+1] ; 2 cycles
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rra ; 1 cycle ; this should populate with the carry flag
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ld [rLFSR+1], a ; 2 cycles
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ld a, [rLFSR] ; 2 cycles
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rra ; 1 cycle
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ld [rLFSR], a ; 2 cycles
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ret ; 37 cycles total. can call roughly 12? of these per scanline
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OneRandomByte: ; clocks LFSR 8 times so a is a fresh random byte
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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call ClockLFSR
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ret
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RandomUnderD: ; takes d as limit (inclusive)
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; check what the highest bit of d is and dispatch to thee number of bits
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; we're going to generate
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ld d, a
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ld e, %1111_1111
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bit 7, d
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jr nz, .inEight
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ld e, %0111_1111
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bit 6, d
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jr nz, .inSeven
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ld e, %0011_1111
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bit 5, d
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jr nz, .inSix
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ld e, %0001_1111
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bit 4, d
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jr nz, .inFive
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ld e, %0000_1111
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bit 3, d
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jr nz, .inFour
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ld e, %0000_0111
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bit 2, d
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jr nz, .inThree
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ld e, %0000_0011
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bit 1, d
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jr nz, .inTwo
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ld e, %0000_0001
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bit 0, d
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jr nz, .inOne
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ld a, 0
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ret
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.inEight
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call ClockLFSR
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.inSeven
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call ClockLFSR
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.inSix
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call ClockLFSR
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.inFive
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call ClockLFSR
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.inFour
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call ClockLFSR
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.inThree
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call ClockLFSR
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.inTwo
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call ClockLFSR
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.inOne
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call ClockLFSR
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and a, e
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cp a, d
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ret z
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ret c
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jr .inOne
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SwapCards: ; takes hl as array, c and e as indices to swap (uses b and d)
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inc hl ; first element of array
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push hl ; save it for later
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ld b, 0
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ld d, 0
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add hl, bc
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ld a, [hl]
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ld b, a ; stash old [hl+0c] in b
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pop hl
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push hl
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add hl, de
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ld a, [hl]
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ld d, a ; stash old [hl+0e] in d
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ld [hl], b ; put old [hl+0c] in [hl+0e]
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pop hl
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ld b, 0 ; return 0 to b
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add hl, bc
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ld [hl], d ; put old [hl+0e] in [hl+0c]
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ret
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