Image noise: Difference between revisions

Line 3,202:

rrca

xor (hl) ;crude xorshift rng used to select a "random" location to draw to.

ld h,0

ld L,a ;this random value becomes the low byte of HL

rlc L

rl h ;reduce the RNG's bias towards the top of the screen by shifting one bit into H.

ld a,&98 ;high byte of gb vram

add h ;h will equal either 0 or 1.

ld H,a ;now H will either equal &98 or &99

ld a,(frametimer) ;it's possible that this value is different from before since every vblank it increments.

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wait:

halt ;waits for an interrupt.

cp (hl) ;

cp (hl) compare 0 to vblankflag

jr z,wait ;vblank flag is still 1 so keep waiting.

ld (hl),a ;clear vblank flag.

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And the interrupt handler located at memory address <code>&A000</code>:

<lang z80>TV_Static_FX:

;this code runs every time the game boy finishes drawing one row of pixels, unless the interrupt is disabled as described above.

push hl

push af

ld hl,(COSINE_INDEX

ld hl,(COSINE_INDEX)

inc (hl)

ld a,(hl)

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Line 3,269:

; returns cos(A) into A

~~LDH~~ (&43),A ;output A to horizontal scroll register.

LD (&FF43),A ;output cos(frametimer) to horizontal scroll register

done_TV_Static_FX:

pop af

pop hl

reti ;return to main program

TV_Static_FX_End:</lang>