Spectrum Computing

Code: Select all

1 FOR a=0 TO 14444: POKE PEEK a+PEEK (a+1) +PEEK (a+2) +22528,RND*56: NEXT a: GO TO 1

Code: Select all

org 28000

c_mosaicfade:
		ld a,39					; Galois LFSR seed
		ld d,$38				; LFSR mask (and mask for attributes)
c_mosaicfade_l1:
		ld b,3					; 9 squares per frame
c_mosaicfade_l2:
		call c_mosaicfade_plot			; Plot square
		djnz c_mosaicfade_l2
		halt
		jr c_mosaicfade_l1

c_mosaicfade_plot:
		rrca					; Rotate
		jr nc,c_mosaicfade_plot_l1
		xor d					; XOR if last bit cycled was 1 (feedback part)

c_mosaicfade_plot_l1:
		ld l,a
		ld h,$58

		call c_matchcolours
		ld (hl),a				; 1st attr. page
		
		ld a,l
		inc h
		xor %00110110				; Randomise position
		ld l,a
		call c_matchcolours			; Set ink to be same colour as paper
		ld (hl),a

		ld a,l					; Get page position value
		inc h
		xor %11001001				; Randomise position
		ld l,a
		call c_matchcolours
		ld (hl),a
		
		ld a,l
		cpl					; Restore to original value (equiv. to XOR $FF)

c_mosaicfade_plot_l2:
		cp 39					; Are we back to the start of the period?
		ret nz					; Return if not
		or a
		ret z
		pop hl					; We want to return directly to the caller

		xor a
		jr c_mosaicfade_plot_l1

c_matchcolours:
		and d					; Keep only the paper colour
		ld c,a
		rrca
		rrca
		rrca					; Rotate paper colour into ink colour slot
		or c					; Combine paper and ink colours
		ret

end 28000