A few days ago I coded The Matrix digital rain effect, a fictional representation of the code for the virtual reality of The Matrix. The technique is simple: fill the screen with random characters and scroll down columns of attributes, occasionally switching between black and green.
Here's the final code - 147 bytes of Z80 using the default Sinclair font:
org 08000h
; black border / black attributes
xor a
out (0FEh),a
ld hl,05AFFh
attr: ld (hl),a
dec hl
bit 2,h
jr z,attr
; fill screen with random characters
ld e,a
fillscr:ld d,040h
fill: call rndchar
ld a,d
cp 058h
jr nz,fill
inc e
jr nz,fillscr
; digital rain loop
frame: ld b,06h
halt
column: push bc
; randomize one character
call random
and 018h
jr z,docol
add a,038h
ld d,a
call random
ld e,a
call rndchar
; select a random column
docol: call random
and 01Fh
ld l,a
ld h,058h
; ~1% chance black -> white
ld a,(hl)
or a
ld bc,0247h
jr z,check
; white -> bright green
white: cp c
ld c,044h
jr z,movecol
; bright green -> green
cp c
ld c,04h
jr z,movecol
; ~6% chance green -> black
ld bc,0F00h
check: call random
cp b
jr c,movecol
ld c,(hl)
; move column down
movecol:ld de,020h
ld b,018h
down: ld a,(hl)
ld (hl),c
ld c,a
add hl,de
djnz down
pop bc
djnz column
; test for keypress
ld bc,07FFEh
in a,(c)
rrca
jr c,frame
ret
; display a random glyph
rndchar:call random
crange: sub 05Fh
jr nc,crange
add a,a
ld l,a
ld h,0
add hl,hl
add hl,hl
ld bc,(05C36h)
add hl,bc
ld b,8
char: ld a,(hl)
ld (de),a
inc d
inc hl
djnz char
ret
; get a byte from the ROM
random: push hl
ld hl,(seed)
inc hl
ld a,h
and 01Fh
ld h,a
ld (seed),hl
ld a,(hl)
pop hl
ret
seed:
