ZX81 Listing for zxscroll.asm


ZX81 assembly listing for *ZX SCROLL**SLR/2022*

ZX81 Assembly Program Listing

*ZX SCROLL**SLR/2022* (zxscroll.asm)


;
; Scroll ZX
; v1.00 Steven Reid (c) 2022
; A very simple scroll demo in assembly.
;

; +++
;
; Header and startup
;

        ; start up stuff
        org 16514               ; stored in REM at top (ZX81)
        jr start                ; needed for z80asm

; title and copyright (will show on listing)
copy:
        db $17,$3f,$3d,$00,$38,$28,$37,$34,$31,$31
        db $17,$17,$38,$31,$37,$18,$1e,$1c,$1e,$1e
        db $17,$76  ; *ZX SCROLL**SLR/2022*

start:
        call slow               ; SLOW is required.
        call cls                ; clear screen

; end header and startup
;
; ---

; +++
;
; Main Loop
;

mainloop:

; replicate this in assembly!
;  20 FOR X=0 TO 6.3 STEP .3
        ld b,21                 ; 21 positions!

next:
        push bc                 ; save position var

        ld hl,50                ; short pause
        call newpause           ; and pause (also allows)

;  30 SCROLL
        ld hl,(d_file)          ; get top of screen memory
        inc hl                  ; move to top of screen
        ld d,h                  ; copy value into
        ld e,l                  ; de
        ld bc,33                ; set hl to next line
        add hl,bc               ; by adding33
        ld bc,693               ; prepare to copy lines 1-23
        ldir                    ; starting at top

;  40 LET L=15+14*SIN X
        pop bc                  ; restore position (b)
        ld hl,positions         ; location of computed vaules
        ld d,0                  ; set d to zero
        ld e,b                  ; set e to position (b)
        add hl,de               ; add to get location
        dec hl                  ; subtract 1
        ld e,(hl)               ; put tab vaule in e

;  50 PRINT TAB L;"ZX"
        ld hl,(d_file)          ; get top of screen memory
        add hl,de               ; add vaule from above
        ld de,661               ; move to bottom of screen
        add hl,de               ; postion on bottom row
        ld a,63                 ; load a with Z
        ld (hl),a               ; and print it
        inc hl                  ; move over one
        ld a,61                 ; load a with X
        ld (hl),a               ; and print it

;  60 NEXT X
        djnz next               ; loop through positions

;  70 RUN
        jp mainloop             ; run again!

; End of loop!
;

; +++
;
; Routines
;

; Print string at HL
; stops when reaches $FF
;
print:
        ld a,(hl)               ; load character
        inc hl                  ; increment memory
        cp $ff                  ; last character?
        ret z                   ; yep, return
        rst $10                 ; nope, print it!
        jr print                ; loop

;
; Delay
;
; set bc to speed
; press space to 

pframe: dw $0000
newpause:
        ld (pframe),hl

        call kscan              ; get key press
        inc l
        jr z,lppause            ; not yet loop
        dec l
        ld b,h
        ld c,l
        call findchar           ; yep, grab character pressed
        ld a,(hl)
        and a                   ; pressed space (break or 0)?
        jr nz,lppause           ; not yet loop

        call wait               ; make sure user has let go of key
        jp stop                 ; stop!

        ; loop!
lppause:
        ld hl,(pframe)
        dec hl
        ld a,h
        or l
        jr nz,newpause          ; not zero, keep going!

        ret                     ; pause is done!

wait:   call kscan              ; Wait for human to take finger off of key.
        inc l
        jr nz,wait
        ret

; +++
;
; Data and Defines
;

; ZX81 system vars
d_file:         equ $400c
d_fcc:          equ 16398
frames:         equ 16436

; ZX81 ROM functions
kscan:          equ $02bb
findchar:       equ $07bd
stop:           equ $0cdc
slow:           equ $0f2b
fast:           equ $02e7
save:           equ $02f9
printat:        equ $08f5
pause:          equ $0f35
cls:            equ $0a2a

; Calculated positions
positions:
        db 15,19,23,26,28
        db 29,29,27,24,21
        db 17,13, 9, 5, 3
        db  1, 1, 2, 4, 7
        db 11

; end defines
; ---