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|
.export init
.feature c_comments
numL := $02 ; low byte of a 16-bit number
numH := $03 ; low byte of a 16-bit number
key_buf_len := $c6
key_buf := $0277
GETIN := $ffe4
CHROUT := $ffd2
CLRCHN := $e544
; initialisation: clear screen and set num to point to screen mem start
init:
; clear screen
lda #$93
jsr CHROUT
ldy #$00
lda #$04
sta numH
lda #$00
sta numL
jsr CLRCHN
jmp loop
; main loop
loop:
jsr GETIN
beq loop
cmp #$1d ; right arrow
beq if_right
cmp #$9d ; left arrow
beq if_left
cmp #$91 ; up arrow
beq if_up
cmp #$11 ; down arrow
beq if_down
cmp #$2d ; reset if - pressed
beq if_reset
cmp #$20 ; fill cell if space pressed
beq if_space
cmp #$30 ; end program if 0 pressed
bne loop
jmp end
if_right:
jsr inc_num
jmp loop
if_left:
jsr dec_num
jmp loop
if_up:
lda #40
jsr s16b
jmp loop
if_down:
lda #40
jsr a16b
jmp loop
if_reset:
lda #$93
jsr CHROUT
jmp loop
if_space:
ldx #$00
lda (numL,x)
cmp #$a0
beq if_cell_filled
jmp if_cell_empty
if_cell_filled:
lda #$20
jsr print_byte
jmp loop
if_cell_empty:
lda #$a0
jsr print_byte
jmp loop
; print byte in A at location in num
print_byte:
; init: store original A in X and push Y to stack
tax
tya
pha
txa
ldy #$00
sta (numL),y
; deinit: pull Y from stack and transfer original A from X to A
pla
tay
txa
rts
; adds A to num
a16b:
clc
adc numL
sta numL
lda #$00
adc numH
sta numH
rts
; subtracts A from num
s16b:
pha
lda numL
tsx
inx
sec
sbc $0100,x
sta numL
lda numH
sbc #$00
sta numH
pla
rts
inc_num:
inc numL
beq inc_H
rts
inc_H:
inc numH
rts
dec_num:
pha
dec numL
lda numL
cmp #$ff
beq dec_H
pla
rts
dec_H:
dec numH
pla
rts
end:
brk
|
