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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
CHROUT := $ffd2
; 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
jmp loop
; main loop
loop:
jsr key_wait
lda key_buf
cmp #$1d ; right arrow
beq if_right
cmp #$9d ; left arrow
beq if_left
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_reset:
lda #$04
sta numH
lda #$00
sta numL
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
; loops until keyboard buffer has any number of characters in it
key_wait:
pha
key_wait_loop:
lda key_buf_len ; shows how many keys are in key buffer
cmp #$00
beq key_wait_loop
lda #$00
sta key_buf_len
pla
rts
; 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 (16 bit) (NOT WORKING)
; destroys: X
a16b:
pha
tsx
lda numL
adc $0100,x
sta numL
bcs a16b_carry
jmp a16b_end
a16b_carry:
inc numH
a16b_end:
clc
pla
rts
inc_num:
inc numL
beq inc_H
rts
inc_H:
inc numH
rts
dec_num:
pha
dec numL
lda numL
cmp #$ff
beq dec_H
pla
rts
dec_H:
dec numH
pla
rts
end:
brk
|
