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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
; initialisation: clear screen and set num to point to screen mem start
init:
jsr clear_screen
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
jmp else_left
if_left:
jsr dec_num
jmp loop
else_left:
jsr print_byte
if_right:
jsr inc_num
cmp #$2d ; reset if - pressed
beq if_reset
jmp else_reset
if_reset:
pha
lda #$04
sta numH
lda #$00
sta numL
pla
jmp loop
else_reset:
cmp #$30 ; end program if 0 pressed
bne loop
jmp end
; loops until keyboard buffer has any number of characters in it
key_wait:
; init: pha to retrieve before rts, to keep A intact
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
; clear the screen by filling it with spaces
; destroys: A, Y, numL & numH (in memory)
clear_screen:
; init: y = 0 because you can't use indirect addressing without a register,
; load start of screen memory into numL/numH
ldy #0
lda #$04
sta numH
lda #$00
sta numL
jmp cs_loop
; add 1 to high bit, return if $08 (screen mem only goes to $07ff)
add_carry:
inc numH
lda numH
cmp #$08
bne cs_loop
rts
; store a space in current screen mem address as specified by num, branch if
; number overflows to 0 (i.e. add 1 to high bit of 16 bit number)
cs_loop:
lda #$20
sta (numL),y
inc numL
beq add_carry
jmp cs_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
sbc #$3f ; convert (very vaguely) to c64 screen codes
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
|
