.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