; // Summary
; // A Very Basic NES ROM that displays 1 color: Blue

.segment "INESHDR"
  .byt "NES",$1A
  .byt 1                ; 1 x 16kB PRG block.
  .byt 1                ; 1 x 8kB CHR block.

.segment "VECTORS"
  .addr nmi, reset, irq

.segment "ZEROPAGE"
  retraces:        .res 1

; Variables
.segment "BSS"
  xframes: .res 1
  buttons: .res 1 ; Reserve 1 byte for storing the data read from controller.


.segment "CODE"
  ; Procedures
  ; -----------------------------------------------------------------------------

  .proc reset
    ; Disable Interrupts
    SEI

    ; Basic Initialization
    LDX #0
    STX $2000       ; General init state; NMIs (bit 7) disabled.
    STX $2001       ; Disable rendering, i.e. turn off background & sprites.

    ; PPU Warmup (29658 Cycles)
    BIT $2002   ; Clear the VPL Flag if it was set at reset time.
  vwait1:
    BIT $2002   ; 27384 Cycles has passed.
    BPL vwait1
  vwait2:
    BIT $2002   ; 57165 Cycles has passed.
    BPL vwait2

    ; Clear lingering interrupts since before reset.
    ; BIT $2002     ; Ack VBLANK NMI (if one was left over after reset); bit 7.

    LDA #%00100000 ; Intensify reds
    STA $2001

    LDA #%10000000
    STA $2000



  waitfora:
    ; call subroutine to read current button state
    JSR readjoy

    ; check if bit 7 of buttons is set
    LDA #$80
    BIT buttons

    ; if not, loop to wait
    BEQ waitfora

    LDA #%01000000 ; Intensify greens
    STA $2001

    LDA #%10000000
    STA $2000

    LDX #30
    JSR waitxframes

    LDA #%10000000 ; Intensify blues
    STA $2001

    LDA #%10000000
    STA $2000

    LDX #30
    JSR waitxframes

    forever:
      JMP forever     ; Infinite loop
  .endproc

  .proc readjoy
    LDA #$01
    STA $4016
    STA buttons ; We use our variable!
    LSR A
    STA $4016

  loop:
    LDA $4016
    LSR A       ; Bit 0 -> Carry
    ROL buttons ; Carry -> Bit 0; Bit 7 -> Carry
    BCC loop    ; Branch if Carry Clear
    RTS
  .endproc

  .proc nmi
    INC xframes
    RTI
  .endproc

  .proc irq
    RTI
  .endproc

  .proc waitxframes
    wait:       ; Wait for a frame
      LDA xframes
    waitonce:
      CMP xframes
      BEQ waitonce
      DEX       ; X = X - 1
      BNE wait  ; if X is not zero, keep going...
      RTS
  .endproc