INCSRC  "memory.asm"

        ORG $8000
Start:      SEI         ;disable IRQs
        CLD         ;disable BCD
        LDX #$FF        ;init stack
        TXS
        JSR InitPPU
        JSR InitAPU
        JSR ClearNametable
        JSR LoadPalette
        JSR LoadNametable
        JSR ScreenOn
@gameLoop:  JSR WaitForNMI
        JMP @gameLoop

InitPPU:    LDA #$00
        STA $2000       ;disable NMI
        STA $2001       ;disable rendering
        LDA #$FF
        BIT $2002       ;clear vblank flag by reading
                    ;wait two frames to stabilize PPU
@waitForPPU1:   BIT $2002       ;check vblank flag
        BPL @waitForPPU1
@waitForPPU2:   BIT $2002       ;check vblank flag
        BPL @waitForPPU2
        BIT $2002
        JMP ResetScroll

InitAPU:    LDA #$00
        STA $4010       ; disable DMC IRQ
        RTS

ResetScroll:    LDA #$00
        STA $2005
        STA $2005
        RTS

LoadPalette:    LDA #$3F        ;tell PPU to write to $3F00
        STA $2006
        LDA #$00
        STA $2006
        LDA #$0F        ; code for black
        STA $2007       ; write to background
        LDA #$30        ; code for white
        STA $2007       ; fill the first palette with this
        STA $2007
        STA $2007
        RTS

ClearNametable: LDA #$20        ; write to VRAM 2000
        STA #$2006
        LDA #$00
        STA #$2006
        LDY #$04        ; write 256 * 4 (clear from 2000-2400)
@writeBlock:    LDX #$00
@writeLoop: STA $2007
        DEX
        BNE @writeLoop
        DEY
        BNE @writeBlock
        RTS

HelloTxt:   DB  "HELLO" - "A" + 1, 0
LoadNametable:  LDA #$20        ; write to VRAM 2022 - first visible
        STA $2006
        LDA #$42
        STA $2006
        LDX #$00        ; index into txt
@copyToVRAM:    LDA HelloTxt, X ; fetch char
        BEQ @copyDone   ; stop if 0
        STA $2007       ; write to screen
        INX
        BNE @copyToVRAM ; always loop
@copyDone:  RTS

ScreenOn:   LDA #$80        ; enable NMI
        STA $2000
        LDA #$08        ; enable BG
        STA $2001
        RTS

NullVector: RTI

NMI:        INC retraces
        RTI

WaitForNMI: LDA retraces
@loop       CMP retraces
        BEQ @loop
        RTS

Vectors:    ORG $BFFA
@nmi:       DW  NMI
@reset:     DW  Start
@irq:       DW  NullVector