*       = $0801
        .word (+), 2005  ;pointer, line number
        .null $9e, format("%d", start);will be sys 4096
+   .word 0          ;basic line end

* = $1000

                            ; CONSTANTS
SCREEN_SCROLL = $D016
WRITE_PTR = $BB
READ_PTR = $FB
SCREEN_PAGE_PTR = $0288

BORDERCOLOR = $D020
BGCOLOR = $D021
CHRCOLOR = $0286
SPACE = #$20

SCREENSTART = $0400
SCREENEND = $07FF

CIA_TIMER_ORA = $DC0E
IO_MEM_TAPE_CTRL = $01

CHAR_ROM = $D000
CHROUT = $FFD2
                           ; GLOBAL VARIABLES

colIndex .byte 0
fillDone .byte 0

xLimit .byte ?
screenIndex .byte ?
byteToRead .byte ?
frontLimit .byte ?

inter .word ?

fillBuffer
    ldx #0
    stx byteToRead

nextRow lda colIndex
    cmp #0
    bne skip0
    sta screenIndex
    lda #40
    sta frontLimit
    jmp fillFront

skip0
    sec
    lda #40
    sbc colIndex
    sta frontLimit
    sta screenIndex

fillBack
    ldy byteToRead
    lda (READ_PTR),y    ;y starts always at 0
    ldy screenIndex     ;y start always at 40 - colIndex
    sta (WRITE_PTR),y
    inc byteToRead
    inc screenIndex
    ldy screenIndex
    cpy #40
    bne fillBack
    lda colIndex
    sta byteToRead
    lda #0
    sta screenIndex

fillFront
    ldy byteToRead
    lda (READ_PTR),y
    ldy screenIndex
    sta (WRITE_PTR),y
    inc byteToRead
    inc screenIndex
    ldy screenIndex
    cpy frontLimit
    bcc fillFront
    ;update pointers (change row)
    clc
    lda WRITE_PTR
    adc #40
    sta WRITE_PTR
    lda WRITE_PTR+1
    adc #00
    sta WRITE_PTR+1
    clc
    lda READ_PTR
    adc #40
    sta READ_PTR
    lda READ_PTR+1
    adc #00
    sta READ_PTR+1
    ;check rownumber and reset offsets
    lda #0
    sta byteToRead
        inx
    cpx #25
    bcc nextRow
rts

notDefault
    ora #128
    sta $D018
    lda #04
    sta SCREEN_PAGE_PTR
    jsr initPointers
rts

setScreenColors
    ; set color
    lda #$0b
    sta $0286
    ; set border color
    lda #$00
    sta $D020
    ; set background color
    lda #$0c
    sta $D021
rts

initPointers
    lda #00
    sta WRITE_PTR
    lda SCREEN_PAGE_PTR
    sta WRITE_PTR+1
    lda #<screen
    sta READ_PTR
    lda #>screen
    sta READ_PTR+1
rts

swapBuffers
    lda $D018
    tax
    and #240
    cmp #16
    bne noDefault
        txa
    and #15         ; x%0000 1111
    ora #128
    sta $D018
    lda #$04
    sta SCREEN_PAGE_PTR
        rts

noDefault
    txa
    and #15         ; x%0000 1111
    ora #16
    sta $D018
    lda #$20
    sta SCREEN_PAGE_PTR
        rts

*       =           $1600
start
    lda #7
    sta SCREEN_SCROLL
                ; clear the screen
        jsr setScreenColors
    jsr $e544
    jsr initPointers
    jsr fillBuffer
    lda #$20
    sta SCREEN_PAGE_PTR
        jsr initPointers
    inc colIndex
;   jmp endlessloop

sei        ;disable maskable IRQs

lda #$7f
sta $dc0d  ;disable timer interrupts which can be generated by the two CIA chips
sta $dd0d  ;the kernal uses such an interrupt to flash the cursor and scan the keyboard, so we better
           ;stop it.

lda $dc0d  ;by reading this two registers we negate any pending CIA irqs.
lda $dd0d  ;if we don't do this, a pending CIA irq might occur after we finish setting up our irq.
           ;we don't want that to happen.

lda #$01   ;this is how to tell the VICII to generate a raster interrupt
sta $d01a

lda #251   ;this is how to tell at which rasterline we want the irq to be triggered
sta $d012

lda #$1b   ;as there are more than 256 rasterlines, the topmost bit of $d011 serves as
sta $d011  ;the 9th bit for the rasterline we want our irq to be triggered.
           ;here we simply set up a character screen, leaving the topmost bit 0.

lda #$35   ;we turn off the BASIC and KERNAL rom here
sta $01    ;the cpu now sees RAM everywhere except at $d000-$e000, where still the registers of
           ;SID/VICII/etc are visible

lda #<irq  ;this is how we set up
sta $fffe  ;the address of our interrupt code
lda #>irq
sta $ffff

cli        ;enable maskable interrupts again

;   lda $314
;   sta inter
;   lda $315
;   sta inter+1
;   sei
;   lda #<IRQ_loop
;   sta $314
;   lda #>IRQ_loop
;   sta $315
;   cli

endlessloop
;   lda #250
;       cmp $d012
;   bcc skip9
;   jsr IRQ_loop
;skip9
    lda fillDone
    cmp #1
    beq endlessloop
    jsr fillBuffer
    inc fillDone
    jmp endlessloop

IRQ_loop
    dec SCREEN_SCROLL
    lda SCREEN_SCROLL
    cmp #$FF
    bne out

resetScroll
    lda #07
    sta SCREEN_SCROLL
    jsr swapbuffers
    jsr initPointers
    lda #0
    sta fillDone
    inc colIndex
        lda colIndex
    cmp #40
    bne out
    lda #0
    sta colIndex
out
rts
    jmp (inter)


screen
.byte 160,160,160,160,160,160,160,160,160,160,160,160,160,160,232,230,230,230,230,230,230,230,230,232,160,160,160,160,160,232,230,230,230,230,230,230,232,160,160,160
.byte 230,230,230,230,232,232,160,160,160,232,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,232,160,232,230,230,230,230,230,230,230,230,230,230,230,230
.byte 230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,104, 46, 46, 46, 46, 46,104,104,230,230,230,230,230,230,230,104, 46, 46, 46, 46,104,230,230,230,230
.byte 104,104,104,104,104,104,104,104,104,104,104,104,104, 46, 46, 46, 46, 46, 46, 46, 46, 46,104,104,104,104,104,104, 46, 46, 46, 46, 46, 46, 46, 46, 46, 46,104,104
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,100,100,100,100,100,100,100,100,100,100,100, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte 100,100,100,100,100,100,100, 32, 32, 32, 32, 32,100,100,100,100,100, 45, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 45,100,100,100,100,100,100,100,100,100,100
.byte  32, 32, 32, 32, 32, 32, 32, 45,100,100,100, 45, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32,104,104,104,104,104,104,104,104, 32, 32, 32, 32, 32,104, 32, 32,104, 32, 32, 32, 32, 32,104, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte 102,102,102,102,102,102,102,102,102,102,102,102,104,104, 32,102,102,102,102, 32,102,104, 32, 32,102,102,102,102, 32,102, 32,102, 32,102,104, 32,104,102,102,102
.byte 102,102,104, 32, 32, 32, 32, 32,104,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,104, 32,102,102,102,102,102,102,102,102,102,102,102
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,102,104, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,104,102,102,102,102,102,102,102,102,102,104, 32,104,102,102,102,104, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,230,230,230,230,230,230,230,230,230,230,230, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte  32, 32, 32, 32, 32, 32, 32, 32, 32,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230, 32, 32, 32, 32, 32, 32, 32, 32, 32
.byte 230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230,230
.byte 230,230,230,230,232,160,232,230,230,230,230,232,160,160,160,160,160,160,160,232,230,230,232,160,160,160,160,160,160,232,230,230,232,160,160,232,230,230,230,230
.byte 160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160
.byte 160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160






irq

;Being all kernal irq handlers switched off we have to do more work by ourselves.
;When an interrupt happens the CPU will stop what its doing, store the status and return address
;into the stack, and then jump to the interrupt routine. It will not store other registers, and if
;we destroy the value of A/X/Y in the interrupt routine, then when returning from the interrupt to
;what the CPU was doing will lead to unpredictable results (most probably a crash). So we better
;store those registers, and restore their original value before reentering the code the CPU was
;interrupted running.

;If you won't change the value of a register you are safe to not to store / restore its value.
;However, it's easy to screw up code like that with later modifying it to use another register too
;and forgetting about storing its state.

;The method shown here to store the registers is the most orthodox and most failsafe.

pha        ;store register A in stack
txa
pha        ;store register X in stack
tya
pha        ;store register Y in stack

jsr IRQ_loop

lda #$ff   ;this is the orthodox and safe way of clearing the interrupt condition of the VICII.
sta $d019  ;if you don't do this the interrupt condition will be present all the time and you end
           ;up having the CPU running the interrupt code all the time, as when it exists the
           ;interrupt, the interrupt request from the VICII will be there again regardless of the
           ;rasterline counter.

           ;it's pretty safe to use inc $d019 (or any other rmw instruction) for brevity, they
           ;will only fail on hardware like c65 or supercpu. c64dtv is ok with this though.

pla
tay        ;restore register Y from stack (remember stack is FIFO: First In First Out)
pla
tax        ;restore register X from stack
pla        ;restore register A from stack

rti        ;Return From Interrupt, this will load into the Program Counter register the address
           ;where the CPU was when the interrupt condition arised which will make the CPU continue
           ;the code it was interrupted at also restores the status register of the CPU