; Use CIO to set up S: as GR.8 or GR.15
; Relocatable code.
; Assemble with:
; cl65 -t none -o loader.xex loader.s

 .include "atari.inc"

dataptr = FR0
tmp = FR0+2

start = $0400

; segment header
 .org start-6
 .word $FFFF
 .word start
 .word endmain-1

 .org start
 .byte $c0  ; default RAMTOP
 .byte $0f  ; GR. mode (8 or 15)
 .byte $04  ; COLOR0 (01 pixels in GR.15)
 .byte $08  ; COLOR1 (10 pixels in GR.15, 1 pixels in GR.8)
 .byte $0c  ; COLOR2 (11 pixels in GR.15, 0 pixels in GR.8)
 .byte $00  ; COLOR3 (unused)
 .byte $00  ; COLOR4 (00 pixels in GR.15, border in GR.8)
 .byte $22  ; SDMCTL ($21 for narrow, $22 for normal)

main:
; find our datablock's start address. this song and dance routine
; allows us to be fully relocatable, and have all the data in a
; single block (easily modified by the shell script that uses this).

 sei        ; disable interrupts so they don't clobber the stack
 lda #$00
 sta NMIEN
 lda #$60   ; RTS opcode
 sta dataptr
 jsr dataptr
ret:        ; here is where the RTS returns to

; address is now in the stack area, fix it up and save it
 tsx
 dex
 lda $100,x    ; lo byte
 sec
 sbc #ret-start-1
 sta dataptr
 lda $101,x    ; hi byte
 sbc #0
 sta dataptr+1 ; $d4/$d5 is now a pointer to 'start'

 cli        ; re-enable interrupts
 lda #$40
 sta NMIEN

; set RAMTOP
 ldy #0
 lda (dataptr),y
 sta RAMTOP

; close device #6
 ldx #$60
 lda #CLOSE
 sta ICCOM,x
 jsr CIOV

; open device #6 (GRAPHICS command basically)
 ldx #$60
 lda #$0c  ; read/write, no +16 or +32
 sta ICAX1,x
 ldy #$01
 lda (dataptr),y
 sta ICAX2,x  ; just the mode (8 or 16)
 lda #OPEN
 sta ICCOM,x
 lda #'S'
 sta tmp
 lda #<tmp
 sta ICBAL,x
 lda #>tmp
 sta ICBAH,x
 jsr CIOV
 
; set up color regs
 ldy #$02
cloop:
 lda (dataptr),y
 sta COLOR0-2,y
 iny
 cpy #$07
 bne cloop

; set up display width
 lda (dataptr),y
 sta SDMCTL

 rts
;;; put some stuff in screen RAM so we can see it
;; lda #$ff
;; sta $a150
;; lda #$aa
;; sta $a151
;; lda #$55
;; sta $a152
;;hang: jmp hang

endmain:

; segment header
 .word INITAD
 .word INITAD+1
 .word main