cc65/libsrc/plus4/crt0.s

;
; Startup code for cc65 (Plus/4 version)
;

	.export		_exit
        .export         brk_jmp
        .export         __STARTUP__ : absolute = 1      ; Mark as startup

	.import		callirq_y, initlib, donelib
	.import	     	callmain, zerobss
	.import	       	__INTERRUPTOR_COUNT__
	.import         __RAM_START__, __RAM_SIZE__     ; Linker generated
	.import         __STACKSIZE__                   ; Linker generated
	.importzp       ST

        .include        "zeropage.inc"
	.include	"plus4.inc"


; ------------------------------------------------------------------------
; Constants

IRQInd 	       	= $500	; JMP $0000 - used as indirect IRQ vector

; ------------------------------------------------------------------------
; Startup code

.segment       	"STARTUP"

Start:

; Save the zero page locations we need

        sei                     ; No interrupts since we're banking out the ROM
        sta     ENABLE_RAM
       	ldx   	#zpspace-1
L1:	lda	sp,x
   	sta	zpsave,x
  	dex
       	bpl	L1
        sta     ENABLE_ROM
        cli

; Switch to second charset

  	lda	#14
  	jsr	$FFD2           ; BSOUT

; Save system stuff and setup the stack. The stack starts at the top of the
; usable RAM.

       	tsx
       	stx    	spsave         	; save system stk ptr

        lda     #<(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
        sta     sp
        lda     #>(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
        sta     sp+1

; Setup the IRQ vector in the banked RAM and switch off the ROM

        ldx     #<IRQ
        ldy     #>IRQ
        sei                     ; No ints, handler not yet in place
        sta     ENABLE_RAM
        stx     $FFFE           ; Install interrupt handler
        sty     $FFFF
        cli                     ; Allow interrupts

; Clear the BSS data

  	jsr	zerobss

; Initialize irqcount, which means that from now own custom linked in IRQ
; handlers (via condes) will be called.

        lda     #.lobyte(__INTERRUPTOR_COUNT__*2)
        sta     irqcount

; Call module constructors

  	jsr	initlib

; Push arguments and call main()

  	jsr    	callmain

; Back from main (this is also the _exit entry). Run module destructors.

_exit: 	pha		       	; Save the return code
        jsr	donelib         ; Run module destructors

; Disable chained IRQ handlers

	lda     #0
        sta     irqcount        ; Disable custom IRQ handlers

; Copy back the zero page stuff

  	ldx	#zpspace-1
L2:	lda	zpsave,x
  	sta	sp,x
  	dex
       	bpl	L2

; Place the program return code into ST

	pla
	sta	ST

; Restore the stack pointer

       	ldx	spsave
       	txs

; Enable the ROM and return to BASIC

        sta     ENABLE_ROM
       	rts

; ------------------------------------------------------------------------
; IRQ handler. The handler in the ROM enables the kernal and jumps to
; $CE00, where the ROM code checks for a BRK or IRQ and branches via the
; indirect vectors at $314/$316.
; To make our stub as fast as possible, we skip the whole part of the ROM
; handler and jump to the indirect vectors directly. We do also call our
; own interrupt handlers if we have any, so they need not use $314.

.segment        "LOWCODE"

IRQ:    cld			; Just to be sure
  	pha
        txa
        pha
  	tya
  	pha
        tsx                     ; Get the stack pointer
        lda     $0104,x         ; Get the saved status register
        and     #$10            ; Test for BRK bit
        bne     dobreak

; It's an IRQ and RAM is enabled. If we have handlers, call them. We will use
; a flag here instead of loading __INTERRUPTOR_COUNT__ directly, since the
; condes function is not reentrant. The irqcount flag will be set/reset from
; the main code, to avoid races.

   	ldy    	irqcount
    	beq	@L1
        jsr     callirq_y       ; Call the IRQ functions

; Since the ROM handler will end with an RTI, we have to fake an IRQ return
; on stack, so we get control of the CPU after the ROM handler and can switch
; back to RAM.

@L1:    lda     #>irq_ret       ; Push new return address
        pha
        lda     #<irq_ret
        pha
       	php		       	; Push faked IRQ frame on stack
	pha		       	; Push faked A register
	pha		       	; Push faked X register
	pha		       	; Push faked Y register
        sta     ENABLE_ROM      ; Switch to ROM
        jmp     (IRQVec)        ; Jump indirect to kernal irq handler

irq_ret:
        sta     ENABLE_RAM      ; Switch back to RAM
	pla
	tay
        pla
        tax
        pla
        rti

dobreak:
        lda     brk_jmp+2       ; Check high byte of address
        beq     nohandler
        jmp     brk_jmp         ; Jump to the handler

; No break handler installed, jump to ROM

nohandler:
        sta     ENABLE_ROM
        jmp     (BRKVec)        ; Jump indirect to the break vector

; ------------------------------------------------------------------------
; Data

.data

; BRK handling
brk_jmp:        jmp     $0000

spsave:	        .res	1

irqcount:       .byte   0

.segment        "ZPSAVE"

zpsave:	        .res	zpspace