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cc65/libsrc/vic20/crt0.s

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;
; Startup code for cc65 (Vic20 version)
;
.export _exit
.export __STARTUP__ : absolute = 1 ; Mark as startup
.import initlib, donelib, callirq
.import zerobss, push0
.import callmain
.import RESTOR, BSOUT, CLRCH
.import __INTERRUPTOR_COUNT__
.import __RAM_START__, __RAM_SIZE__ ; Linker generated
.include "zeropage.inc"
.include "vic20.inc"
; ------------------------------------------------------------------------
; BASIC header with a SYS call
.segment "EXEHDR"
.word Head ; Load address
Head: .word @Next
.word .version ; Line number
.byte $9E ; SYS token
.byte <(((Start / 1000) .mod 10) + '0')
.byte <(((Start / 100) .mod 10) + '0')
.byte <(((Start / 10) .mod 10) + '0')
.byte <(((Start / 1) .mod 10) + '0')
.byte $00 ; End of BASIC line
@Next: .word 0 ; BASIC end marker
; ------------------------------------------------------------------------
; Startup code
.segment "STARTUP"
Start:
; Save the zero page locations we need
ldx #zpspace-1
L1: lda sp,x
sta zpsave,x
dex
bpl L1
; Switch to second charset
lda #14
jsr BSOUT
; Clear the BSS data
jsr zerobss
; Save system stuff and setup the stack
tsx
stx spsave ; Save the system stack ptr
lda #<(__RAM_START__ + __RAM_SIZE__)
sta sp
lda #>(__RAM_START__ + __RAM_SIZE__)
sta sp+1 ; Set argument stack ptr
; If we have IRQ functions, chain our stub into the IRQ vector
lda #<__INTERRUPTOR_COUNT__
beq NoIRQ1
lda IRQVec
ldx IRQVec+1
sta IRQInd+1
stx IRQInd+2
lda #<IRQStub
ldx #>IRQStub
sei
sta IRQVec
stx IRQVec+1
cli
; Call module constructors
NoIRQ1: jsr initlib
; Push arguments and call main()
jsr callmain
; Back from main (This is also the _exit entry). Run module destructors
_exit: jsr donelib
; Reset the IRQ vector if we chained it.
pha ; Save the return code on stack
lda #<__INTERRUPTOR_COUNT__
beq NoIRQ2
lda IRQInd+1
ldx IRQInd+2
sei
sta IRQVec
stx IRQVec+1
cli
; Copy back the zero page stuff
NoIRQ2: 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
; Reset changed vectors, back to basic
jmp RESTOR
; ------------------------------------------------------------------------
; The IRQ vector jumps here, if condes routines are defined with type 2.
IRQStub:
cld ; Just to be sure
jsr callirq ; Call the functions
jmp IRQInd ; Jump to the saved IRQ vector
; ------------------------------------------------------------------------
; Data
.data
IRQInd: jmp $0000
.segment "ZPSAVE"
zpsave: .res zpspace
.bss
spsave: .res 1