;
; Startup code for cc65 (Plus/4 version)
-;
-; This must be the *first* file on the linker command line
;
.export _exit
.export brk_jmp
+ .export __STARTUP__ : absolute = 1 ; Mark as startup
- .import condes, initlib, donelib
- .import push0, callmain, zerobss
- .import __IRQFUNC_TABLE__, __IRQFUNC_COUNT__
+ .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"
IRQInd = $500 ; JMP $0000 - used as indirect IRQ vector
; ------------------------------------------------------------------------
-; Place the startup code in a special segment to cope with the quirks of
-; plus/4 banking.
+; Startup code
.segment "STARTUP"
- .word Head ; Load address
-Head: .word @Next
- .word 1000 ; Line number
- .byte $9E,"4109" ; SYS 4109
- .byte $00 ; End of BASIC line
-@Next: .word 0 ; BASIC end marker
+Start:
-; ------------------------------------------------------------------------
-; Actual code
+; 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 ; save the zero page locations we need
- dex
+ sta zpsave,x
+ dex
bpl L1
sta ENABLE_ROM
cli
-; Close open files
+; Switch to second charset
- jsr $FFCC ; CLRCH
+ lda #14
+ jsr $FFD2 ; BSOUT
-; Switch to second charset
+; 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 #14
- jsr $FFD2 ; BSOUT
+ 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
- lda #<IRQ
- sta $FFFE ; Install interrupt handler
- lda #>IRQ
- sta $FFFF
+ stx $FFFE ; Install interrupt handler
+ sty $FFFF
cli ; Allow interrupts
; Clear the BSS data
- jsr zerobss
+ jsr zerobss
-; Save system stuff and setup the stack. The stack starts at the top of the
-; usable RAM.
+; Initialize irqcount, which means that from now own custom linked in IRQ
+; handlers (via condes) will be called.
- tsx
- stx spsave ; save system stk ptr
-
- lda #<$FD00
- sta sp
- lda #>$FD00
- sta sp+1
+ lda #.lobyte(__INTERRUPTOR_COUNT__*2)
+ sta irqcount
; Call module constructors
- jsr initlib
-
-; If we have IRQ functions, chain our stub into the IRQ vector
-
- lda #<__IRQFUNC_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
+ jsr initlib
; Push arguments and call main()
-NoIRQ1: jsr callmain
-
-; Back from main (this is also the _exit entry). Reset the IRQ vector if
-; we chained it.
-
-_exit: lda #<__IRQFUNC_COUNT__
- beq NoIRQ2
- lda IRQInd+1
- ldx IRQInd+2
- sei
- sta IRQVec
- stx IRQVec+1
- cli
+ jsr callmain
-; Run module destructors.
+; Back from main (this is also the _exit entry). Run module destructors.
-NoIRQ2: jsr donelib ; Run module destructors
+_exit: pha ; Save the return code
+ jsr donelib ; Run module destructors
-; Restore system stuff
+; Disable chained IRQ handlers
- ldx spsave
- txs
+ lda #0
+ sta irqcount ; Disable custom IRQ handlers
; Copy back the zero page stuff
- ldx #zpspace-1
+ ldx #zpspace-1
L2: lda zpsave,x
- sta sp,x
- dex
+ sta sp,x
+ dex
bpl L2
-; Enable the ROM, reset changed vectors and return to BASIC
+; Place the program return code into ST
- sta ENABLE_ROM
- jmp $FF8A ; RESTOR
+ pla
+ sta ST
+
+; Restore the stack pointer
+
+ ldx spsave
+ txs
+; Enable the ROM and return to BASIC
+
+ sta ENABLE_ROM
+ rts
; ------------------------------------------------------------------------
-; IRQ handler
+; 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: pha
+IRQ: cld ; Just to be sure
+ pha
txa
pha
+ tya
+ pha
tsx ; Get the stack pointer
- lda $0103,x ; Get the saved status register
- tax ; Save for later
+ lda $0104,x ; Get the saved status register
and #$10 ; Test for BRK bit
bne dobreak
- lda #>irq_ret ; Push new return address
+
+; It's an IRQ and RAM is enabled. If we have handlers, call them. We will use
+; a flag here instead of loading __INTERRUPTOR_COUNT__ directly, since the
+; condes function is not reentrant. The irqcount flag will be set/reset from
+; the main code, to avoid races.
+
+ ldy irqcount
+ beq @L1
+ jsr callirq_y ; Call the IRQ functions
+
+; Since the ROM handler will end with an RTI, we have to fake an IRQ return
+; on stack, so we get control of the CPU after the ROM handler and can switch
+; back to RAM.
+
+@L1: lda #>irq_ret ; Push new return address
pha
lda #<irq_ret
pha
- txa
- pha
+ php ; Push faked IRQ frame on stack
+ pha ; Push faked A register
+ pha ; Push faked X register
+ pha ; Push faked Y register
sta ENABLE_ROM ; Switch to ROM
- jmp ($FFFE) ; Jump to kernal irq handler
+ jmp (IRQVec) ; Jump indirect to kernal irq handler
irq_ret:
sta ENABLE_RAM ; Switch back to RAM
+ pla
+ tay
pla
tax
pla
; No break handler installed, jump to ROM
nohandler:
- tya
- pha ; ROM handler expects Y on stack
sta ENABLE_ROM
jmp (BRKVec) ; Jump indirect to the break vector
-; ------------------------------------------------------------------------
-; Stub for the IRQ chain. Is used only if there are IRQs defined. Needed in
-; low memory because of the banking.
-
-.segment "LOWCODE"
-
-IRQStub:
- cld ; Just to be sure
- sta ENABLE_RAM ; Switch to RAM
- ldy #<(__IRQFUNC_COUNT__*2)
- lda #<__IRQFUNC_TABLE__
- ldx #>__IRQFUNC_TABLE__
- jsr condes ; Call the IRQ functions
- sta ENABLE_ROM
- jmp IRQInd ; Jump to the saved IRQ vector
-
; ------------------------------------------------------------------------
; Data
.data
-zpsave: .res zpspace
; BRK handling
brk_jmp: jmp $0000
-.bss
spsave: .res 1
+irqcount: .byte 0
+
+.segment "ZPSAVE"
+
+zpsave: .res zpspace
projects / cc65 / blobdiff