; Startup code for cc65 (Plus/4 version)
;
- .export _exit
+ .export _exit
.export brk_jmp
.export __STARTUP__ : absolute = 1 ; Mark as startup
- .import callirq_y, initlib, donelib
- .import callmain, zerobss
- .import __INTERRUPTOR_COUNT__
+ .import callirq_y, initlib, donelib
+ .import callmain, zerobss
+ .import __INTERRUPTOR_COUNT__
+ .import __MAIN_START__, __MAIN_SIZE__ ; Linker generated
+ .import __STACKSIZE__ ; Linker generated
+ .importzp ST
.include "zeropage.inc"
- .include "plus4.inc"
+ .include "plus4.inc"
; ------------------------------------------------------------------------
; Constants
-IRQInd = $500 ; JMP $0000 - used as indirect IRQ vector
-
-; ------------------------------------------------------------------------
-; 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) + $30)
- .byte <(((Start / 100) .mod 10) + $30)
- .byte <(((Start / 10) .mod 10) + $30)
- .byte <(((Start / 1) .mod 10) + $30)
- .byte $00 ; End of BASIC line
-@Next: .word 0 ; BASIC end marker
+IRQInd = $500 ; JMP $0000 - used as indirect IRQ vector
; ------------------------------------------------------------------------
; Startup code
-.segment "STARTUP"
+.segment "STARTUP"
Start:
-; Close open files
-
- jsr $FFCC ; CLRCH
-
-; Save the zero page locations we need
+; Save the zero-page locations that 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
+ ldx #zpspace-1
+L1: lda sp,x
+ sta zpsave,x
+ dex
+ bpl L1
sta ENABLE_ROM
cli
-; Switch to second charset
+; Switch to the second charset.
- lda #14
- jsr $FFD2 ; BSOUT
+ lda #14
+ jsr $FFD2 ; BSOUT
-; Save system stuff and setup the stack. The stack starts at the top of the
-; usable RAM.
+; Save some system stuff; and, set up the stack. The stack starts at the top
+; of the usable RAM.
- tsx
- stx spsave ; save system stk ptr
+ tsx
+ stx spsave ; Save system stk ptr
- lda #<$FD00
+ lda #<(__MAIN_START__ + __MAIN_SIZE__ + __STACKSIZE__)
+ ldx #>(__MAIN_START__ + __MAIN_SIZE__ + __STACKSIZE__)
sta sp
- lda #>$FD00
- sta sp+1
+ stx sp+1
-; Setup the IRQ vector in the banked RAM and switch off the ROM
+; Set up the IRQ vector in the banked RAM; and, switch off the ROM.
ldx #<IRQ
ldy #>IRQ
sty $FFFF
cli ; Allow interrupts
-; Clear the BSS data
+; Clear the BSS data.
- jsr zerobss
+ jsr zerobss
-; Initialize irqcount, which means that from now own custom linked in IRQ
-; handlers (via condes) will be called.
+; Initialize irqcount, which means that, from now on, custom linked-in IRQ
+; handlers will be called (via condes).
lda #.lobyte(__INTERRUPTOR_COUNT__*2)
sta irqcount
-; Call module constructors
+; Call the module constructors.
- jsr initlib
+ jsr initlib
-; Push arguments and call main()
+; Push the command-line arguments; and, call main().
- jsr callmain
+ jsr callmain
-; Back from main (this is also the _exit entry). Run module destructors.
+; Back from main() [this is also the exit() entry]. Run the module destructors.
-_exit: pha ; Save the return code
- jsr donelib ; Run module destructors
+_exit: pha ; Save the return code
+ jsr donelib ; Run module destructors
-; Disable chained IRQ handlers
+; Disable the chained IRQ handlers.
- lda #0
+ lda #0
sta irqcount ; Disable custom IRQ handlers
-; Copy back the zero page stuff
+; Copy back the zero-page stuff.
- ldx #zpspace-1
-L2: lda zpsave,x
- sta sp,x
- dex
- bpl L2
+ ldx #zpspace-1
+L2: lda zpsave,x
+ sta sp,x
+ dex
+ bpl L2
-; Place the program return code into ST
+; Place the program return code into BASIC's status variable.
- pla
- sta ST
+ pla
+ sta ST
-; Restore the stack pointer
+; Restore the stack pointer.
- ldx spsave
- txs
+ ldx spsave
+ txs
-; Enable the ROM, reset changed vectors and return to BASIC
+; Enable the ROM; and, return to BASIC.
sta ENABLE_ROM
- jmp $FF8A ; RESTOR
-
+ 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
+; 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.
+; 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
+IRQ: cld ; Just to be sure
+ pha
txa
pha
- tya
- 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
+; 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
+ 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.
+; on the stack, so that 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
+ 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
+ jmp (IRQVec) ; Jump indirect to Kernal IRQ handler
irq_ret:
sta ENABLE_RAM ; Switch back to RAM
- pla
- tay
+ pla
+ tay
pla
tax
pla
beq nohandler
jmp brk_jmp ; Jump to the handler
-; No break handler installed, jump to ROM
+; No break handler installed, jump to ROM.
nohandler:
sta ENABLE_ROM
; BRK handling
brk_jmp: jmp $0000
-spsave: .res 1
+spsave: .res 1
irqcount: .byte 0
-.segment "ZPSAVE"
+.segment "INIT"
-zpsave: .res zpspace
+zpsave: .res zpspace