2 ; Startup code for cc65 (CBM 500 version)
6 .export __STARTUP__ : absolute = 1 ; Mark as startup
8 .import _clrscr, initlib, donelib, callirq_y
9 .import push0, callmain
10 .import __CHARRAM_START__, __CHARRAM_SIZE__, __VIDRAM_START__
11 .import __BSS_RUN__, __BSS_SIZE__, __EXTZP_RUN__
12 .import __INTERRUPTOR_COUNT__
15 .include "zeropage.inc"
20 ; ------------------------------------------------------------------------
21 ; BASIC header and a small BASIC program. Since it is not possible to start
22 ; programs in other banks using SYS, the BASIC program will write a small
23 ; machine code program into memory at $100 and start that machine code
24 ; program. The machine code program will then start the machine language
25 ; code in bank 0, which will initialize the system by copying stuff from
26 ; the system bank, and start the application.
28 ; Here's the basic program that's in the following lines:
35 ; 60 data 120,169,0,133,0
37 ; The machine program in the data lines is:
41 ; sta $00 <-- Switch to bank 0 after this command
43 ; Initialization is not only complex because of the jumping from one bank
44 ; into another. but also because we want to save memory, and because of
45 ; this, we will use the system memory ($00-$3FF) for initialization stuff
46 ; that is overwritten later.
51 .byte $03,$00,$11,$00,$0a,$00,$81,$20,$49,$b2,$30,$20,$a4,$20,$34,$00
52 .byte $19,$00,$14,$00,$87,$20,$4a,$00,$27,$00,$1e,$00,$97,$20,$32,$35
53 .byte $36,$aa,$49,$2c,$4a,$00,$2f,$00,$28,$00,$82,$20,$49,$00,$39,$00
54 .byte $32,$00,$9e,$20,$32,$35,$36,$00,$4f,$00,$3c,$00,$83,$20,$31,$32
55 .byte $30,$2c,$31,$36,$39,$2c,$30,$2c,$31,$33,$33,$2c,$30,$00,$00,$00
57 ;------------------------------------------------------------------------------
58 ; A table that contains values that must be transfered from the system zero
59 ; page into our zero page. Contains pairs of bytes; first one is the address
60 ; in the system ZP, second one is our ZP address. The table goes into page 2,
61 ; but is declared here because it is needed earlier.
77 ;------------------------------------------------------------------------------
78 ; Page 3 data. This page contains the break vector and the bankswitch
79 ; subroutine that is copied into high memory on startup. The space occupied by
80 ; this routine will later be used for a copy of the bank 15 stack. It must be
81 ; saved, since we're going to destroy it when calling bank 15.
85 BRKVec: .addr _exit ; BRK indirect vector
109 lda #.hibyte(excrts-1)
112 lda #.lobyte(excrts-1)
118 sta $1FF ; Save new sp
146 ldy $1FF ; Restore sp in bank 15
148 lda #.hibyte(expull-1)
151 lda #.lobyte(expull-1)
172 .if (expull <> $FF26)
173 .error "Symbol expull must be aligned with kernal in bank 15"
180 ;------------------------------------------------------------------------------
181 ; The code in the target bank when switching back will be put at the bottom
182 ; of the stack. We will jump here to switch segments. The range $F2..$FF is
183 ; not used by any kernal routine.
189 ; We are at $100 now. The following snippet is a copy of the code that is poked
190 ; in the system bank memory by the basic header program, it's only for
191 ; documentation and not actually used here:
197 ; This is the actual starting point of our code after switching banks for
198 ; startup. Beware: The following code will get overwritten as soon as we
199 ; use the stack (since it's in page 1)! We jump to another location, since
200 ; we need some space for subroutines that aren't used later.
204 ; Hardware vectors, copied to $FFFA
210 .word nmi ; NMI vector
211 .word 0 ; Reset - not used
212 .word irq ; IRQ vector
215 ; Initializers for the extended zeropage. See extzp.s
233 ; Switch the indirect segment to the system bank
238 ; Initialize the extended zeropage
240 ldx #.sizeof(extzp)-1
246 ; Save the old stack pointer from the system bank and setup our hw sp
251 sta (sysp1),y ; Save system stack point into $F:$1FF
252 ldx #$FE ; Leave $1FF untouched for cross bank calls
253 txs ; Set up our own stack
255 ; Copy stuff from the system zeropage to ours
257 lda #.sizeof(transfer_table)
260 ldy transfer_table-2,x
261 lda transfer_table-1,x
269 ; Set the interrupt, NMI and other vectors
271 ldx #.sizeof(vectors)-1
273 sta $10000 - .sizeof(vectors),x
279 lda #.lobyte(callbank15::entry)
281 lda #.hibyte(callbank15::entry)
284 ; Setup the subroutine and jump vector table that redirects kernal calls to
287 ldy #.sizeof(callbank15)
288 @L1: lda callbank15-1,y
289 sta callbank15::entry-1,y
293 ; Setup the jump vector table. Y is zero on entry.
295 ldx #45-1 ; Number of vectors
296 @L2: lda #$20 ; JSR opcode
299 lda #.lobyte(callbank15::entry)
302 lda #.hibyte(callbank15::entry)
308 ; Set the indirect segment to bank we're executing in
313 ; Zero the BSS segment. We will do that here instead calling the routine
314 ; in the common library, since we have the memory anyway, and this way,
331 inc ptr1+1 ; Next page
335 ; Clear the remaining page
337 Z2: ldx #<__BSS_SIZE__
345 ; ------------------------------------------------------------------------
346 ; We are at $200 now. We may now start calling subroutines safely, since
347 ; the code we execute is no longer in the stack page.
351 ; Copy the character rom from the system bank into the execution bank
357 lda #<__CHARRAM_START__
359 lda #>__CHARRAM_START__
361 lda #>__CHARRAM_SIZE__ ; 16 * 256 bytes to copy
365 sta IndReg ; Access the system bank
367 sta __VIDRAM_START__,y
372 ccopy2: lda __VIDRAM_START__,y
377 inc ptr2+1 ; Bump high pointer bytes
381 ; Clear the video memory. We will do this before switching the video to bank 0
382 ; to avoid garbage when doing so.
386 ; Reprogram the VIC so that the text screen and the character ROM is in the
387 ; execution bank. This is done in three steps:
389 lda #$0F ; We need access to the system bank
392 ; Place the VIC video RAM into bank 0
403 ; Set bit 14/15 of the VIC address range to the high bits of __VIDRAM_START__
404 ; PC6/PC7 (VICBANKSEL 0/1) = 11
410 ora #<((>__VIDRAM_START__) & $C0)
413 ; Set the VIC base address register to the addresses of the video and
420 ora #<(((__VIDRAM_START__ >> 6) & $F0) | ((__CHARRAM_START__ >> 10) & $0E) | $02)
422 ; ora #<(((>__VIDRAM_START__) << 2) & $F0)
425 ; Switch back to the execution bank
430 ; Activate chained interrupt handlers, then enable interrupts.
432 lda #.lobyte(__INTERRUPTOR_COUNT__*2)
436 ; Call module constructors.
440 ; Push arguments and call main()
444 ; Call module destructors. This is also the _exit entry and the default entry
445 ; point for the break vector.
447 _exit: pha ; Save the return code on stack
448 jsr donelib ; Run module destructors
450 sta irqcount ; Disable custom irq handlers
452 ; Address the system bank
457 ; Switch back the video to the system bank
471 ; Copy stuff back from our zeropage to the systems
474 lda #.sizeof(transfer_table)
477 ldy transfer_table-2,x
478 lda transfer_table-1,x
487 ; Place the program return code into ST
493 ; Setup the welcome code at the stack bottom in the system bank.
496 lda (sysp1),y ; Load system bank sp
499 lda #$58 ; CLI opcode
502 lda #$60 ; RTS opcode
509 ; -------------------------------------------------------------------------
510 ; The IRQ handler goes into PAGE2. For performance reasons, and to allow
511 ; easier chaining, we do handle the IRQs in the execution bank (instead of
512 ; passing them to the system bank).
514 ; This is the mapping of the active irq register of the 6525 (tpi1):
516 ; Bit 7 6 5 4 3 2 1 0
518 ; | | | ^ SRQ IEEE 488
531 sta IndReg ; Be sure to address our segment
533 lda $105,x ; Get the flags from the stack
534 and #$10 ; Test break flag
541 ; Call chained IRQ handlers
545 jsr callirq_y ; Call the functions
547 ; Done with chained IRQ handlers, check the TPI for IRQs and handle them
552 lda (tpi1),y ; Interrupt Register 6525
557 cmp #%00000001 ; ticker irq?
559 jsr scnkey ; Poll the keyboard
560 jsr UDTIM ; Bump the time
564 irqend: ldy #TPI::AIR
565 sta (tpi1),y ; Clear interrupt
578 ; -------------------------------------------------------------------------