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Improvements to c64_0.oph startup/teardown code 

16 additional bytes from the zero page are now available; the ZP
locations from $02-$8F are now free for your program's use.

Approximately 128 additional bytes in main RAM are now available,
giving you free reign from $0800-$CFFF. Zero Page backup is now
handled underneath the KERNAL's ROM, with the program epilogue
safely handling swapping out the KERNAL for the duration of the
switch. (IRQs are disabled, and NMI handling code is replicated
and modified to not hurt anything.)

Program exit is now handled by keyboard buffer and jumping
through BASIC's warm reset vector. This technique lets programs
play more nicely with PUCRUNCH and onefilers (which otherwise
often confused BASIC as the BASIC prologue would change as part
of decompression or link-loading).
This commit is contained in:
Michael Martin 2014-05-14 07:03:46 -07:00
parent 5b64c9701e
commit 60f03a34af
2 changed files with 45 additions and 17 deletions
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8
platform/README.txt
View File

@ -6,11 +6,13 @@ c64_0.oph: A Commodore 64 equivalent to a modern compiler's "crt0.s" - it
contains a .PRG file header, a short BASIC program that launches
the machine language program, and a prologue and epilogue that
prepare memory for your use and then clean it up again when you
are done. Memory locations $02 through $7F on the zero page are
are done. Memory locations $02 through $8F on the zero page are
available for your use, and the program lives at the beginning
a contiguous block of RAM from $0800 through $CF81. The BASIC
a contiguous block of RAM from $0800 through $CFFF. The BASIC
ROM is swapped out of memory (leaving $A000-$BFFF as RAM) for
the duration of your program.
the duration of your program. BASIC's working storage on the
zero page is backed up in the RAM underneath the KERNAL ROM
while your program runs.
c64kernal.oph: A collection of standard aliases for the KERNAL routines on the
Commodore 64. Names for these routines have been chosen to match

54
platform/c64_0.oph
View File

@ -5,8 +5,8 @@
;; Include this at the TOP of your C64 program, and it will handle
;; hiding away the BASIC ROM and data and restoring it at the end.
;;
;; You will have a contiguous block of RAM from $0800 to $CF81, and
;; Zero Page access from $02 to $7F in the segment "zp".
;; You will have a contiguous block of RAM from $0800 to $CFFF, and
;; Zero Page access from $02 to $8F in the segment "zp".
.word $0801
.org $0801
@ -24,11 +24,16 @@ _next: .word 0 ; End of program
.text
.scope
; Cache BASIC zero page at top of available RAM
ldx #$7E
* lda $01, x
sta $CF81, x
dex
; Cache BASIC zero page underneath the KERNAL, while also
; making RAM copies of the NMI routines
ldx #$00
* lda $00, x
sta $e000, x
lda $fe00, x
sta $fe00, x
lda $ff00, x
sta $ff00, x
inx
bne -
; Swap out the BASIC ROM for RAM
@ -40,20 +45,41 @@ _next: .word 0 ; End of program
; Run the real program
jsr _main
; Restore BASIC ROM
lda $01
ora #$07
; Swap out KERNAL to expose cached BASIC ZP values
; Block IRQs during this period. NMIs cannot be blocked,
; but we copied enough of the processing code into the
; RAM under the KERNAL that we can disable NMI processing
; during this period
sei ; Disable IRQs
lda #$c1 ; Defang NMIs
sta $318
lda $01 ; Swap out KERNAL
and #$fd
sta $01
; Restore BASIC zero page
ldx #$7E
* lda $CF81, x
ldx #$8E
* lda $e001, x
sta $01, x
dex
bne -
; Back to BASIC
rts
; Restore BASIC ROM, KERNAL, and interrupts
lda $01
ora #$07
sta $01
lda #$47 ; Restore NMI vector
sta $318
cli ; Re-enable interrupts
; Back to BASIC. We do this by clearing the keyboard
; buffer and then jumping through the warm start
; vector. This will more cleanly handle case where
; the program has somehow modified BASIC's state,
; such as running through PUCRUNCH or a onefiler.
stx $c6 ; .X is zero from previous loop
jmp ($a002)
_main:
; Program follows...