79 lines
1.6 KiB
Plaintext
79 lines
1.6 KiB
Plaintext
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;
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;; Commodore 64 Basic Runtime File
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;;
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;; Include this at the TOP of your C64 program, and it will handle
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;; hiding away the BASIC ROM and data and restoring it at the end.
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;;
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;; You will have a contiguous block of RAM from $0800 to $CFFF, and
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;; Zero Page access from $02 to $8F in the segment "zp".
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.include "c64header.oph"
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.data zp ; Zero Page memory segment.
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.org $0002
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.text
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.scope
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; Cache BASIC zero page underneath the KERNAL, while also
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; making RAM copies of the NMI routines
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ldx #$00
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* lda $00, x
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sta $e000, x
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lda $fe00, x
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sta $fe00, x
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lda $ff00, x
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sta $ff00, x
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inx
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bne -
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; Swap out the BASIC ROM for RAM
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lda $01
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and #$fe
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ora #$06
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sta $01
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; Run the real program
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jsr _main
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; Swap out KERNAL to expose cached BASIC ZP values
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; Block IRQs during this period. NMIs cannot be blocked,
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; but we copied enough of the processing code into the
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; RAM under the KERNAL that we can disable NMI processing
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; during this period
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sei ; Disable IRQs
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lda #$c1 ; Defang NMIs
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sta $318
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lda $01 ; Swap out KERNAL
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and #$fd
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sta $01
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; Restore BASIC zero page
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ldx #$8E
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* lda $e001, x
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sta $01, x
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dex
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bne -
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; Restore BASIC ROM, KERNAL, and interrupts
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lda $01
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ora #$07
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sta $01
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lda #$47 ; Restore NMI vector
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sta $318
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cli ; Re-enable interrupts
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; Back to BASIC. We do this by clearing the keyboard
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; buffer and then jumping through the warm start
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; vector. This will more cleanly handle case where
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; the program has somehow modified BASIC's state,
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; such as running through PUCRUNCH or a onefiler.
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stx $c6 ; .X is zero from previous loop
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jmp ($a002)
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_main:
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; Program follows...
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.scend
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