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Allow _sys() to call ROM routines. 

_sys() is supposed to be (primarily) intended to call ROM routines. Leveraging the "file overlay" mechanism of the cc65 build system allows to provide a Apple II specific _sys() implementation that temporarily switches in the ROM.
This commit is contained in:
Oliver Schmidt 2016-03-25 21:57:06 +01:00
parent c9734004ee
commit 29d1400340
1 changed files with 81 additions and 0 deletions
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libsrc/apple2/_sys.s Normal file
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;
; void __fastcall__ _sys (struct regs* r);
;
.export __sys
.import jmpvec
.include "zeropage.inc"
.segment "LOWCODE"
__sys: sta ptr1
stx ptr1+1 ; Save the pointer to r
; Fetch the PC and store it into the jump vector
ldy #5
lda (ptr1),y
sta jmpvec+2
dey
lda (ptr1),y
sta jmpvec+1
; Remember the flags so we can restore them to a known state after calling the
; routine
php
; Get the flags, keep the state of bit 4 and 5 using the other flags from
; the flags value passed by the caller. Push the new flags and push A.
dey
php
pla ; Current flags -> A
eor (ptr1),y
and #%00110000
eor (ptr1),y
pha ; Push new flags value
ldy #0
lda (ptr1),y
pha
; Get and assign X and Y
iny
lda (ptr1),y
tax
iny
lda (ptr1),y
tay
; Switch in ROM
bit $C082
; Set A and the flags, call the machine code routine
pla
plp
jsr jmpvec
; Back from the routine. Save the flags and A.
php
pha
; Switch in LC bank 2 for R/O
bit $C080
; Put the register values into the regs structure
tya
ldy #2
sta (ptr1),y
dey
txa
sta (ptr1),y
dey
pla
sta (ptr1),y
ldy #3
pla
sta (ptr1),y
; Restore the old flags value
plp
; Done
rts