
PROGRAM LISTING: 82-06/VBICALL.SRC
;
; Listing #2
; by Stan Ockers
; for ANTIC The ATARI Resource
; a Vertical Blank Interrupt routine
; call from BASIC A=USR(1536)
;
ORG $600 ; page six location
PLA ; ignore parameter count
LDY #$0A ; lo-byte VBI routine
LDX #$06 ; hi-byte VBI routine
LDA #07 ; deferred VBI
JMP $E45C ; O.S. inserts VBI
VBI LDA #00 ; zero ?
CMP $D0 ; move flag plyr 0
BEQ SKP ; forget plyr 0
INC $CE ; one space right
LDX $CE ; update
STX $D000 ; horiz pos. plyr 0
SKP CMP $D1 ; mov flag 1 0 ?
BEQ OUT ; yes forget p1yr 1
DEC $CF ; one space left
LDX $CF ; update
STX $D001 ; horiz pos. plyr I
OUT JMP $E462 ; VBI exit thru O.S.
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