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From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Interrupt
Message-ID: <1990Dec1.093832.11196@nntp-server.caltech.edu>
Date: 1 Dec 90 09:38:32 GMT
References: <2253.apple2.info.apple@pro-micol>
Organization: California Institute of Technology, Pasadena
Lines: 16

8 bit II's don't have a VBL interrupt vector. The official way to do it is to
add an interrupt handler to Prodos 8 that services VBL's.

For music, VBL isn't going to do you any good. Since the speaker output is a
flip flop (or equivalent), the sound is not going to change unless the CPU is
doing it -- i.e. music takes an ungodly percentage of the CPU and is too 
inconvenient to do with interrupts.

This is why we bought MockingBoards. The Mockingboard has two programmable
sound generators and plenty of interrupt and timing facilities. The GS uses
the sound chip for all of that, which means that one of the voices is used to
provide a tempo interrupt, and the others sit there pumping out sound while
the computer goes about its merry business.

Todd Whitesel
toddpw @ tybalt.caltech.edu