Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!ucbvax!ADMIN.HumberC.ON.CA!GRAY
From: GRAY@ADMIN.HumberC.ON.CA (Kelly Gray)
Newsgroups: comp.sys.apple2
Subject: Re: GS/OS / ProDOS 8 SSC
Message-ID: <90Jun6.211042edt.57799@ugw.utcs.utoronto.ca>
Date: 7 Jun 90 01:07:26 GMT
References: <info-apple-request@APPLE.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 28


> Is there a vector that won't be reset, which I can use for such
> interrupts?  Is there a way to turn off SSC interrupts without dropping
> the online user?  An answer to either one will help greatly.  I haven't
> been able to find an answer in any documentation I have, so maybe an
> Apple person can help me find an answer if nobody else can.

According to the databook I have on the 6551 ACIA chip (The one used in the SSC)
the interrupts can be turned off by writing 1s into bits 1 & 3 of the Command
Register. Be careful not to change bit 0 of the command register though, since
that will change the DTR setting, and could force the modem to hang up the
phone.
 The ACIA Command register on the SSC is located at $C08A+$n0
where n is the slot number the SSC is in.
 In Assembler, the following code should do the job, assuming that the X
register has been loaded with $n0.

    LDA $C08A,X      ;read current command value
    ORA #$0A         ;set bits 1 & 3
    AND #$F0         ;optional, clear bit 2
    STA $C08A,X      ;set new value back in the command register

You may also want to make sure that bit 2 is a zero when you disable the
interrupts, since setting bits 2 and 3 high at the same time will transmit a
BREAK, possibly causing problems.

      <o_o>
           Kelly Gray (GRAY@ADMIN.HUMBERC.ON.CA)