Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!nosc!crash!jcs
From: jcs@crash.cts.com (John Schultz)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Bypassing the Audio Filter
Message-ID: <3667@crash.cts.com>
Date: 25 Jul 90 03:35:13 GMT
References: <90195.001845L98CC@CUNYVM.BITNET> <3519@crash.cts.com> <826@bilver.UUCP>
Organization: Crash TimeSharing, El Cajon, CA
Lines: 27

In article <826@bilver.UUCP> alex@bilver.UUCP (Alex Matulich) writes:
>In article <3519@crash.cts.com> jcs@crash.cts.com (John Schultz) writes:
>>In article <90195.001845L98CC@CUNYVM.BITNET> L98CC@CUNYVM writes:
>>>   I want to write a program to turn off the hi-frequency filter on A500.
>>>I do know that setting a certain bit in certain address will turn it off.
>>>What is that address and bit?  But even better, is there a system call I can
>>
>>  In C:
>>  unsigned char * filter = (unsigned char *)0xBFE001;
>>     *filter = (*filter) & 0xfe; /* Turn off */
>>     *filter = (*filter) | 1;    /* Turn on */
>
>No, NO!  You only need to switch the second bit!
>
>For example, to toggle the bit on and off, you would say,
>
>*filter ^= 2;

  Whoops. Looks like I goofed the translation from assembly to C. Good eye,
Alex. The above example only changes one bit, albeit the wrong bit. Should
read:

  *filter = (*filter) & 0xfd; /* Turn off */
  *filter = (*filter) | 2;    /* Turn on */

  Toggling will work as well, but the prior state cannot be guaranteed, so
explicit on/off is best.