Re: VBL on an Apple //c

[phoenyx <quazar@dcnet2000.com>]

Pim Blokland wrote:

> In one of my BASIC programs, I use the VBL signal (PEEK(49177) for timing
> purposes. This works on //es and IIgses, but lately I found out that it
> does not work on a //c.
> Apparently, on a //c you cannot see the VBL signal directly, but you have
> to let the computer do it for you.
>
> So my question is: could someone show me a small machine language routine
> that I cann call from BASIC and which waits for the start of the vertical
> blanking? (Or the end for that matter) And I would really appreciate it if
> you can assure me that it does actually do the job, for I don't have a //c
> and can't test it here myself.

Well, I don't have the specifics but, you have to work with the //c's
mouse port. There are several program's around which show how
to do this. At least one of which is in the Nibble magazines. You
could also check a //c reference manual. I think it is also mentioned
in one of the technical notes from Apple. You might also look around
for code which places a soft clock in the //c. These also use the same
method since this is the only standard way to get a timed interrupt
signal in the //c.

Phoenyx