Imagedisk 1.17

[Chuck Guzis]

On 1 Aug 2007 at 18:56, Jim Leonard wrote:

> Bit 4 of Port B at I/O address 61 hex on an AT and later machine is a
> read-only bit carrying a signal called Refresh Detect. This signal
> comes from a 'T' (toggle) flip-flop which is clocked by the refresh
> trigger signal, which comes from CTC channel one.  Assuming that the
> RAM refresh rate has not been changed, this bit will toggle (change
> from 0 to 1 or from 1 to 0) once every 15.0857 microseconds (the exact
> value is 216/14.31818), and Port B can be polled in a loop to
> implement a delay of any length.  For short delays, with interrupts
> locked out, this gives an accurate and very convenient relative delay
> mechanism. However, for long delays, it would be naughty to leave
> interrupts locked out for the entire delay period, and interrupts will
> cause gaps in the polling process, slightly lengthening the delay (it
> will wait longer than expected).

I believe I mentioned that it wasn't present on the PC-XT and that it 
was AT-and later.   In any case, it's easy--a simple 65K loop 
checking this bit for toggling will root out the aberrent machines.  
Fortunately for everyone, these are mostly the slower (x88/86/186) 
systems that are very amenable to a simple counted CPU loop.

When I do my delay, I wait for the bit to flip once, then again, then 
again.  Simple--even my old NT and 98 diskette device drivers worked 
with it.

Cheers,
Chuck