Re: Calling all DOS-heads

From: Nate Lawson <>
Date: Fri, 30 Mar 2012 15:02:24 -0700
Message-Id: <>
On Mar 30, 2012, at 12:37 PM, Ingo Korb wrote:

> Spiro Trikaliotis <> writes:
>> The "broken" here is relative - at least, we thought so.
> Ah, so you've been playing with hidden cards all the time and expected
> people to guess the answers you were looking for without all the
> information they need to find those answers? Nice.

Sorry, Jim should have given more background. Nothing was intentionally hidden and this device was discussed publicly on other forums.

As Spiro said, it's a custom version of the ZoomFloppy that will be installed in the drive, between the socket and the VIA chip. It assumes exclusive control by isolating the VIA. Only one drive is supported in this mode, no IEC chain.

>> Obviously, we neglected an effect when ATN is deasserted (put high
>> again). Thus, we try to find out the reason of this effect, and if
>> anything can be done about it.
> With all the additional information that has been revealed by now I'm
> not sure why the drive doesn't react to ATN at a later point. If the
> interrupt is triggered[1] $7c should be set to 1 and the only place
> where it is reset according to the cross-reference in AAY1541 is in the
> ATN handler which will pull DATA_bus low unconditionally.
> If you want to figure out what the CPU actually does I recommend adding
> a few of the lowest address lines (A0-A2 should be enough, A0-A3 are a
> bit more convenient) to the logic analyzer trace and possibly SYNC to
> see which cycles are instruction fetches. Tracing IRQ may show if the
> interrupt is accidentally cleared before it is handeled - but the only
> access to $1801 in a stock rom is in the subroutine that sets $7c and I
> really hope you're not experimenting with a drive with any kind of
> parallel speeder in it.
> -ik
> [1] I don't see why it wouldn't be - IRQ is level-sensitive, so even if
> the drive is running with interrupts disabled it will show up when
> interrupts are reenabled.

I agree with this analysis. We need to know what's going on in the drive CPU. It is a stock 1541 ROM.


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Received on 2012-03-30 23:00:14

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