Cool AppleSoft BASIC trick I never thought of before

From: Vintage Computer GAWD! <foo_at_siconic.com>
Date: Thu May 18 16:44:30 2000

On Thu, 18 May 2000, Eric J. Korpela wrote:

> In the case of disk and tape I/O, the Apple ][ is heavily dependent on
> instruction timing. What it comes down to, is that no interrupts can be
> allowed while a disk block read/write or any portion of a tape read/write
> is happening. It also means that a multitasking OS can't block on I/O the
> way a system that supported interupt driven port I/O or DMA could.
>
> I assume that this was fixed somewhere in the Apple ][ line. Perhaps in
> Unidisk type controllers?

There was never really anything to "fix". You just don't interrupt the
Apple when it's reading or writing to disk. The regular IRQ signal is
turned off before a read or write with the SEI and CLI instructions. But
there's no way to disable the Non Maskable Interrupt, which some hardware
copy protection cracking aids gave you access to. Unless these cards did
something to specifically avoid interrupting during a disk write (I always
wanted one but never got around to getting one during their heyday) the
results would be unpredictable.

Sellam International Man of Intrigue and Danger
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Received on Thu May 18 2000 - 16:44:30 BST

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