ZIP Chip Heartbreak

[jeff.brielmaier@yob.com (Jeff Brielmaier)]

>: DMA compatibility has nothing to do with speed.  The problem is that
>: DMA works by controlling several signals available on the slots which
>: cannot be detected by the microprocessor (they are handled by external
>: circuitry).  In a DMA cycle, a byte is transferred directly from
>: memory to the slot peripheral (or vice versa) while the processor is
>: disconnected from the bus.

>: In theory, the same could be said for IIe card-based accelerators such
>: as the TransWarp IIe, but none of them implement DMA support.

The reason that the TransWarp //e (and Accelerator //e; and ][+ versions of 
the accelerator card) doesn't work w/ DMA devices is that the accelerator card 
uses the DMA signals itself to take control of the ][-][+-//e at power up.

Since the accelerator grabs the DMA control lines, if another device tries to 
grab the DMA lines the computer "crashes" (typically into the monitor).

GT>I realize this is dreaming here but even though the CPU pins don't
GT>receive the data accessed by a DMA device (I'm not so sure the thses
GT>signals don't make it to the CPU pins, but never the less are not
GT>supported by all //e accelerators) couldn't the CPU when it detects a DMA
GT>signal (that I know makes it to the CPU!) just flush its cache?  I know
GT>you'ld have some speed loss.  Just a thought.

Nope.  The CPU (or running software) can not detect if an add-in card has 
enabled DMA mode in the Apple // since the on-board CPU chip becomes
disconnected from the address and data buses.

* KingQWK 1.05 # [PK] * I'm not as think as you stoned I am.

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