Re: DMA at 2.8 MHz?

[Jim Murphy <murph@apple.com>]

[ NOTE: I'm reposting this because of a small fight I had with my
        newsreader. This should be more intelligible. And it won't
	happen again. I promise. :-) ]


In article <m0nsW4V-0000J7C@crash.cts.com> Neal Pitts,
     ncp@gnh-starport.cts.com writes:

[ Quote from Drew Vogan of CV Tech ]

>  The motherboard won't allow a DMA speed above 1 meg/second.  I wish it
> could because the RF could actually do DMA at a tad over 3 meg/sec. The
> IO connectors are designed to use 2e peripherals that run at 1Mhz and
> apple's design guys are boneheads and didn't allow for high speed
> peripherals that could run at 2.8Mhz.

     Seeing that I'm not a hardware guru, I can't talk from my own
technical knowledge, but quoting from the Apple IIgs Hardware Reference,
Second Edition, p. 178 (in the 1st ed. this is on page 168):

-----------
DMA Cards

[ Uninteresting stuff removed - jim ]

     NOTE: Normally the system should be running at 1.024 MHz when
     performing DMA; otherwise, DMA to I/O or Mega II video areas will
     not work properly. However, DMA can be performed while the system
     is running fast as long as the following warnings are heeded:

     - Only high-speed RAM or ROM can be accessed (access to I/O, video,
       or the Mega II banks does not work properly).

     - Fast DMA may cause a repeated cycle to occur to the location
       currently being accessed by the processor. This repetition could
       cause a malfunction if the processor is accessing I/O when the DMA
       occurs; however, a repeated access to a RAM or ROM location will
       have no effect. The 65C816 can be stopped indefinitely for DMA and
       does not require any processor refresh cycles from a DMA card.

-----------

     So it appears that our hardware engineers weren't "boneheads" as Drew
so eloquently put it, or I could be completely confused. You make the
call.


Jim Murphy        Apple II System Software Engineer        murph@apple.com

"...never quite said what I wanted to say to you,                 The Cure
  never quite managed the words to explain to you..."             Untitled