Re: DMA at 2.8 MHz?

[Jim Murphy <murph@apple.com>]

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 wil 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, it may indeed be the other way around.


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