Re: Second Sight SVGA Board Tech Info

[bazyar@netcom.com (Jawaid Bazyar)]

jpenne@ee.ualberta.ca (Jerry Penner) writes:

>I would like to know if there is going to be some preliminary
>information available about such technical issues as memory management
>on the SS VGA board (from what I understand there is no VRAM, just
>DRAM), limitations on how fast bits can be pushed around on the screen
>(general case, specific cases), if this Z180 CPU was chosen for any
>special reason, or just because the designer was familiar with it and
>it costs little.

  1) We could get 20MHz versions of the part
  2) the designer was familiar with it, as were our firmware engineers
  3) It costs very little

>More specifically, has a organizational scheme been implemented that
>allows developers from all over the place to cooperate with regards to
>downloaded software routines on the board?  

>Is there firmware on the Z180
>side that takes care of ugly stuff like setting video modes while
>checking for illegal modes based on the current monitor?

  Yes.

>Is this
>firmware written in such a way that new routines can be added without
>breaking existing (downloadable) code?  

  I believe I have a way to do it, and if I get some time I'll put
that in the prelim developer docs and get that info out here soon.

>Is Z180 code in general
>relocatable, so that if my program has downloaded some stuff and your
>program also downloads some, they won't conflict?

  No, not generally relocatable, although you can certainly write 
relocatable code, just as you can with most processors.

>I don't know anything about the Z180 (but it sounds an awful lot like
>a Z80 that's somehow had a bus/instruction set upgrade), so I would
>like to know why it was chosen.  TMS34010's are quite cheap now and do
>memory cycles in 4 CPU cycles, which is what I have been reading is
>the same operation as the Z180.  The 34010, however, can do logical
>pixel operations like Add with Saturate, etc, that would take a bunch
>of instructions on a general purpose CPU to emulate.  

  The 34010 is fine and dandy, but suffers from a number of problems,
to wit: you need external hardware to handle horizontal resolutions
greater than 512 pixels; you have to use expensive VRAM instead
of cheap DRAM; it may be "cheap", but it's not _cheap_. The choice
is academic at this point, since the card has been designed. Hindsight
will no doubt tell us where we goofed :-)

>Maybe 5 MB/s is
>enough video bandwidth, but not if it takes the CPU 10 cycles to
>figure out what to stuff onto the screen.  Does the Z180 have an
>instruction cache?

  No, it doesn't. I'd have pushed for the Z280 (which does have a cache)
and might have gotten it if it hadn't been an expensive part.
The Z180 is also an 8-bit part, which is important in cost-effective
interfacing to the Apple II bus.

>Lots of questions.  A nice reasonably complete technical summary would
>be nice, since if I buy this board, I do believe I'm going to have to
>program it myself for the most part.  I would really like to have a
>good idea of what I'm buying before I lay down the cash.

  I was hoping the Q&A file would have enough information. What exactly
are you after? Detailed specs on the Z180, video subsystem, VGA chip?
If so, I can work on those today.

-- 
 Jawaid Bazyar              |   Like UNIX? Like your Apple IIGS? Then ask
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