🚀 go-pugleaf

In article <7182@sugar.hackercorp.com>, peter@sugar.hackercorp.com
(Peter da Silva) writes:
|> In article <23699@grebyn.com> ckp@grebyn.UUCP (Checkpoint
Technologies) writes:
|> > The 8 meg expansion address space of the Amiga is too precious  to
|> > devote a meg or more of it to "just" a frame buffer.
|> 
|> I don't know about that. Every A2000 already has a megabyte allocated to
|> pretty much that very purpose. If you're going to shove more than a few megs
|> of RAM into your 2000 you're likely to be running a 68020 or so and have
|> almost 2 gig of RAM-only address space available on your CPU card.

Maybe a 68020, '030, or '040 coprocessor (as exist on the accelerator boards
that are available) could be an integral part of the board, so that the
CPU could
access all 2 or 3 ports (or more) ports of the RAM, and 1024x1024x24 or larger
bit planes could be supported.  One would have to make some speed calculations
to make sure the frame buffer board would not be slowed down so much by the
Amiga's system bus speed that the board would not be a winning peripheral.

|> (yes, I know that most 68020 cards don't take advantage of >16M memory, but
|>  it's in the CPU's address space and should be used. The only problem
with it
|>  is that you can't access it from the Zorro bus so you can't DMA to it, but
|>  that's a SMOP.)

For the extra address space to be accessible to the system CPU, the system bus
needs the extra address bits.  How many address bits are there on the A2000's
system bus (Zorro II, right?)?  Isn't it just 24?  That would provide a
16 MByte
address space.  Often, the addresses in a computer are not fully decoded, thus
wasting some address space.  I don't think more than 9 MBytes of configurable
address space is gauranteed by the specifications of the A2000 to exist in an
A2000.  By the way, some calculation:  3 bytes (24 bits) x 1024 x 1024 x
3 ports =
9 MBytes.

|> I'd think that paged I/O cards would be a bit of a pain for any
hypothectical
|> future device-independent graphics.library, too. Another reason to go to a
|> flat address space, no?
|> -- 
|> Peter da Silva.   `-_-'
|> <peter@sugar.hackercorp.com>.

Tom