Re: Microsoft Softcard and IIgs

[vladitx <vladitx@nucleusys.com>]

On Mar 12, 1:36 pm, Steven Hirsch <snhir...@gmail.com> wrote:
> vladitx wrote:
> > Definitely the PCPI design takes the least-resistance path and
> > decouples the Apple II and the Z80 system as much as possible, at the
> > cost of  losing the direct access to Apple II memory.
>
> And why is that a problem?

It's a "cost", not a problem per se. :-)

Z80 loses all (direct) Apple hardware, including video. On the other
hand if done with a proper driver API (and reasonable transfer speed)
it gains sort of portability.

> Actually, with a good software implementation the screen scrolling is far
> quicker with a decoupled coprocessor.  The host processor has control all the
> time and simply needs to read bytes from a port and write them to the screen.
>   With the softcard, regardless of who is writing to the screen (and I don't
> recall offhand whether this is 6502 or Z80 code) they share bus cycles with
> the other guy.

Agreed. AFAIR with SoftCard the 6502 is mostly idle, doing only disk I/
O (but I could be totally wrong here).

> PCPI actually missed a few opportunities for improving performance in this
> area, but out of the box it's quicker than a softcard.  When I did the
> CardZ180 BIOS, I was able to able to trim a lot of overhead from screen output
> and it could easily keep up with a 19k2 baud modem.  Stock Applicard would
> drop characters in this mode and the Softcard was hopeless.

Agreed again. I can understand why Applicard has it's own UART.

> Add an accelerator to the host and sky's the limit.  You cannot use an
> accelerator with a DMA device.  Applicard works with any accelerator on any
> version of the A2.

The IIgs case is an example of accelerator and DMA, but I guess it's
DMA is different than the original II.

> There were very few A2 cards that used DMA.  The only others I can think of
> are the Transwarp I (DMA with a vengeance - once it grabs on, it doesn't ever
> let go) and the Vista 8" floppy controller.

Useful info, thanks.