RE: Article PC-card

From: Ethan Dicks (
Date: 2001-03-29 16:39:50

--- wrote:
> Hallo Nick,
> For accessing the memory or I/O different lines are used: MEMW/MEMR and 
> IOWR/IORD. You only activate that line which is needed for the specified 
> operation and area. A C= does not have these lines. So you have to generate 
> all four of them. One thing you should know too that these lines are used 
> for timing as the 65xx ICs use CLK2. A C= does not have a sepperate I/O and 
> memory area. So I used an I/O bit to make a difference between a memory- or 
> an I/O-operation.
> So I combined the R/W, CLK and an I/O bit using a 74LS138 
> (3-to-8-multiplexer) to generate the four needed signals.

Interesting solution.  I make and sell the GG2 Bus+, an ISA adapter for
the Amiga.  Because of the larger address space (6502 vs MC68000), it has
no problem using memory address lines from the 68K to map the entire ISA
memory space and the entire ISA I/O space into a 1Mb block of AUTOCONFIG space.
An older version mapped just the I/O space into a 64Kb AUTOCONFIG block.

It's interesting watching the discussion here.  I see the same sorts of
issues brought up now as in the past.  The GG2 Bus+ does not support DMA;
it's I/O and ISA memory only.  Nobody ever completed a VGA driver, but there
are drivers for IDE, network cards of various flavors (NE2000, 3c503, 3c509
and 80x3, primarily), serial ports, printer ports and a few odd devices
under AmigaDOS and UNIX.

To add fuel to the fire, there are some network cards that use memory space,
some that don't.  If I were designing a new ISA bridge, I'd make sure to
support memory space (although for the Amiga, it would be nice to disable
that and use a 64Kb AUTOCONFIG block rather than a 1Mb block for those A2000
users who want a full 8Mb of RAM in their systems).

If this discussion ever results in any hardware being designed, I'd love to
build one.  


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