10 Aug
2011
10 Aug
'11
2:29 p.m.
On 10 Aug 2011 at 18:55, Tony Duell wrote:
That;s the polite way of putting it!. IBM was even
worse than some
othr 8088 machines in that they only had page registers (to provide
the top 4 bits of the address), one of which served for 2 channels.
Of course they should have used an 8089 line Apricot did, but that's
another story...
Remember that the 8237 DMAC furnished dual functionality--regular DMA
services and also memory refresh. With an 8089 having only two DMA
channels, you're forced to go to something like an 8203/8207 DRAM
controller. The use of both the 8089 (and its support circuitry) and
the memory controller would have offended the bean counters mightily,
I suspect. (Witness the cost-cutting done on the Peanut).
As far as PC-type 8-bit expansion cards using DMA, there were
several. I don't know how long they were on the drawing board, so
it's pretty difficult saying whether they were or weren't in the
original game plan.
* MFM hard disk controller (uses DMA 3)
* GPIB adapter
* SDLC adapter
--Chuck