11 Aug
2011
11 Aug
'11
12:08 p.m.
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).
I have noi idea of the cost of an 8089 at the time, but th esupport
circuity would have been a lot less than that needed by the 8237 (no pag
register and associated logic). Yes, you'd have needed some memory
refresh hardware, but countless other machines provided that without
needed a DMA channel to do it (and you didn't _have_ to used a
single-chip DRAM controller). Remmber that in the PC, all the DMAC did
for the RAM was the refresh operation, the timing of RAS/CAS, addres
multiplexing, etc was done in separate hardware. Adding a refresh counter
and th eassociated logic would npt have been that much exta.
-tony