21 May
2013
21 May
'13
2:39 p.m.
On 05/21/2013 01:05 PM, Tony Duell wrote:
The problem is that the higehr data rate on the floppy
drive leads to a
higher byte transfoer rate on the processor side of the disk controller.
Msot 8 bit machiens (certain of the Spectrom type) didn't use any form of
DMA fro the floppy controller, it relied o nthe processor to transfew the
bytes as neeeded. Doing this at the DD data rate normally involved some
tight code. I think doing it at the HD rate would be very hard/impossible/
A few years back, Herb Johnson opened a discussion of this on his web
site. The conclusion was that a 4 MHz Z80 was indeed fast enough to do
this with programmed I/O and DD 8" disks (500KHz data rate) and that it
was *barely* possible to do at 2 MHz Z80 set up correctly and not at all
with a 2 MHz 8080.
I suggested a few schemes where the 8080 might be able to keep up, but
they were big (straight-line code) and used some odd techniques (e.g.
PUSH instead of STAX to do a 16-bit store and decrement). I never was
curious enough to try them--just hand-timed them.
--Chuck