Northstar Horizon

From: Allison J Parent <>
Date: Tue Nov 2 18:29:00 1999

<I have never seen a Z-80A system that needed DMA for disk I/O. The require
<loop is simple enough to synchronize using the nWAIT line. The CCS and SD
<FDC's both did PIO, and since the OS didn't have anything better to do
<during disk I/O, the wasted CPU cycles, if there were any, were going to b
<wasted anyway.

Well I have several. What is the cpu doing? Running CPM and more. The
cycles it would ahve been doing PIO are now, supporting interrupts real
time, printer buffering and disk caching for the disk (hard and floppy).

Keep in mind CPM didn't rule out much so background tasks and even
multiprocessing were possible with CP/M-80 with a few minor limitations.
The big thing that annoyed me to the max with many of the boxen of the time
was that go to the disk ment stop typing and compared to my PDP8, PDP-11
experience this was stupid. PIO at 4mhz really limited the number of
interrupts yu could take while servicing a DD floppy (worst case a byte
every 27uS) or DD 8" (worst case a byte EVERY 13uS). Sure you could sync
a z80 to that but it was pretty much dead waiting for a sector to come
around. DMA was a solution, that and use of interrupts (mode 2) made for
a much smoother system that felt smoother and faster to the user. It was
particulary noticeable for apps that either ran overlays or like VEDIT
virtualized the file on the disk.

<few locations of memory as opposed to a large (2K) refresh buffer in the
<already small memory map. It's just that N* (and VECTOR) were not among

NS* didn't do a video board. Many people used a VDM-1 in it or the
VectorGrapahic card. In 1978 a ADM1A was ~$800, good TTY $400, VDM1
$199 (if I remember right). The SD systems board was better and looked
more like a parallel device.

<stated. I would not, however, pretend that the CCS or SDS stuff I liked t
<use was enough "better" that anyone would be making a mistake to use it.

I thought they were pretty neat too. thats why in '96 I got two CCS systems
complete and running!

Received on Tue Nov 02 1999 - 18:29:00 GMT

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