25 Nov
2006
25 Nov
'06
10:29 a.m.
On 25 Nov 2006 at 3:27, jim stephens wrote:
Well - then I guess NorthStar was obscene... :-)
The single-density controller does indeed have 256 byte of ROM (in three chips!)
The double-density one has 512 bytes of ROM.
The controller occupies 1K of memory space from E800-EBFF
E800-EFFF ROM on DD, unused or mirror ROM on SD
E900-E9FF ROM on both SD and DD
EA00-EAFF Hardware control (address is write value)
EB00-EBFF Write data (address is data)
I might have EA00 & EB00 reversed, it's been a while since I was in the
nuts and bolts of the N* controller.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
And my memory of 256 bytes may be off, I vaguly
recall maybe on 32 bytes
are visible and used by the processor at boot. People had to use pretty small proms
in the early days of the hobby.
I recall that the disk controller had a small bipolar prom and that
it was indeed 32 bytes. It was miles better than toggling the boot
code by hand... A 256-byte boot prom back then would have been an
almost obscene luxury... ;)