Rebirth of IMSAI

From: Richard Erlacher <>
Date: Wed Mar 31 20:03:03 1999

Have a look at the imbedded comments below, please.


-----Original Message-----
From: Allison J Parent <>
To: Discussion re-collecting of classic computers
Date: Wednesday, March 31, 1999 4:50 PM
Subject: Re: Rebirth of IMSAI

><This would require a variable length or variable preset counter sourcing
><clock. The problem was knowing when it was going the generate an M1 cycle
>I used a shift register and an or gate.
and I used a counter and a gate. Today I'd use a part of a PAL, as you
probably would as well.
><counts. Either that or you'd have to look for the clock edge after the
><appearance of the M1 strobe and KNOW it wasn't part of the interrupt
><acknowledge. Neither was thrifty with logic, nor was it fun.
>M1 made it easy. the logic was if M1 then hod clock one cycle.

Yup, that's more or less what's needed.

><I had one friend whose NorthStar convinced me every time I saw it, that I
><didn't want one. We were using CP/M, and you really didn't have even one
><byte to spare in your measly 64K. His NorthStar only had 48K of memory
><space, for some reason. Maybe it was because they'd mapped that region fo
>Funny mine has 56k to the base of BDOS. The trick is CCP and BDOS resides
>below the controller at E800h and the BIOS in the 4K at F000h. I also
>have a PROM burnt for F800 (trivial). The first banking scheme I did had
>mappable 4k pages in the F000h space. Very nice really. Later I went
>softsector using a uPD765 (I worked for NEC sooo...) and pulled the hard
>sector controller.

That was along time ago. Perhaps it was 56K rather than 48. All my CPM
systems, and I had several at the time, used 64K, though that's a small
difference nowadays.

Received on Wed Mar 31 1999 - 20:03:03 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:32:23 BST