80-Bus News


July–August 1984 · Volume 3 · Issue 4

Page 19 of 43

* For some reason, which may be historical, SYS supports only 2 Virtual memory or RAMDISK pages, and these are Pages 2 and 3 rather than 1 and 2. If GM802 boards are used their read and write address switches MUST be set accordingly. Because GM862 boards are always supplied fully populated, the standard switch settings as shown in the Manual for a Nascom with four memory pages (one system page and three virtual pages) may always be used. However unless SYS is modified, only two pages of RAMDISK will be available, reducing its effective size to 118K or less.


As such restrictions were intolerable, SYS (Version 18.1) was modified so that it could be assembled to run with 1, 2, or 3 Pages of RAMDISK and also allow these pages to be contiguous. That is, starting with the system memory on page 0 ,​the first disk page will be 1, the next 2, and the last 3. Besides enabling all three pages of the GM862 to be accessed, this hardware configuration is compatible with that employed with Gemini CONFIG.COM so that either operating system can be used without resetting switches or unplugging boards. The modifications, made to Modules SYSB3 and SYSB8 of SYS 18.1, are shown below. Although, sadly, no longer marketed, there must be quite a few SYSes of differing ages and pedigree out there in the vast void of space and time, and hopefully this information will also be of use with some at least of these:–

N.B. To aid identification of the line to be modified, the nearest previous label name is stated, followed by the (inclusive) number of lines to count from the label. Blank lines are NOT included in the count. Generally only one or two parameters need to be changed, making it easy to identify the correct line. Additions are inserted BEFORE the original Line indicated.

SYSB3 Modifications – if vgem

change vinit:+11told a,vnorm​*16​+02h;Read from page 1
change vinit:+18told a,0e0​+vnorm;Write pages 1, 2, 3
change be2: +7told a,20h​+vnorm;Write to page 1

SYSB8 Modifications – if vgem

change dpbv:+5todefb 0e0h,0;ALO 3 directory blks.
;change is optional
change vb: +4told a,vnorm​*16​+02h;Read from page 1
add at vr: +8ld a,vnorm​*16​+02h;Read page 1
jr c,vr2;Next page ?
add at vw: +8ld a,20h​+vnorm;Write page 1
jr c,vw2;Next page ?


That should have been the end of the story, but during the first few weeks of operation both my systems suffered odd failures at times. The original 3A power supplies were overloaded by the additional memory, even though the supplies had been ‘boosted’ to deliver more power and had yielded stirling service in the past. Two 6A switchmode units had to be bought (sob sob) and were carefully wired in, but there were still a few odd ‘happenings’ which were finally exorcized by adding more shielding around the disk drives in the Vero rack and fitting an 80mm cooling fan in the console machine. Since then all has been sweetness and light.

Page 19 of 43