* 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.
SYS Modifications
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:+11 | to | ld a,vnorm*16+02h | ;Read from page 1 |
| change vinit:+18 | to | ld a,0e0+vnorm | ;Write pages 1, 2, 3 |
| change be2: +7 | to | ld a,20h+vnorm | ;Write to page 1 |
SYSB8 Modifications – if vgem
| change dpbv:+5 | to | defb 0e0h,0 | ;ALO 3 directory blks. |
| ;change is optional |
| change vb: +4 | to | ld a,vnorm*16+02h | ;Read from page 1 |
| add at vr: +8 | | ld a,vnorm*16+02h | ;Read page 1 |
| | jr c,vr2 | ;Next page ? |
| add at vw: +8 | | ld a,20h+vnorm | ;Write page 1 |
| jr c,vw2 | ;Next page ? |
HEAT POWER and LIGAT
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.