Text 127, 105 rader
Skriven 2006-05-31 18:56:24 av Peter Knapper (3:772/1.10)
    Kommentar till text 124 av Bob Ackley (1:2905/3)
Ärende: eCS 3 of 4.
===================
As an aside, the mechanism used to implement microcode branches was
quite cute. The 4k of microcode was divided into 256-word pages, and
4-way interleaved. A given microcode word specified only the top 6 bits
of the next instruction's address within the current page, the bottom 2
bits were determined by two separate function codes, giving each
instruction up to 4 possible successors (the function codes included
force 0 and force 1 where no branch was needed). Since the four possible
destinations were already read by the end of the cycle there was no
branching delay.

There really were lots of 'blinkenlights' - a section for the microcode,
a section for the selector channel, as well as the conventional IP, data
and status displays. Quite a few of the lamps had blown, so they got
moved around depending on what I was working on - I never got round to
buying any new ones.

I now had a system that would start and stay running, even if it
wouldn't do anything. I hadn't worried about the disk side of things
yet. At this stage I had to start learning about what the 360 could do.
I hadn't had to worry at all about the intricacies of 360 machine code
or CCW programming, so now I had to find everything I could. All those
books in the library on 'Learning IBM 360 Assembler Language' suddenly
had a great new significance, though most of my initial work was derived
from the listings in the diagnostic manuals, plus the fact that the 1802
mnemonics seemed to mirror those of the 360.

I wrote a few simple programs, and dialled them in on the front panel.
There were four sixteen-position switches for the address, two for data,
and 'Store' and 'Read' buttons. Things seemed to work much as expected,
but there were a few anomalies which I eventually traced to a few faulty
SLT components.

In one case I managed to trace the fault to a particular SLT gate, and
verified the fault by swapping its board with an identical one elsewhere
in the CPU (there was a cross-reference in the manual of where each type
of board was used.) I eventually isolated it to a single input to a
three or four input gate, and worked out that a single diode
strategically placed on the back of the board would resurrect it. It worked!

In another case I managed to swap a faulty board into a position which
didn't use the failed portion. Other than these, and the CCROS problems,
the machine had survived about 10 years in storage remarkably well.

With the CPU sorted out, my attention turned to the disk storage. I had
got sick of manually entering programs, though at least they stayed
there forever (I had several programs scattered through the 64k space.)

As far as I can recall, the 2841 worked straight away. It didn't have
CCROS microcode, but rather TROS, transformer ROS, which was a sort of
second cousin which used small transformers to sense copper tracks on
plastic strips. Depending on where a small hole was punched, the track
would either go through, or bypass, a ferrite loop. I think that TROS
was faster than CCROS, but wasn't so readily modified.

Each drive was connected to the 2841 by two cables, one data cable
directly between the two, and one control cable daisy- chained from one
to the next, somewhat like the ST506 cabling arrangement. Each drive
could seek independently, but only one could be transferring data at a
time. The 2841 had no buffering capability - data had to go down the
channel and into the CPU as soon as it left the drive, at the rate of
145kB/second.

The heads on the 2311 drives are hydraulically actuated, and for their
size move at astonishing speeds. With the weight of the head assembly,
it's no wonder that whole drives can move during vigorous disk
accessing. A lot of the hydraulic fluid had leaked out, but I drained
one drive which gave me enough to top up the other three.

I picked a disk pack that didn't look important, fitted it, and hit the
'Power' button. It spun up as I watched nervously. Once it was up to
speed, the heads slowly moved out onto the edge of the disk, loaded, and
then there was a bang-bang as the head assembly seeked into the centre
and back again. This turned out to be quite normal, but at the time it
gave me a great fright, and I still don't know the real reason for it.
Maybe it's just to get any head crashes over and done with right away.

By this time I knew that I should be able to set the address '190' on
the front panel switches, and hit the 'IPL' button to boot the system
up. I found the pack marked 'SYSVOL' and mounted it on drive 190. Once
it had spun up I pressed IPL. The console typewriter sprung into life,
banging out a prompt for the current date. I think it took me a while to
get past this as I didn't have any manuals and didn't know exactly what
it wanted, but eventually I had my own running DOS/360 system.

This was quite amazing, as the only DOS systems I had ever used were
CP/M and TRSDOS, and here was something with a whole new terminology. At
the same time, with only a console typewriter, there wasn't much I could
do with it. There must have been a line printer and card reader with it
at some stage, and the whole operation of the system was centred around
being able to feed JCL in through the reader.

Luckily, a friend of a friend lent me a small DOS handbook which listed
all the commands and proved invaluable. John Machin, I'm sorry I never
gave it back! I went through the disk packs I had, looking for
interesting programs. As far as I could tell, the machine had been used
in a typical commercial billing environment. I never found any
interesting data, not that I looked very hard. I was more interested in
the programs, and trying each one out, as well as trying to extract
COBOL source using the library utilities.

Contd...


--- Maximus/2 3.01
 * Origin: Another Good Point About OS/2 (3:772/1.10)