12 Mar
2007
12 Mar
'07
1:03 p.m.
not know
how long development of such an aircraft takes but
the first take-off was in 1974 I think, so this would match
the design of this box wuite well - what do you think?
Aircraft development takes
a long long time. To make it worthwhile
the aircraft has a long service life. Whilst the computers are bullet
proof in an office/home, the temperature extremes, vibration, high G
forces etc in service means that many failures occur and PCBs could
be replaced many times over the years, so do not pay too much notice
of the dates on components of the boards currently installed. There were at least four 12 bit versions, and one
13 bit apparently.
Then 12 bit models were the 902, 102C, ARCH 105 and Minim or 12/12.
Thank you - I
will try to find information on the
web in the next days. Compared to the Elliott machines
the DECs are very common stuff ;-) Yes, that
matches - the core memory module already has the
label GEC. So this is (as I suspected) older than the CPU and
taken maybe from a different design?!
You have it the wrong way around. The GEC
name replaced the Marconi
name, which had earlier replaced Elliott. Core memory was very convenient for military applications and was in
use long after it was
replaced in commercial applications. Think of a missile or torpedo
held in stock for many years, stick a power plug into it and program
the target, pull out the plug and then launch it. Only then does the
internal power supply come up, the processor boots up and does its
thing.
The internet claimes sometimes, that core memory is less sensitive
to radiation, too. This might have been an advantage in certain
military applications (really bad applications, honestly speaking). Data retenttion of the core memory should be quite
good,
but since reading is always destructive, there is the
rist, that bad data gets written back. In the case of
Programmer Electronic Control, they did quite a lot
to adjust the core memory's dirver current automatically
according to a temerature measurement of the core. As
my experiments show, you have to reduce the current if
temperatuere goes up. This physically understandable
(1/T).
My old mainframes also have a temperature sensor in the core store,
and they are only allowed to be used in a narrow temperature band,
about 10 to 30 degrees C.
bit 920s before moving on to the Zilog Z8001.
Hey, than you are a valuable expert on these - do you have
got any source of intormation about the 920s? 9 Jump if negative
RIGHT - all three
jumps are relative ones 11 Store program counter (for function return)
SIMILAR - This stores PC and then does a
table jump
The Programmer Electronic Control (Let's call it PEC in the future)
stores PC+1 (as your 18bit) to the specified location into
core. The new address is taken from the position in memory
where the index register points to. So in my case this is
a perfect table jump.
Ah, so you do this ?
0 word with address of routine
11 link word
12 Multiply
RIGHT, unsigned multiplication
13 Divide
HMMM, the command takes very long
but the results are
very strange. I thought it might be some type of
random number generation by an irreducible polynomal,
but it is definitively not Divide. Maybe here is something
different or wrong with the microcode. 14 Shift
In PART: Here exist many subgroups
of commands including
shift left/right. Also the Q as you call it can be transferred
to Accu and vice versa. There als is a MTA (MoveToAccu as I
call it)
which is a two-word instruction (most others are one-word) and
transfers the word following this command into Accu. About
16 bit patterns have (at least to me now) the same meaning in
this segment. (but the data disappears from the source). A load
immediate
instruction would have been very handy.
Yes, the load immediate is really nice. A
block
move does not exist on PEC. Essentially the immediate
"MTA - Move To Accu" is one of two "two-read-cycles"
instructions. Up to now I did not fugire out, what
the other instruction does with the data read. Do
you remember any "two-read" instructions in the Elliotts? orders were encoded in the 14 order on the 12 bit
machines with only
256 numbers available compared with 8k on the 18 bitters.
Yes, this seems to be
absolutely correct!
15 Input/Output and special (like interrupt return)
16 to 31, as above but indexed by B register.
NOPE - The box is 12-bit and does not have got this block.
EVERYTHING is done via the index register I as I called it. Does it get cleared automatically somehow?
You are clever - that is exactly what appens:
Every instruction which is affected by this
register (ADD, SUB, LDA, STA, ...) clear
the register. So you have to load it right before
each instruction. A severe constraint in my
opinion is, that the index register can only
be loaded from words 0-127 in the lower 4k. So
this might be a bottleneck in memory since the
table jump (I called it IDXCALL since it used
the index register to get ne new address from)
requires memory for each routine called, too.
perhaps
studying the Elliot would help to solve the remaining
problems??!?!?!!
I am willing to try. By the way, there are two 't's in
Elliott.
Thanks - is stored into my brain now ;-)
bootstrap, in later versions, this was just copied
into core
when the
machine was initialised.
Interesting. The Programmer Electronic Control starts
execution
at 0x0a0 after reset. But of
course the application was different
and the operation software was completely loaded via the big plug
boefore operation.
I suspect the big plug is for the OMP (Operator's Monitor
Panel), and yes the program would be loaded via this once,
probably in the
factory or at a maintenance depot, and the machine would probably be
rebooted many times afterwards.
If data gets corrupted, than the complete aircraft
has
to return to a maintenance depot for reprogramming
the machine? So if there are 10 or 20 of these boxes in
one aircraft, they need service quite often I think.
Roger, how was your experience - was this reliable with
the Elliotts or did you have to reporgram the pater tape
loader quite often?
In my case, the transputer setup generates some spikes
doring it bootup. If PEC is already turned on, this
corrupts the data in core memory. So I always do a
memory-test on PEC after turn on (to adjust the core
driver current) and than freshly program my test
routines.
The 920 was unusual in that it booted into the
highest level
interrupt, level 1, so no interrupts will be serviced until you go
down an interrupt level or two or three.
So 15 7168 increases the interrupt level
and
after executing this once, the interrupt 1 can be
serviced, right?
What happens in the Elliott, if an interrupt occurs?
Is there a jump to a certain address in core or is the
new program counter itself read from core?
For a level 1 interrupt when the processor is on level 2, it stores
the SCR in location 2, and the B in location 3. Then it loads the SCR
from location zero and the B from location 1.
I have to think how to figure this out for PEC.... Of
course I have to apply signals to all possible interrupt
sources, load a program trying the yet unknown instructions
and watch wheter any irregularities or jumps occur. I will
have to trap PEC if the idle-loop is left. This is a bigger
project and I will need some weeks to check this.
Yes bit not within the CPU box.
The 15 order is coded thus:
15 0 to 2047 Input
15 2048 Shift A register left 7 places and OR in a character
from the
paper tape reader (via OMP)
15 2052 (IIRC) Wait until character pressed on teletype and
read into
A register (via OMP)
15 4096 to 6143 Output
15 6144 Output character to paper tape punch (via OMP)
15 6148 Output character to teletype (via OMP) if I remember
correctly
15 7168 Terminate current program level i.e. return from
interrupt
15 7169 Test if standardised IIRC, skip the next instruction
if the
accumulator is zero or top two bits are different
15 7170 Increment B and skip the next instruction if bottom
13 bits
are zero
15 7171 Read the value of the control keys on the OMP
15 7172 Move l.s. 17 bits of A to the m.s. 17 bits of Q.
Bottom bit
of Q is zeroed
15 7173 Move m.s. 17 bits of Q to l.s. 17 bits of A. Top bit
of A is
zeroed
15 7174 Move A to B
15 7175 Move B to A
15 7176 Set relative addressing mode
15 7177 Set absolute addressing mode
Thank you very much for this list. This gives me
valuable hints and ideas what to try out next.
BTW: Have there been timers on the Elliotts you
know about?
Since you are such an expert I am sure you
have a nice collection of vintage hardware,
right? Perhaps you even have got an Elliott
up and running?
No, when I started working for Elliotts, all computers were out of my
reach, but then I was offered an old mainframe (which originally cost
247,000 pounds, for scrap value, 150 pounds). It was built in 1962. I
now have two of them in a barn at home. LOTS of lights and switches,
each has a card reader, card punch, line printer and one has a paper
tape reader and punch. Oh they both have drum stores and ten track
magnetic tape.
Roger Holmes.