23 Dec
2015
23 Dec
'15
4:03 p.m.
Noel,
Comments inline. This has got to be the most helpful mailing list ever.
Thank you for responding so carefully.
On 12/23/15 12:59 AM, Noel Chiappa wrote:
This one, I worked through in my dreams :). I woke up this morning with
the full-fledged thought, 000777 isn't halt, it's a branch to the new PC
+ (2 * offset, 377) - I have been reading the PDP-11/40 handbook, much
too much :):
0004 BR to PC+2 + (2* Offset of the next six bits, 377)
3 7 7 - offset in octal
11 111 111 - binary equivalent
00 000 000 - 1's complement
00 000 001 - 2's complement
----------
-1
So: BR, PC+2 +( 2 * - 1), or .+2-2, or BR . as you say.
Actually, after reflecting on your comments and walking through it
again, this is really elegant code. The guys who thought this up were
amazing. Using the MTCMA is brilliant, as you said, it's a big enough
negative value to cause the entire block to be read, but it's also the
value that is used to obtain the destination for that byte count through
it's decrement and further, to obtain the destination for the command to
read the data after it's second decrement.
From: Will
Senn
000777 HALT
That's actually "BR ."; the difference is important, since the CPU (IIRC)
doesn't honour DMA requests when it is halted, and DMA needs to be working for
the controller to read that first block (a secondary tape bootstrap) into
memory. This seems
like gobbledegook to me.
:-)
It moves the MTCMA (Magtape Current Memory
Address) into R0, then it
moves the MTCMA into the MTBRC (Magtape Byte Record Count)
"The address of the MTCMA into", etc. Looking quickly at the programming spec
for the TM11 controllers, it wants a negative of the byte count to transfer in
this register; the address of the MTCMA just happens to also be a large enough
negative number to be usable as the (negative) size of the transfer request.
(The first record is probably shorter than that, but that doesn't matter.)
Note that this code could probably also have been written:
MOV #172524, R0
MOV R0,@R0
and been equally functional.
then it moves 60003 into the MTC (Magtape control
register), which
causes a read operation with 800BPI 9 Channel density.
I'm too lazy to look at the programming spec for the details, but that sounds
right.
Am I misinterpreting the byte codes or is this
some idiosyncratic way to
get the Magnetic tape to rewind or something (the TM11 has a control
function to rewind, so it seems unlikely that this is the case
No, it's just the shortest possible program to read the first block off the
tape. It depends on i) the operator having manually set the
tape to the right point
(the start of the tape), so that it's the first block that gets read, and ii)
the fact that the reset performed by hitting the 'Start' key on the CPU clears
the TM11 registers, including the Current Memory Address register, so the
block that's read is read into memory location zero.
Hence the direction to 'once the tape has stopped moving, re-start the CPU at
0'.
Noel
In order to test this and armed with my newfound knowledge, I fired up
SimH and attached the v6 distribution tape and set it locked for read
only. I examined memory from 0-100, which was empty. I then deposited
the bootstrap and ran it. It hung, allowing the NPR. I then stopped the
CPU with CTRL-E and examined the memory starting at location 0, voila, a
407 program starting at byte 0. My understanding at this point is that
the program never touches MTCMA (other than to use it for the byte count
and decrements) so it is initially 0 based on how SimH works and the
program simply reads the first block into memory in location 0 and hangs
until the simulator is suspended.
The next step in the install is to g 0, which runs a program that prints
the = prompt to which tmrk is a reasonable response. I gather the 407
program is some kind of minimalist shell that includes tmrk as one of
its commands.
Thanks again for your help.
Will