PDP 11/73 is ALIVE!

From: Ian Primus <ian_primus_at_yahoo.com>
Date: Sun Nov 23 20:21:17 2003

> Well, except for s/BME/BNE/ on the third line (address 2004) and
> s/.R4/,R4/ on the twelfth (address 2034), this looks not entirely
> unreasonable.

Hehe... oops... typo's. Sorry about that. I typed that in by hand, and
I must have made a couple futz keystrokes.

> So it had sent the command to the tape but was sitting in the spin loop
> at locations 2026 and 2032, waiting for the ack.
>
> Given how small this is, and the control structure of the code, it
> appears to be expecting the device at 172552 to DMA some boot code into
> low memory; when this is done, as signaled by finding 122204 in what I
> assume is the device's CSR, it jumps to it with the CLR PC.

I checked my connections again, and lo and behold, I had the tape drive
cables reversed. The pin 1 markings on the back of the drive threw me
off, since the connectors are so close together, I got the labels mixed
up and was reading the pin 1 from the adjacent connector, thus both
connectors were rotated 180 degrees. I swapped them, and typed in the
program again. I the typed 2000g, and the tape drive started to whir!
It seeked back and forth in tiny and equal increments. Then, the drive
stopped and 012710 was displayed, then the _at_ prompt. I typed g, and got
: *** THIS VOLUME DOES NOT CONTAIN A HARDWARE BOOTABLE SYSTEM ***. It
still didn't boot, but at least it's doing something! (Lesson for
today, never connect hardware while tired, and always use a flashlight
to see behind racks.)


> It might be instructive to clear low memory first, then run the above
> code, and see if low memory has changed.
>
> For example,
>
> 001764 012700 b: mov #b,r0
> 001766 001764
> 001770 012701 mov #b/2,r1
> 001772 000772
> 001774 005040 x: clr -(r0)
> 001776 077102 sob r1,x
>
> the rest of the code as above, and then use 1764G. When you break it,
> examine low memory (type 0/ and just hit return a bunch of times). If
> low memory isn't all 0, _something_ has changed it, and if you send me
> the contents (a capture of the output when you hit return until you
> find a bunch of all-0 memory locations) I can disassemble it for you.
> Not that that will _necessarily_ help, but if the disassembly makes
> sense, there is an excellent chance the hardware is connected
> correctly.

I might end up doing that, I can write down the output, and email it to
you. Right now though, it looks like the program _is_ running, it's
just not clear what it's looking for on the tape. I was told that the
tapes I have are bootable, so I must be doing something else wrong.
Either that, or all three tapes I tried have gone bad.

I have two blank tapes, as well as the SCSI nine track tape drive that
goes with my Prime. I should be able to connect the other tape drive to
a PC, and dd a tape image to a real tape. Is there a known working,
bootable tape image out there on the 'net that I could use to test?
I've been looking around, and so far I've found only a couple, and I'm
not sure that they would work on my machine. I found a couple different
places that had BSD 2.11, one of which had it as one tape image,
another had it as three. I only have two tapes, so installing from
three would be a pain (write first two, install one, install two while
rewriting one into three, install three, then scream when installer
asks for one again...)


Ian Primus
ian_primus_at_yahoo.com
Received on Sun Nov 23 2003 - 20:21:17 GMT