[TUHS] PDP-11 questions

Johnny Billquist bqt at update.uu.se
Mon Jan 25 04:34:17 AEST 2016

On 2016-01-24 19:01, Mark Longridge<cubexyz at gmail.com> wrote:
> Ok, I got a few questions about PDP-11.
> First, I was wondering when Bell Labs got that first PDP-11/20 what
> software (if any) came with it? I assume when one bought a PDP-11/20
> you would get some type of OS with it.

No. You might get diagnostics, but any kind of OS you would have to buy 
separately, and there were several to choose from, depending on your needs.

> According to the folks at alt.sys.pdp11 the PDP-11 computer doesn't
> have anything equivalent to a PC's BIOS. But I know a bit about what a
> PC's BIOS does and that includes RAM Initialization. Wouldn't the DRAM
> on the PDP-11/something need to be initialized too? Perhaps an older
> PDP-11 doesn't have DRAM but surely the later models did?

RAM don't need to be initialized. Maybe you mean clearing it, so it 
don't contain random information?

ECC memory, on the other hand needs to be initialized, but for those 
PDP-11s who has that, the initialization is done in hardware.

> Now the last question has to do with what made the PDP-11 architecture
> so great. Part of that had to be the relatively affordablility of the
> PDP-11 and of course it was the machine that made Unix possible. It
> seems though that there should have been a PDP-11 based desktop and as
> far as I can tell that didn't happen. Instead we got a bunch of micros
> with 8080, z80 and 6502 cpus, but nothing that could run Unix, at
> least not a Unix v7 with source code.

The architecture is very easy to program on, and rather intuitive. You 
have general registers, an orthogonal instruction set, and the machine 
can be programmed as a stack based, a register based, or just plain 
memory-to-memory style equally well.

In addition, I/O is pretty simple, as there are no special I/O 
instructions. Same instructions as for anything else are also used for I/O.

Also, the memory model on the PDP-11 is pretty nice, with a proper MMU 
which allows you to write reasonable OSes.

There were in fact desktop based PDP-11s, but DEC shot themselves in the 
foot there. They were afraid of eating into their own business, so they 
made the desktop PDP-11 incompatible in some ways with all other 
PDP-11s, so you could in general not run much PDP-11 software on the 
desktop, but had to develop specific programs for that platform. That, 
and the fact that it took DEC too long to enter the market, meant that 
the IBM PC had already become the standard by the time DEC came with the 
Professional (the PDP-11 desktop).

DEC also made a couple of other PDP-11 based systems that were sortof 
desktop, such as the VT-103, which was a VT100 with a PDP-11 inside. 
Interesting idea, but the VT103 didn't have good mass storage, and had a 
very slow and limited PDP-11 CPU. The PDT-11 was another attempt, with 
similar issues as the VT-103.

If we were to examine prototype things, DEC did a lot more as well, 
including a portable PDP-11 with an LCD display. Never became a product.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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