Several comments on crashed drive in PDP-11/23

Johnny Billquist bqt at Update.UU.SE
Sun Jun 19 17:42:02 CDT 2005

On Sun, 19 Jun 2005 Philipp Hachtmann <hachti at> wrote:


> > RSX (don't know if the other OSes do the same) write a special boot block
> > on non-bootable disks. This boot block basically just prints the above
> > message, and halts. All that fits fine into 512 bytes.
> Aha. I also have an RSX-11M license..... But no media or a clue how to
> get it into my machine.

Media is usually on TK50 or 1/2" tape.

> >>What is the "modern way" to get software into the PDP11?
> > Phew. Tape normally. Small, modern PDP11s usually have TK50, and software
> > is distributed on TK50.
> TK50...???

You might know it under the name DLT. TK50 was the original DLT.
It's plain and simple just a tape.

> >>Is there anything like a RIM or BIN loader?
> > Yes, but that is very old technology. People don't have paper tape on
> > PDP-11, and haven't for a very long time.
> Hm.... But I'm a paper tape enthusiast..... I just think about giving
> the 11 away again - perhaps it's too modern for me.....

If you look for old diagnostics, old systems like DOS-11 and the like,
you'll find that paper tape plays a role.
RT-11, RSX and RSTS/E (as well as Unix) are still current products, still
being sold. Of course they have moved on from paper tape. They actually
never were paper tape based, even though you have device drivers for that
medium as well.

> > If you're on a bare system, with only disk drives to play with, then your
> > options are limited.
> I have a RX02 double diskette drive and a lot of disks....

Well, your problem is one of how to get something in there in the first

> > The best might actually be to pretend you have a
> > TU58, which speaks through a normal serial line, but is a block device,
> > which most PDP11 can boot from.
> May the original bootstrap code boot from a serial port directly??

Quite possibly. You can probably boot from a number of devices. Which one
is selected by a block of switches on the CPU board, or if you talk
interactively with the CPU, you tell it which device to boot from.

> > Yes. The TU58 is a cheap block device, for which the hardware on the
> > PDP-11 side actually is just a serial interface. Most boot roms know how
> > to boot from one, and an emulator for the actual TU58 exist.
> The standard ROM in my 11/23+ does not boot from tu58?

I would suspect it does.

> >>Can I boot RT11 or RSX11 from floppy???
> > Yes.
> Ok. that's the point where I try to start. I will make a floppy. Need an
> image and then I'll try to put the image to onto a floppy.

That can be a problem. How do you get the image onto a floppy before you
have a system in which you can access the floppy?

> > Oh, and I would probably try to align the drive correctly if I fooled
> > around with it. You never know when another pack shows up which you might
> > want to read.
> I can't realign the drive because I don't have the alignment tools,
> control panel for the drive and an alignment pack. But I did not change
> the head alignment when I removed the heads. Left them together without
> moving relatively to each other..

Well, if you can't, then you can't.

> Does anyone have experience with that "vtserver" program?
> It puts it's bootcode into the pdp (via odt) and then the CPU stops.
> Sometimes a single 'r' character shows up. And sometimes I see an error
> message (non-existent record 13)....
> I start to think about a problem with my serial port.... Are there known
> issues with the console port? I have tried 300,2400,9600 and 19200.
> Always errors. But NEVER while vtserver puts its startup code. Always
> after vtserver has started the pdp.

Can't help you there. I just play with big hardware, and have tape drives.
Never tried vtserver.

> And a simple question: How does the addressing in pdp11 work? R7 seems
> to be only 16 bit wide. And when I boot up the system, the MMU should be
> off and the memory mapping should be direct. How do I access the upper
> bits of my PC (i.e. "changing the segment")?

Yes. The PDP-11 is a 16-bit machine.
There are no "upper" bits to the PC. The PC is plain and simple just 16

Here is how it works. The MMU can be disabled or enabled. If enabled, it
can be in 18-bit addressing or 22-bit addressing.
When an address is gated onto the bus, it's mapped from the virtual
address to a physical address.
If you MMU is disabled, addresses 0-157777 (octal) gets mapped to the same
physical addresses. Address 160000-177777 gets mapped to physical
If the MMU is enabled, the high three bit of a virtual address selects the
PAR/PDR register pair used to relocate the virtual address to a physical
address. That means your memory is divided into eight pages, and each page
can map anywhere in physical memory. In 18-bit mode, only parts of the PAR
register is used, while in 22-bit mode, the full content of the PAR is
To get the physical address from a virtual address, you do:

(written in C)
(PAR << 6) + (VA & 017777)

And the right PAR is (VA >> 13)

PAR is also 16 bits. This will tell you that the page will start on a
multiple of 64, and that the 6 low bits of a virtual address is unchanged.


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

More information about the cctalk mailing list