howto bitsavers BASIC-11 v2 disk image under simh

Glen Slick glen.slick at
Thu Aug 12 17:31:40 CDT 2021

On Wed, Aug 11, 2021 at 5:59 PM Paul Koning <paulkoning at> wrote:
> > On Aug 11, 2021, at 7:06 PM, Fritz Mueller via cctalk <cctalk at> wrote:
> >
> >> ...
> >
> > Ah ha!  Thanks much, Glen!
> >
> > simh is usually pretty good about padding out attached images,
> No, I don't think that is true.  What is true is that recent versions of SIMH will create full size container files for disk containers.  But sufficiently old versions did not, so it is not too strange to run into, say, an RK05 image that's shorter than 4800 blocks.
> But those normally work; all that happens is that reading too far either gives you zeroes or a read error, I don't remember which.  Since the blocks were never written they should be marked as unused in the file system and nothing will read them, so that's fine.
> If there's an unused track at the start of a SIMH image file and in your copy that was missing, that's a different case.  Is that the issue?
> Or is the RX02 emulation different from regular disks?
>         paul

It's been a while since the last time I did anything with a SIMH
PDP-11. I just fired up the simh-4.0-Current--2020-06-09-0912a927
version that I last used to take a look at these RX-02 disk images.

As far as I can tell by default PUTR expects to work with logical
sector order RX-02 disk images that are 512,512 bytes in size. The
BASIC-11 RX-02 disk image available here is in logical sector order,
but is less than 512,512 bytes in size:
PUTR appears to be unhappy with the disk image unless it is padded to
512,512 bytes in size.

On the other hand as far as I can tell by default SIMH expects to work
with physical sector order RX-02 disk images. If I mount the logical
sector order RX-02 disk image that works with PUTR in SIMH, then RT-11
gives a "?DIR-F-Invalid directory" error. If I translate the logical
sector order RX-02 disk image back into a physical sector order disk
image (dealing with track shifting, sector interleaving, and track to
track sector skewing) then RT-11 on SIMH is happy with the disk image.

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