[TUHS] Determining what was on a tape back in the day
will.senn at gmail.com
Sun Nov 19 11:47:25 AEST 2017
On 11/18/17 4:53 PM, Clem Cole wrote:
> On Sat, Nov 18, 2017 at 4:07 PM, Will Senn <will.senn at gmail.com
> <mailto:will.senn at gmail.com>> wrote:
> I thought disk (RK05) and tape (magtape) blocks were different...
> For simh they are, but not once UNIX sees them.
> Physically 7/9-tapes were variable formatt
> ed and could have multiple 'files' on them. UNIX reveals all of this
> to user (as do most OSs), so you need put in the simh 'virtual' tape
> format support for the size of the 'blocks' and all of the extra
> things that the HW supports.
> But after the simh 'mounts' the 'virtual tape file' on the host when
> it reads the 'tape', simh strips the meta-data out and presents on the
> blocks to the OS. Or on write, simh takes the raw blocks, adds the
> simulated metadata and writes that to host file system as a 'virtual
> tape file.'
> In the old days disks physically could also be different formats.
> But the 'controller' was used to format the disk. Each disk block
> included metadata that the controller used. On DEC (and most other
> systems of the day), the disk controller had some way to set this up,
> usually with the diagnostic system. The OS saw the disk after
> formatting (as we do now). The diagnostics would have decided how
> big a block was etc... DEC standardized on 512 bytes per block.
> simh could have taken the approach like disks, and then 'virtual
> disks' would need the meta data; but could have supported all sorts of
> file formats (like Apollo's and Xerox's). But the simulated disk
> controller would then need to handle the meta data.
> Since, most OSs just looked at disk as 'block streams' simh only needs
> to provide for the OS to work properly, is map a UNIX file of bytes
> into 512 byte blocks. This works for most OSs. As I said, it will
> not work for Aegis or any of the Xerox systems which put some of what
> the OS normally did in the microcode of the disk controller.
Thanks, Clem, this is very helpful information. I have a better sense
now of what's going on.
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