[Simh] PDP-10/RP06 Disk Image Size

[Cory Smelosky]

On Sun, 12 Apr 2015, Rob Doyle wrote:

> I'm designing an RH11 disk controller for my KS10 FPGA. I'd like to
> use the SIMH data format for the media.
>

Add RP07 support too!

> I noticed that an RP06 disk image from SIMH is 315,187,200 bytes.
>

Wait, what? Isn't an RP06 around ~180M?

> Assuming an RP06 has 815 cylinders, 19 heads, 20 sectors (confirmed
> in SIMH code) - I calculate that disk image should be 317,132,800
> bytes.

http://www.columbia.edu/cu/computinghistory/rp06.html

                         128    Words/sector
                          20    Sectors/track
                          19    Tracks/cylinder
                         815    Cylinders/pack
Number of heads:         20

What am _I_ missing? ;)

>
> If use 810 cylinders (instead of 815), I get the size of the of the
> image file.

Is your file system considering the file to be "sparse" by chance?

>
> SIMH doesn't pack data: there are 128 words per sector and each
> word occupies 64-bits or 8 bytes.  Therefore a sector is 1024 bytes.
>
> What's am I missing?
>
> Rob
>
> --
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-- 
Cory Smelosky
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