PDP-11 disk image question

[Paul Koning]

> On Feb 18, 2019, at 11:22 PM, Charles Anthony via cctalk <cctalk at classiccmp.org> wrote:
> 
>> ...
> Looking at the SIMH code:
> 
> /*
>    type sec     surf    cyl     tpg     gpc     RCT     LBNs
>    RA81 51(+1)  14      1258    14      1       2856    891072
> */
> 
> #define RA81_SECT       51                              /* +1 spare/track */
> #define RA81_SURF       14
> #define RA81_CYL        1258                            /* 0-1247 user */
> #define RA81_TPG        RA81_SURF
> #define RA81_GPC        1
> #define RA81_XBN        2436                            /* cyl 1252-1254? */
> #define RA81_DBN        2436                            /* cyl 1255-1256? */
> #define RA81_LBN        891072                          /* 51*14*1248 */
> #define RA81_RCTS       2856                            /* cyl 1248-1251? */
> #define RA81_RCTC       1
> #define RA81_RBN        17472                           /* 1 *14*1248 */
> 
> I am presuming that (without actually checking) that these are the numbers
> returned to RSTS when it queries the disk; if these numbers are different
> then the h/w, then that could be an issue. I am a bit curious about "+1
> spare/track" - I wonder if the h/w reports 51 or 52?
> 
> -- Charles

RSTS doesn't care about geometry, though MSCP does return something it calls geometry, for optimization purposes.  All RSTS wants to know is the device size, in 512-byte blocks.  Whatever the controller returns is what it uses.

This is why in SIMH you can use "RAUSER" devices with MSCP, any size you like up to 8 megablocks.  RSTS doesn't limit you to the sizes of the DEC MSCP products, and in fact you won't find any list of those model codes or any of their attributes in the RSTS sources.

	paul