RT11 sysgen and disk partitions

Jerome H. Fine jhfinexgs2 at compsys.to
Sun Mar 13 22:11:30 CST 2005


 >Megan wrote:

>What you need to understand is that RT-11 has a maximum volume size
>of 65536 blocks.  Therefore, for volumes which are larger than
>this (typically only the DU drives, like RD53 and RD54, or SCSI or
>DSSI which act like MSCP devices), the drive is logically partitioned
>into units of 32mb each...
>  
>
Jerome Fine replies:

I am pleased you note that each RT-11 partition is
65536 blocks - except maybe (usually) for the last
RT-11 partition.  It is again helpful to note that
internally, RT-11 reports the size as 65535 blocks
with the first block being zero and the last block
being block 65534.  This is so that a full 16 bit
word can hold that largest size of 65535. blocks
even though the actual size is 65536. blocks.

If anyone thinks to test the above information, try:
COPY/DEVICE  DU0:/START:65535./END:65535.  DU1:/START:65535.
or any other block for DU1: for that matter.  NOTE
that any other "START" block number for DU0: will result
in the END block 65535. not being used.  This is because
there is a bug in DUP.SAV which ignores block 65535. except
when there is ONLY a single block to copy.  In that case,
I presume that the person who wrote the code forgot to
check that block 65535. should also be ignored in that
case as well.  I conclude that the likely reason is that
2 mistakes resulted in what the user requested being done.

For example, try:
COPY/DEVICE  DU0:/START:0./END:65535.  DU1:/START:0.
the result is the same for both of the following:
COPY/DEVICE  DU0:  DU1:
COPY/DEVICE  DU0:/START:0./END:65534.  DU1:/START:0.
with NO warning message being provided in the last
case.

Sincerely yours,

Jerome Fine
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