[Simh] RT-11 Storage Strategy

[Paul Koning]

> On Feb 22, 2016, at 3:50 AM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> On 2016-02-21 16:51, Paul Koning wrote:
>> ...
>> True in principle.  But a number of OS used a different file structure for DECtape than for disks -- because of the performance issues.  DOS does, for example, and RSTS copied the DOS DECtape directory structure (but not the disk one).  RT11, on the other hand, uses the same format for both, and the RT11 format works well.  That's why RT runs from DECtape without any extra work, while DOS and RSTS don't.
> 
> Interesting. I didn't know that RSTS/E used a different file system for DECtapes. RSX do not. It's the same file system used on both large disks, floppies and DECtapes.

The RSTS file systems uses directories that are linked lists, which means things get really horribly slow if you can't do decent random access.  That includes the file windows (pointers to where the file blocks are).  DOS ditto, though the details differ.  So for DECtape, DOS took the aproach of having each data block contain the link to the next, rather than having those links in the directory as it does for disks.  This is why DECtape file blocks in the DOS format are only 510 bytes long.

	paul