[Simh] Way out idea for simh

[Paul Koning]

> On Apr 20, 2016, at 11:43 AM, Ken Cornetet <Ken.Cornetet at kimballelectronics.com> wrote:
> 
> A common theme on this list is how to get files copied between the host and the emulated machines. I have a crazy idea for a simh feature to help in that regard: Add an FTP server to simh that would write to a “universal” file system on a simh block device file (disk, tape, drum)  that the guest OS would have attached. You could fire up your favorite ftp client and copy files into and out of this file system.
>  
> Obviously, the guest OS would need to have tools written to read/write this universal file system, but with a simple enough file system, that wouldn’t be a huge hurdle. I have to admit, outside of unix and RTE, I have no notion of how many operating systems that run on simh emulated machines allow direct disk access. I am assuming there is a way to do it on most all of them. If not, tape or drum could be an option.
>  
> For this “universal” file system, I nominate Hewlett-Packard’s LIF. It is simple and well documented. A fixed flat directory at the beginning of the image, fixed size directory entries, and linear space allocation (no allocation tables).

I don't know LIF, but the RT-11 file system is certainly simple.

There are a couple of complications.  First, you'd have to write a file access utility for each guest OS.  Given a simple enough file system that isn't necessarily a huge burden.  Then again, what might be simple, requiringly only modest code, on one machine might be a major burden on another simply because it has much less memory.

Another problem is that there isn't any universal disk format, so you're missing the foundation for a universal file format.  Consider the IBM 1620, with disks that have 200 digit sectors.  Or (not that SimH supports it, but another simulator does) CDC 6000 machines, where the sector size is 322 12-bit words.

Chances are that magnetic tape is more general; there aren't as many encodings there.  Basically it's 6 bits vs. 8 bits per frame.  Everyone understands variable length data, and unlabeled tapes are fairly widely supported.  Even if not, writing a labeled tape with a single file on it isn't too hard.  You're still stuck with machines that have no magnetic tape support, there aren't all that many but certainly some.  

Paper tape is yet a third option, which is presumably unlabeled but often transparent.  (Not always, the 1620 comes to mind as a notorious example of a machine that could read only coded tape with punches conforming to the code it expects.)

	paul