APDL Public Domain 1/APDL_PD1A.iso/program/virtual/VSI_Basic/Letter

Text File | 1992-09-20 | 3.4 KB | 78 lines

Virtual Store Interrupts and Pageing

A recent letter by Maurice Hendrix described how he would
like Virtual memory management under Risc-OS and included
an implementation. I had earlier thought that the job was
too much bother and not useful enough but having looked
at what he was doing have come to accept that he really
does have a point.

I have implemented a simple virtual store interrupt
handler in a Basic program 'VSI_Sieve' accompanying this
note. The sieve program is as in the program by Maurice
Hendrix except that the pages are automatically paged in
as necessary when they are referenced. Up to 8 Mbyte of
virtual store starting at &800000 can be used if the disk
space is available. The big restriction is that references
to the virtual store area must not be passed to Risc-OS.
the virtual store handling only works for programs in
user mode - in particular *LOAD or *SAVE will not work.

The program is single tasking and will not run in a Task
window. However hooks are provided for running under the
Wimp. One way of working might be to put all the Wimp
related processing and data into the low addresses and all
the computation and tables into the virtual store area.

There is obviously endless scope for development and
improvements. The most obvious to me are:

1. Have a virtual store module like in the implementation
by Maurice Hendrix that would allocate space to tasks from
the space of a 'Virtual Store Application' the size of which
could be changed easily.

2. Implement a way of running C programs in the virtual
store space and calling them from programs in lower space.
All the Ansi C calls except fread, fwrite, setvbuf, and
system look like they can be handed over direct to the current
library.

3. Allow use in a task window by simply remapping a page
if it is present but the mapping has changed unexpectedly.
Implementing the handling in a module may mean the job can be
done more elegantly.

4. There is room for efficiency improvements. The page
replacement policy could certainly be improved. An option
could be set to do a Wimp_Poll call when a VSI happens
and do the transfers in the background.

With these developments large C programs could be run
quite easily in virtual store. The only space that would
be locked in store would be the Wimp front ends. In fact I
believe the main use could be to allow large programs to
sleep in the background without consuming space when they
are not actually processing.

So how does virtual store work out in practice in VSI_Sieve?
Sadly this program is not structured to use virtual store
efficiently. The program does repeated linear passes through
the data so every page keeps being loaded in in turn - there
is no real working set at any stage. And there are a lot of
passes when trying to find the millionth prime using a megabyte
of virtual store. I estimate the program should take a couple
of days to do the job but I haven't the patience. The program
could be better structured to count all the primes in a block
at a time - also this would be more sensible than trying to
estimate in advance how many numbers to look at to get to
the millionth prime.

Run VSI_Sieve with a top limit of 1000000 and a current
directory with a quarter megabyte free. This will make the disk
chatter for a while and eventually report the number of primes
below a million. Smaller values than about 800000 don't require
pageing.