reentrant vs. non-reentrant code

[Paul Gillingwater]

In article <1670001 at hpcilzb.HP.COM> tedj at hpcilzb.HP.COM (Ted Johnson) writes:
>
>Can someone please explain the difference between "reentrant" and
>"non-reentrant" code?  Thanks!

Many of us down-under probably don't bother replying to questions
because the delays means that it's already been answered, but what
the heck, here goes anyway....   ;-)

Back in 1979 when I was writing multi-tasking operating systems for
M6800's for process control applications, I found out the hard way 
the difference.  When I wrote modules that might need to be called
concurrently to respond to different sources of interrupts, I found
that if an interrupt was currently being processed (and it had to be
done real fast so no use setting a flag to process later) when another
interrupt came in, the code would  be executed again (i.e. re-entered)
before the first call could finish and clean up after itself.

The result of this was that any local variables (read absolute addresses
to RAM - this is M6800 assembler, remember!) were overwritten.  The
solution was simple - just use the X register to maintain a heap - sort
of like a stack, only starting at the bottom of RAM.  Any local variables
then became just offsets from this pointer (which WAS maintained at a
fixed place, and had to be accessed under special Test-and-set semaphore
control.  Of course any decent HLL will look after local variables in a clean
manner.

So, to summarise, re-entrant code doesn't mind being entered any number of
times (subject to stack/heap space), and won't do things that can't be
undone.  It cleans up after itself (if well written), whilst non-re-entrant
code (like the IBM ROM BIOS) doesn't and isn't.

MS-DOS - Just say Gnu!
-- 
Paul Gillingwater, Computer Sciences	Call this BBS - Magic Tower (24 hours)
paul at csnz.nz  (vuwcomp!dsiramd!csnz)	NZ +64 4 753 561 8N1 TowerNet software
P.O.Box 929, Wellington, NEW ZEALAND	V21/V23/V22/V22bis/Bell 103/Bell 212A
Vox: +64 4 846194, Fax: +64 4 843924	"All things must parse"-ancient proverb