[PicForth] Another gotcha- @ and ! can break interrupts

[Alex Holden]

Here's another gotcha which took me a couple of hours to get to the 
bottom of. Sometimes when you do '@' and '!' picforth will ask you to 
include libfetch.fs / libstore.fs. These files define words called any-@ 
and any-! which are sometimes called when you use '@' and '!'. any-@ and 
any-! use suspend-interrupts and restore-interrupts to prevent 
interrupts from occuring in the middle of a fetch or store. I also used 
suspend-interrupts and restore-interrupts to protect a critical section 
of my own, which happened to call '@' a couple of times. If 
suspend-interrupts is called twice without a restore inbetween, the next 
restore-interrupts will restore the state after the first 
suspend-interrupts (ie. interrupts-off). So what was happening was that 
every time my word with the critical section in it was called, the 
interrupt system on the PIC froze solid.

I can think of a few possible solutions:
* Make suspend/restore-interrupts take an argument which is a number 
between 0 and 7 that points to which bit in the isr-enabled variable to 
use (so system-type stuff could use a different bit in isr-enabled to 
user-type stuff).
* Make suspend/restore-interrupts nestable by creating a new variable 
which is used to automatically keep track of which bit in the 
isr-enabled variable was used by the last save-interrupts.
* Have two versions of suspend/restore-interrupts which use different 
bits in isr-enabled - one intended for 'system' type words with critical 
sections in them, and one intended for user defined words.
* Simply revise the documentation to warn against using 
suspend/restore-interrupts unless you're absolutely certain none of the 
words in your critical section also use them (pointing out that even @ 
and ! aren't safe).

-- 
------------ Alex Holden - http://www.linuxhacker.org ------------
If it doesn't work, you're not hitting it with a big enough hammer