32-bit mapper ops across pages fix

[rakslice]

Divide the 32-bit mapper reads and writes into 16-bit ops to handle ops across page boundaries; check address alignment in 16-bit reads

[ewenmcneill]

To record what we discovered in Discord:

more --version

seems to be a good trigger of this cross-page read issue.

With this fix, it now seems to run correctly:

# cd /usr/tmp
# ls -l core
core not found
# more --version
Usage: more [-dfln] [+linenum | +/pattern] name1 name2 ...
# 

and without this fix it would always crash (usually bus error trying to access a wild address in ROM from user space, sometimes "illegal instruction" in a way that didn't make sense).

Since the 68010 is a 16-bit address bus processor, with 32-bit internals, it appears the compiler used by Plexus quite happily generated 16-bit aligned accesses to 32-bit values. And if those spanned a page boundary, the real hardware would do two map lookups, but the emulator was previously only doing one map lookup.

Great find!

Ewen

[rakslice]

Closing already fixed :D