You two recent bug reports

[Alan Bawden <ALAN at MIT-MC>]

Well you don't give a very clear picture of the symptoms of the bug
are suffering from, but here is some advice that might help you out:

1) When a macro (such as your TTYVAR macro) calls a function (such as
your SIXBIT function) as MACRO EXPAND TIME it should be enclosed
within an (EVAL-WHEN (... COMPILE ...) ...).  When the compiler
compiles an ordinary top-level DEFUN form is just compiles it, it
doesn't arrange to define it in the compile-time lisp world unless you
ask it to.  Since the body of a macro is evaluated at compile-time,
you must take care to define any functions called there.

2) I notice that you do (car (syscall ...)) a fair amount.  This is
probably not the source of your MPVs (taking the car and cdr of small
fixnums being relatively safe), but if you are in the habit of taking
car and cdr of other things that are likely to not be conses, then you
are living dangerously.