gnu@sun.uucp (John Gilmore) (08/14/84)
Tom Breuel reported earlier that LisaBug seems to strip high order bits
sometimes, perhaps it is mucking around with his PC too. Any real
Apple people know?
Or anybody got a 68000 board with another debugger?
P.S.: About choice of implementation of type tags: given that the LISP
runs on an MacIntosh with very limited memory resources, given that the
Mac has a "fixed" architecture and is unlikely to profit from
Motorola's corrections to the 68000 (as in the 68010), and given that
the LISP performs very well speedwise, I think the choice of putting
type tags into the "unused" msb of pointers, and not explicitely
stripping them before dereferencing (except for the case of indirect
jumps :-) is justified. Since all type reconisers and selectors are
defined as macros, this efficiency hack even leaves the program
portable.
Thomas M. Breuel
...{genrad!wjh12!tardis,allegra!harvard}!gallifrey!tmb
Well, if your Lisp is worth its salt it will survive the Mac. What
would prevent its running on, say, a Sun? The problem is not that
you're depending on 68000 bugs, it's that you depend on a 24 bit
address bus. Various commercially available 68000 based systems
provide anywhere from 20 bits to 32 bits of addressability. Given a
network of machines (linked by AppleNet or Ethernet or whatever), why
should there need to be N different copies of your Lisp, based on the
address bus size of the machine that wants to run it? (The source may
be portable but the binary isn't.)
Also, I've heard rumors of a "Big Mac". Given that Apple seems to like
the 68000 family, they might make a machine with the faster 68020 once
it's available in large volumes. Of course, if too many people write
software like yours, they won't be able to offer binary compatability
with Mac programs. Reminds me of the people doing the 16-bit 6502 chip
who had to put the 6502 bugs back in because all the Apple ][ programs
depended on them...