Programmed code generation (was: Self-modifying code)

[Barry Shein]

Did I miss something critical here? Most lisp systems generate and
execute there own machine code. They usually do so by loading the code
into the data segment but not for any particular reason other than
that few OS's support the extension and modification of a text
segment, and that it probably doesn't much matter other than a) not
being able to mark it read-only which would usually be desireable (tho
this could be done in data or text) and b) possibly taking advantage
of a different paging strategy for text vs data pages, if relevant.
None of that is critical tho both could be very useful.

Exploiting shared library designs might make this more feasible in the
future, it occurs to me that it might be possible to dynamically build
a file object that the OS will consider a shared library text and then
page it back into the text section. I would imagine some changes might
be needed to current designs (eg. do the shareable objects and symbols
have to be fully specified at initial load-time?)

Speaking of lisp and self-modifying code I worked on an IBM370 lisp
which accomplished trace and stepping modes (debugging) by
self-modifying the top of the eval loop. Of course it didn't save much
more than a test and branch, but eval loops are entered for every
operator/function (eg add, subtract etc etc etc), on a 370/158 with
less than a MIP and 100 logged in users one becomes a fanatic about
such things, I don't know how much it helped in fact.

And there was always the IBM370 MVC (move characters) instruction
which only took a fixed number of chars specified at assemble time, a
common sequence was (particular register numbers irrelevant):

	stc	r1,*+5
	mvc	0(r2,0),0(r3)

which overwrote the second zero (,0), the length. The 370 of course
provided an EX (execute) instruction which allowed indirect execution
of an instruction after OR'ing in a particular register's low byte,
naturally that was designed to accomplish the above (SS instructions
which took a fixed length field all had the same basic format using
either the byte or two nibbles as a length indicator.) Why people
tended to use the store char hack above rather than EX was always a
mystery to me, other than tribal practice.

	-Barry Shein, Boston University