On 2015-09-17 18:30, Dave G4UGM wrote:
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From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Paul
Koning
Sent: 17 September 2015 17:02
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk@classiccmp.org>
Subject: Re: Self modifying code, lambda calculus - Re: ENIAC programming
On Sep 16, 2015, at 11:36 PM, ben <bfranc...@jetnet.ab.ca> wrote:
On 9/16/2015 9:25 PM, Toby Thain wrote:
On 2015-09-16 6:18 PM, Dave G4UGM wrote:
...
It is notable that in order to solve all problems, a computer must
permit self modifying code.
Is that true? AFAIK Lambda calculus can describe any computable
function (as can a Turing machine), and it has no concept of "self
modifying code".
I never studied any of that, but you do have to LOAD and RUN the
program ToSolveAnythingBut42 some how so I guess that would count AS
Self Modifying Code.
"load" is an operation in a RAM stored program computer, sure. But self-
modifying code means a program that modifies its own code during
execution. That is a scheme that has on rare occasions been used in
history.
I actually think its pretty common, at least on certain machines, especially
for character manipulation.
There are machines, I think the Honeywell L66 is one, which make character
sting moves interruptible by updating the addresses and lengths as the
instruction is executed.
Say what? So if you run through the code/function twice, it won't work?
That would be pretty ugly.
I know architectures like the VAX keeps such state in registers. So
there are some instructions that destroy some register content. Not that
fun perhaps, but at least it means the code works if you run it several
times, or even if you run it as reentrant code.
As far as self-modifying code though, on some architectures it is more
or less necessary. The PDP-8 for example cannot really work without
self-modifying code. (Except in some limited scope.)
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
