Self modifying code, lambda calculus - Re: ENIAC programming

[Johnny Billquist]

On 2015-09-17 18:30, Dave G4UGM wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
>> Koning
>> Sent: 17 September 2015 17:02
>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk at classiccmp.org>
>> Subject: Re: Self modifying code, lambda calculus - Re: ENIAC programming
>>
>>
>>> On Sep 16, 2015, at 11:36 PM, ben <bfranchuk at 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
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