How many of you like HP41C calculators?

From: William R. Buckley <wrb_at_wrbuckley.com>
Date: Wed Nov 19 15:21:08 2003

> > There are several architectures for computers, with all being the equal
> > of the Turing machine. Frankly, the Turing machine is the definition of
>
> Odd... None of my computers have enough memory to be equivalent to a
> Turing Machine, and I am pretty sure none of yours do either...
>
> > The writing of self-modifying code (a specialty for me) does
> require that
> > the data path have access to the code store, and not all processors
>
> The Turing machine only allows modification of the 'data memory' (tape),
> not the 'program memory' (state transition table). Of course you can make

Neither can you alter the microcode of the x86, yet it is the essence of a
computer. Also, it allows modification of programming.

The program modified on a Turing machine is contained in the Tape of
that Turing machine, not in the state set of that Turing machine. See
"Introduction to Computer Theory," by Daniel I. A. Cohen, pp 557 et seq.

> the latter act as an interpetter for instructions stored on the former,
> but if you allow that, then just about any processor that has any write
> access to memory allows self-modifying programs (it would, for example,

Exactly, and thus the distinction between environments is distracting.

> be possible to write an interpretter in Forty-One CAlculator Language
> that interpretted numbers stored in the user data registers as a program
> in some new language. Since FOCAL programs can edit the contents of data
> registers, it would then be possible to write self-modifying programs in
> this new language. Of course it would be really slow, but it's still
> possible.) I've written PIC programs that interpret the contents of the
> EEPROM memory (on a 16C84, shows how long ago it was) as a set of
> instructions to do things like toggle I/O lines, jump to a new location
> in the EEPROM, modify 'registers' stored in the PICs data memory, etc.
> The result wasn't a turing-complete language by any means, but it was a
> sort-of programming language, and it did allow (at least in principle)
> self-modifying code. The PIC itself is Harvard architacture, and the
> machine code program (the interpreter) could not be modified while
> running, but the 'user program' could.
>
> -tony

William R. Buckley
Received on Wed Nov 19 2003 - 15:21:08 GMT