How many of you like HP41C calculators?

From: Tom Jennings <tomj_at_wps.com>
Date: Tue Nov 18 17:06:17 2003

On Tue, 2003-11-18 at 14:06, Eric Smith wrote:
> Tom Jennings wrote:
> > Strictly-speaking, Microchip's PICs are NOT COMPUTERS. Of course I don't
> > make that distinction when working with them.
>
> I strongly disagree with any definition of "computer" that *requires*
> the machine to have a von Neumann architecture. In fact, most people
> do not even define computer to necessarily be of the stored program
> variety, although I'm willing to accept such a definition.
>

Oh I don't mean to be that insistent about it, especially in practical
terms, but it is the pivotal definition of computing machinery, the
machine that modifies itself, which is what makes it breathtakingly
unique. It's an important distinction to be ABLE to make, not
necessarily on objects we work with though, I agree.


> The earliest PICs (eg. the NMOS PIC1654 from General Instruments in
> the 1970s, and the CMOS PIC16C54 from Microchip in the 1980s) do have
> a strict Harvard architecture. Program memory is in an entirely
> separate address space from data memory, and they have no way for the
> program to be able to read or write an arbitrary location in program
> memory. Table lookup works because there is a way to do a branch to
> a computed address (though it is limited to only a portion of the program
> address space), and there is a single instruction that loads the
> accumulator with a constant and returns (RETLW).

Yup. There's a little blurriness in the Microchip PICs, in that they
have a stack that contains return addresses, but some machines, like the
ancient Signetics 8x300, didnt. It had what appeared to be "subroutine
calls" but they were basically numbered jumps managed by the assembler,
and there were a limited number available.

I cannot recall the mechanism used, and would really like to know if
anyone recalls, how the assembler managed the "return" jump tables!
Received on Tue Nov 18 2003 - 17:06:17 GMT