TTL computing

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Thu Apr 11 21:04:24 2002

On Fri, 12 Apr 2002, Tony Duell wrote:

> > > The point is, I can't see a real difference between a ROM chip and a
> > > decoder + gates. Electronically they're much the same thing. Physically,
> > > to change the program, I have to use a soldering iron. It is _not_ clear
> > > to me why one is called firmware and the other called hardware.
> > >
> > A ROM chip doesn' have firmware, since it's hard-wired at the factory. A
>
> Are you saying that a program in a mask programmed ROM is not firmware?
> Because everybody else that I know would call it that.
>
> > PROM/EPROM/EEPROM, PAL/GAL, CPLD, FPGA or whatever, that you can program to
> > suit your own definition, falls in a different class. A RAM, which doesn't
> > retain that definition between power-on cycles, falls into a different
> > category.
>
> And where do you put core memory? Or core-on-a-rope? :-)
>
> > The distinction between hardware and firmware is in that you can't tell from
> > looking at the part as you buy it, what it's going to be. If you buy a 7474,
>
> That's a dubious definition at best....
>
> If I buy a 7474, then I know it's going to be 2 d-type ff's. But I can
> choose to use those as 2 bits of an register on a CPU I want to build, or
> as a /4 clock divider, or to do the handshake for a centronics input
> port, or...


Or 1/2 as an inverter for an xtal oscillator and 1/2 for div/2
(had to do that one to avoid using 1 more package)

>
> If I buy a PROM, then again I know it's going to store (say) 256 4 bit
> words. But I can use those as a tiny bit of microcode, or as the feedback
> logic for a state machine, or whatever...
>
> -tony
>
>

Peter Wallace
Received on Thu Apr 11 2002 - 21:04:24 BST