IBM PC hacking

[Steve Thatcher]

the original IBM PC Bios was a Motorola 24 pin ROM. In the early 
days, I used a 28 pin socket and wired to a 24 pin header to be able 
to modify the bios and use a standard 28 pin EPROM for changes. There 
were no latches in the ROM. It was probably cheaper for IBM to get a 
masked ROM then to use EPROMs which were relatively expensive for the 
quantity they were buiding. There was no EPROM equivalent for the 
Motorola chip.

best regards, Steve Thatcher

> > >
> > > > The PC used either EProms, (16K I think) and the Roms that were
> > > > shipped with the BIOS were registered.  The standard Data I/O would
> > >
> > > Do you mean there were internal data latches in the IBM ROMs?
> > >
> >
> > Yes, as I understand it and saw, the roms could be enabled, using a
> > line that the data I/O didn't drive (29b variety any did not).  So you got
> > FF  out when you read them.
>
>I can believe there were some odd enables on those ROMs, but I wasn't
>aware of any internal latches. I will have a look again.