IBM PC hacking

[Scott Stevens]

On Wed, 14 Sep 2005 00:45:56 +0100 (BST)
ard at p850ug1.demon.co.uk (Tony Duell) wrote:

> > I remember a little shop here in Santa Ana which had the first clone
> > of the IBM PC.  It was a single board which had 640k memory, allowed
> > using 64k memories, instead of the 16K memory that the PC and XT
> > earlier models used.
> 
> DId any IBM PC/ST motherboards use 16K DRAMs. Yes, I know the standard
> 
> memory mapping PROM could be set up to use 4 rows of 4816s (64K o nthe
> 
> mainboard), but did IBM ever do that? There's no mention of it in my
> TechRef.
> 

The IBM PC (PC-1) had four banks of 4116 DRAM on the motherboard.  The
first row was soldered in, the other three banks were optional.  It was
called the 16/64K motherboard.  My copy of the Technical Reference
Manual is from that generation, as it came with dad's 16/64 system.  The
rest of the 256K was in the I/O channel.

> > 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?
> 
> > not read them since they were not programmable, and needed their
> > output enabled to read the data.
> > 
> > But once someone had them in the 2716's, it was easy to get them
> > running in your superboard.
> 
> Surely it was trivial to use DEBUG or similar to dump the appropriate 
> area of memory to disk...
> 
> 
> > 
> > First systems had a 63 watt P/S, and IBM cards if you could find
> > them for video.  Also there was no floppy controller on the first
> > board.
> 
> AFAIK, no IBM PC, PC/XT, PC/XT-286, PC/AT, or PC-jr had a floppy (or
> hard disk) contorller on the mainboard. None had parallel ports
> either, and the PC-jr was the only one to have video and a serial port
> on the mainboard.
> 

Just to add more lore:
The floppy controller and floppy drive were very optional on the early
PC, and a rather pricey option.  There was a cassette cable for less
wealthy users.  And on any IBM machine up to the late XT era, if you
boot it up with no disk in the A drive or no floppy hardware at all
installed, it will boot into 'cassette basic'. (does anybody know if the
PC-AT had cassette basic resident in ROM?)  My PC Convertable will do
so.  Sadly, you can't save or load anything into 'Cassette Basic' on the
Convertable or models later than the PC as there's no cassette
interface.  I wonder if anybody has ever 'back ported' the Cassette port
to an ISA bus card??  (not that it would help on the PC Convertable, of
course.