IBM PC hacking

[Tony Duell]

> Just to add more lore:

And I'll add some more, I still use such machines...

> 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

The cassette port pinout ios the same as that for a TRS-80 (M1, M3, M4, 
M100, CoCo, etc) (and for that matter a Dragon). The same cable works 
with both.

> 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

Yes it does. AFAIK all IBM PC, PC/XT, PortablePC (which is the same 
mainboard as the PC/XT) and PC/AT machines have ROM BASIC. I am typing 
this on a much hacked 8MHz IBM PC/AT, and it has BASIC in ROM.

> 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.

>From what I remember, cassette I/O is handled via one of the software 
INTs (INT 15, maybe, I can check). The routines are only present in the 
PC BIOS, on all later machines, the routines return (maybe setting an 
error flag, I forget). 

But there would be no reason that a BIOS extension ROM couldn't intercept 
this INT vector and perform suitable I/O, either to a cassette recorder 
or to, say, a paper tape punch/reader. I have sort-of thought about 
making an ISA card for the latter with the necessary ROM on it, but I 
haven't got a round tuit.

-tony