Re: Where does the "16 SECTOR" message come from?

[dempson@actrix.gen.nz (David Empson)]

Tristan Mumford <xtristan.xmumford@xgmail.xcom> wrote:

> Michael J. Mahon wrote:
> 
> > <Whack on side of head>  Thanks for reminding me that it *has*
> > to bank switch the slot space, since that's the only way it
> > can be compatible!
> > 
> > So the only different code is the "reset" 256 bytes, and
> > after it runs, the slot code will be identical to either
> > the 13-sector boot ROM or the 16-sector boot ROM...
> 
> Unless I'm mistaken, which is not entirely unheard of, I should just be able
> to attack it by just poking to 0x180 (sorry about the C style hex notation.
> Force of habit) and BSAVEing each 256 byte page. Sure they are probably
> duplicates of the Apple disk ROMs, but it can't hurt to look, right?
> 
> Jorge said that it had a 2k ROM. I'm going to believe this statement. It
> also means I'm curious of its contents, and how much of it can be accessed.
> 
> I'd be surprised if it's more than just the 3 pages, but curiosity drives
> me!

2KB is the smallest EPROM size that was readily available at the time (a
2708 or equivalent). They are probably only using three pages.

The rest may be addressable: it is likely to be a multi-bit latch
written at $Cx80, with the bank register being cleared by a hardware
reset. Bank 0 will therefore be the reset code, and it seems that bank 2
and 3 contain the 16/13 sector boot code. Bank 1 will probably exist but
be unused (unless needed to extend the code in bank 0) and banks 4-7
might be unimplemented, leaving half the ROM inaccessible.

Oh well, I'm sure someone will find a use for my "copy the C8 area" code
snippet, and it will be in the archives. :-)
-- 
David Empson
dempson@actrix.gen.nz