Re: How to Boot 2nd drive?

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

Tim Shoppa <shoppa@alph02.triumf.ca> wrote:

> In article <550u69$528@darla.visi.com>, Nathan Mates <nathan@visi.com> wrote:
> >   Standard duodisks on a //e can't swap drives without some hardware
> >trickery in the form of moving the drives around, swapping lines on
> >the cable, or mucking around with the EPROMs.
> 
> EPROM's?  You lost me there.  Where is there an EPROM in a Disk II
> and controller?  All I know about is the 256x8 bipolar PROM that
> the boot code resides in.

There are two bipolar PROMs on the Disk ][ Controller: the "P5" and "P6"
PROMs.  One of them (P5) contains the 256-byte boot code for the Apple
][, and the other (P6) contains the logic state sequencer which
implements most of the low-level disk access (such as clocking bits
through for reading or writing a single byte).

By modifying the boot code PROM, you could arrange to boot off drive 2,
but only with ProDOS.  DOS 3.3 will revert to drive 1 unless patched.

[This PROM is replaced when a 13-sector disk controller is updated to
boot 16-sector disks.]

Modifying the logic state sequencer wouldn't help, because the drive
select lines are controlled directly by the Apple II accessing $C0nA or
$C0nB (these set the state of output Q5 of the 74LS259, which is used to
select either of the two drive enable lines).

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand