Re: Questions about IIGS, hard drives, and booting different OSes...

["David Wilson" <mcs6502@gmail.com>]

David Empson wrote:
> It has been a while since I looked into this, but I think the
> fundamental problem is that there is no information available to the
> PRODOS bootstrap loader which tells it the drive from which the file was
> loaded. It can find out which slot was being used courtesy of some zero
> page variables left behind by the block 0 bootstrap loader (which have
> to be replicated if the PRODOS file is launched directly).

ProDOS 8 can boot from drive 2 in one circumstance (that I know of).
According to the IIc technical reference manual, a IIc with the
original ROM and an external 5.25" drive supports this functionality.
It does this with PR#7 which results in the following code being
executed:

c700 dfb $ff ; make it look like slot is empty (1 byte NOP on 65C02)
c701 lda #$e0 ; slot 6 drive 2
c703 ldy #1 ; drive 2
c705 bra $c6fb
c6fb jmp $c60b
c60b stx $2b ; SLOTZ
c60d sta $4f ; BOOTDEV

It looks like the Zero Page location BOOTDEV ($4F) is where the ProDOS
8 boot block code looks to find the boot device number and if  the high
bit is set it knows it was booted from drive 2. I do not know of any
other controller cards with firmware that support booting from anything
but drive/partition 1. I am sure it could be written for the CFFA
firmware if anyone wanted it.