Re: mac kybds on gs

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

Henrik 'Ratte' Gudat <GUDATH@EZINFO.VMSMAIL.ETHZ.CH> wrote:

> In <50q55g$gmu@login.freenet.columbus.oh.us>
> rschaefe@freenet.columbus.oh.us writes:
> 
> > Just outta curiosity, will the power-on key of a Mac keyboard work to
> > turn on a gs?
> 
> Which reminds me of something I've seen in the hardware ref 2nd edition...a
> flag where you could turn off the GS. (Or was it read-only? Where's the point
> :-) ). Maybe Apple intended to squeeze a soft power supply into the Mark
> Twain?

There is a hardware flag on the 1MB (ROM 3) motherboard which indicates
whether the computer has just powered up.  The bit can be set or cleared
by software, so the intended sequence of operations is:

1. On power up, the bit is set.
2. During reset handling, the firmware or operating system tests the bit
to identify whether this was a powerup.  The bit is then cleared.

On subsequent resets, the bit will be clear, so a powerup can be
reliably detected, and distinguished from any other reset.  The software
can set the bit again, so it is possible to trick the firmware/OS into
thinking it has powered up again.

The flag is accessed via bit 6 of the SPEED register ($C036).

I don't know if this bit is actually used by the firmware.  It appears
to be cleared during reset: set the bit, then reboot into ProDOS-8,
launch BASIC.SYSTEM and PRINT PEEK(49206); the value should be 128
(fast) or 0 (normal).  If the "power on" bit is set, then the value is
192 (fast) or 64 (normal).  Other values will appear in this location if
you have any 5.25" disk controllers (including the built-in IIgs disk
port).

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