Re: DOS 3.3 Nibble Encoding

[jpenne@ee.ualberta.ca (Jerry Penner)]

k_bouchard@csc32.enet.dec.com (My name is...) writes:

>Am I out in left field on this? All this verbage to explain something basic? I
>always thought that *any* computer boots from sector0-track0 because early in
>the boot process of *any* computer,a RESET signal is sent out on the computer's
>I/O bus (or whatever bus the various controllers sit on) and this "RESET"
>results in all registers being set to zero. (including both sector and track
>registers) 
>Are Apples different in some way?

The disk controller card has a small amount (256 bytes) of firmware on
it which is a 6502 program that reads track 0 sector 0.  This happens
quite a ways after the computer has restarted from a RESET condition
(quite a ways being a relative term, but it can be up to several seconds
on a IIgs).  If Apple had wanted to make the controller card read
track 22, sector 9, they could have.  It would have been a rather
arbitrary and strange decision however.  There is no "track" or
"sector" register, per se.  Most computers' peripherals have some
firmware on them to allow them to boot.  Either that or the firmware
ROM built into the computer itself has to "know" how to boot various
types of peripherals and the peripheral has to be able to indicate to
the system what type of I/O device it is.
-- 
Jerry Penner   jpenne@ee.ualberta.ca   or   jjp@myrias.ab.ca