Re: DOS 3.3 Nibble Encoding

[sbdocker@acs.ucalgary.ca (Sean Brendan Dockery)]

In article <pingn.753398237@sfu.ca> pingn@malibu.sfu.ca (Albert Ng)
writes:

| sbdocker@acs.ucalgary.ca (Sean Brendan Dockery) writes:
| 
| >It is not explicitly stated in the Beneath Apple DOS manual as to how
| >the boot sector is encoded.  For the same drives to boot both 13
| >sectors/track DOS 3.2.1 disks and 16 sectors/track DOS 3.3 disks, it
| >seems rather unclear as to what encoding scheme would be present in
| >the firmware to overcome.  Can the Apple IIc and Apple IIGS internal
| >firmwares even handle booting DOS 3.2.1 disks directly?
| 
| Well, are you refer[ring] to some dual boot disk ?
|
| If so than a custom Dos 3.3 Track 0 Sector 0 is created and it's almost
| like the image of Dos 3.2 Boot Rom.  
| 
| So the Dos 3.2 , Track 0 Sector 0 is loaded  and proceed as usual.

Well, you lost me with what you were trying to write here, but I'll
try to elaborate on what I wrote above.

First, the boot sector obviously conforms to the format of the rest of
the disk.

My question refers to the fact that the disk controller ROM won't be
able to decode DOS 3.2.1 and DOS 3.3 formatted disks in the same
manner as they use different encoding schemes.

I'm not sure if the built-in disk controller ROMs can handle the
decoding (let alone identifying) both encoding schemes.  The basic
question is whether or not there is enough room for both algorithms.

| One thing that puzzled me is, how can I differentiate from a Dos 3.2 sector 0
| or a Dos 3.3 sector 0 ?? 

This is the exact problem that I am referring to above.  Unless the
disk controller is explicitly trying to identify a DOS 3.2.1 verus a
DOS 3.3 format disk, the encoding format (5 and 3 for DOS 3.2.1 and 6
and 2 for DOS 3.3) is unknown.  I imagine that decoding a 5 and 3
encoded sector with the 6 and 2 decoding algorithm would result in a
checksum error anyway.

It is the same thing that is puzzling me.  :-)

| Remember Dos 3.2 's address mark is D5AAB5, while 3.3 is D5AA96 . That's how
| things works. 

My documentation did not mention any differences in the prologues of
DOS 3.2.1 and DOS 3.3, so this is "news to me."
-- 
Sean Dockery

dockery@griffin.cuc.ab.ca
sbdocker@acs.ucalgary.ca