Re: what sector mapping is this?

[martin_haye <martin.haye@gmail.com>]

Hi Jonno,

My bad! Here's how I found it...

I just did a hex dump of the file http://bitbucket.org/martin.haye/super-mon/downloads/Super-Mon%201.1.dsk
and compared that with an in-memory image generated by reading track
zero this way:

0280-G   20 40 03    JSR  NAKEDOS
0283-H   6D 41 DC    ADC  $DC41
0286-I   A9 00       LDA  #00
0288-J   A9 00       LDA  #00
028A-K   2C 00 20    BIT  $2000
028D-L   A9 10       LDA  #10
028F-M   D8          CLD
0290-N   60          RTS

For each sector from $2000.2FFF I looked at the first few bytes and
scanned the hex dump for those bytes. Here's the mapping I obtained:
(Physical sector  ->  Sector in file)

0->0
1->7
2->E
3->6
4->D
5->5
6->C
7->4
8->B
9->3
A->A
B->2
C->9
D->1
E->8
F->F

This mapping looks to be DOS 3.3 sector interleaving according to
http://mirrors.apple2.org.za/apple.cabi.net/GS.Emulators/A2.Emulator.Image.TIL.txt

It didn't however match the offsets you found. However, if I
subtracted one sector number from yours, it suddenly matched up (i.e.
sector $09 at offset $300, sector $0A at offset $A00, and sector $0B
at offset $200). So then I asked myself why you thought sector A is
the first catalog sector and a little investigation turned up the
answer: the docs are wrong! The online help states that the catalog
occupies sectors $A through $C, when in reality it occupies 9 through
$0B.

My apologies for leading you astray, and I'll fix the documentation
shortly.

--Martin