The reason why the skew is necessary is that the Apple II can not read a 
sector and get the data decoded before the next sector passes underneath 
the drive head.  (Disk continues to rotate.) So the sectors are layed 
out during formatting to allow 1 or two secotors between so when the 
comptuer is ready, the next sector is about to pass under the read head. 
  If you change the skew, then a different sector will be there and 
you'll have to wait for the disk to rotate the wanted sector under the 
head.  It is alo true that if you chnage the DOS or read routine, the 
timing will be different and a different skew may work more efficiently.

Physicaly the sectors are in sequence around the track
A-B-C-D-E-F-G-H-I-J-K-L-M-N-O-P
But are numbered in this sequence according to Bill:
0-7-14-6-13-5-12-4-11-3-10-2-9-1-8-15

With this layout, it looks like 12 secotrs pass by before the next 
logical sector number is physically under the head.  This gives the RWTS 
program time to decode the sector data and move it in memory before it 
needs to read another sector.  If they were physically adjascent, then 
the copmputer would have to wait for another 16 sectors instead of 12 to 
pass by and would slow down the read process even more.

Now that I've counted, this does not look correct.  I think the skew is 
closer to 3, not 12 secotrs...
A-B- C-D-E- F-G-H- I-J-K- L-M-N- O- P
0-11-6-1-12-7-2-13-8-3-14-9-4-15-10-5


If the Rom loader program can read sectors that are physically 
adjascent, then by all means modify the format program to number them 
physically adjascent so you can use standard utilites to put the data in 
  'logical' (and now adjascent) sectors.

A-B-C-D-E-F-G-H-I-J-K- L- M- N- O- P
0-1-2-3-4-5-6-7-8-8-10-11-12-13-14-15

It is probably best to not ignore the sector number when reading, but 
rather force your program to do what the boot loader does and copy the 
Rom sector read code and modify it to look for a specific sector. 
Otherwise you may not be gettting the sectors in the order you wish.

My 2 cents

Thankx,
Ed

> Jim Cullen wrote:
>>So, If I use RWTS to write sector $07, then the boot ROM would read
>>Physical $01?
>>Then It should work... I hope.
>>Can I assume that this table is true for every sector?  Probably a
>>dumb question!
>>If I'm using portions of the Boot ROM code to read from the disk, to
>>read sector $07, I would tell the code to read sector $01 instead and
>>vice-versa?
> 
> 
> Exactly.
> 
> -michael
> 
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