Re: Disk II Family's Phase

["Charlie" <charlied@NOSPAMbboard.com>]

"Bryan Parkoff" <BParkoff@satx.rr.com> wrote in message
gf6cnUFJ3aW3uc2jXTWQkQ@giganews.com">news:gf6cnUFJ3aW3uc2jXTWQkQ@giganews.com...
>     It seems to be interesting that standard Disk Drive only uses Phase0
> that is turned on for reading and writing, but all Phase0, Phase1, Phase2,
> and Phase3 are always turned off.  I wonder why Phase1, Phase2, and Phase 3
> are not used for turning on.
>     Please explain what Phase do.

>I understand that it is used for stepping
> arm to move from outer toward inner.

> Is it true that stepping arm always move from outer toward inner by
> reading SELECTIVE track and sector?

It's been a long time since I delved into the way the track seek routine works
but if I recall correctly all four phases are turned on and then off one at a
time to move the arm one track.  I believe this was done by incrementing or
decrementing the X register and then doing a LDA $C080,X.  Incrementing moves
the arm inward and decrementing moves it outward, I think.  I believe the
timing of these moves must be right although I don't know what it is supposed
to be.

> Does it mean that software has NO WAY
> TO TELL hardware to LOCATE track and sector.

Well, by pulling the arm as far out as it will go I believe the arm would be on
track zero (or near it).  By moving the appropriate number of phases you could
move to any track and reading the address headers would tell you if you are
right.  The software of course would store the track number so that it "knows"
where it is for the next seek.

> The hardware's job is to
> retrieve each bit by reading from outer toward inner.

I'm not sure what you mean here but if you are referring to the LDA $C080,X
(phase on/off) the read in this case is only a softswitch.  The value placed in
the accumulator is meaningless.

> In fact is that it is
> the software's job to SEARCH track and sector before track and sector are
> compared to match user's desire track and sector BEFORE 256 bytes in a
> sector dump into memory.
>     Does it make sense?

Yes.

Charlie