lai@vedge.UUCP (David Lai) (08/24/88)
In article <1223@nunki.usc.edu> mcvey@sal6.usc.edu (Stephanie Salsbury Mc Vey) writes: >In the hardware reference manual for the amiga there is a part about the >registers that control the internal disk . in one of these register >(adcon ? I forgot the name ...) there are bits assigned named >PRECOMP (these are 2 bits) and they have the following lines > 00 ... > 01 150 nsec > 10 450 nsec > 11 <some number> nsec > >well my question is : what do they do ? I think that they are timing selectors, >yes , but for what ? >wait time between each bits to read from the disk ? time needed to toast some >bread if stuck in the disk drive ? > > > Run DMC VEY Here's my shot at it: Precomp is used by disk controllers to compensate for a phenomena that causes magnetic domains to shift. Think about putting 2 magnets' same polarities close. The bits are recorded on disk are very close together, and shifting occurs. If uncompensated, this shifting causes data to read back badly. On the inner tracks this is more apparant because the bits are packed closer together. Precomp(ensation) is simply adjusting the timing of the write data (on specific patterns, like 2 closely spaced pulses) by making them even closer, so when natural shifting occurs, the data read back will be approximately what we originally intended. The precomp number is the time of pre-shift done to the bits prior to writing on the disk, and is a function of the track #. Floppy disk controllers vary the precompensation depending on the current track. Precomp numbers can be obtained from the drive manufacturer, eg: tracks 0-40 none tracks 41-79 150ns (not real numbers for amiga drive) Many drives have built in precomp, which switches on when a certain track is reached, so the comtroller need not worry about that. Well... hope that answers your ques. BTW, normally our site doesn't receive articles with usa distribution... why not use na instead? -- "What is a DJ if he can't scratch?" - Uncle Jamms Army The views expressed are those of the author, and not of Visual Edge, nor Usenet. David Lai (vedge!lai@larry.mcrcim.mcgill.edu || ...watmath!onfcanim!vedge!lai)