I've got the DOS Manual, and I used to have Beneath Apple DOS... If you want more detail than that, you might have to email Woz... I mean, he designed the whole thing... He may be the only one who knows... And, being Woz, he might even answer you. Ah, a *real* OS... I'm getting verklempt... Mark -----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Vintage Computer Festival Sent: 21 March 2005 17:19 To: Classic Computers Mailing List Subject: Good description of Apple ][ disk controller I am pretty surprised that I can't find any good technical descriptions of the Apple ][ disk controller. It's not in the DOS manual and, come to think of it, I can't think of any publication Apple came out with that describes it. I wonder why this is? Does anyone have any good suggestions for reference material on this subject? I've got Beneath Apple DOS, which is of course excellent, and the Bag of Tricks manual (which rehashes a lot of what's in BAD), but nothing outside of those really describes the disk controller circuitry at an even lower level. I want to know what's going on in the PROMs, and why the certain rules of encoding data are why they are (i.e. first bit of every disk byte must be set, no more than one set of consecutive zeros in any byte, etc.) -- Sellam Ismail Vintage Computer Festival ------------------------------------------------------------------------ ------ International Man of Intrigue and Danger http://www.vintage.org [ Old computing resources for business || Buy/Sell/Trade Vintage Computers ] [ and academia at www.VintageTech.com || at http://marketplace.vintage.org ]

Sellam wrote: Because Apple considered it somewhat proprietary. Though there's a patent on it: http://www.brouhaha.com/~eric/patent/us/4210959.pdf There's also a patent on the later IWM chip, which was basically a single chip version with a few improvements: http://www.brouhaha.com/~eric/patent/us/4742448.pdf You want the book "The Apple II Circuit Description", or "The Apple IIe Circuit Description", by Winston Gayler. Out of print, of course, but you can get a copy for the low, low price of only $250 from an Amazon used seller. The bits shift in from the right, and there's no counter. The only way you can tell that the complete "nybble" has been read is that the MSB of the shift register is set. The restriction on no more than one consecutive zero (with the older 13-sector state machine), or no more than two consecutive zeros (with the newer 16-sector state machine) is required because the state machine cannot reliably discrimiate the length of a long period of time with no transitions. For example, it couldn't accurately distinguish four empty bit times on a drive that was slightly fast For the 13-sector state machine, there could never be two consecutive zeros, and there were 34 valid nybbles that met that criterion. Two of those, D5 and AA, were used for provide a unique pattern for the start of address and data marks. The remaining 32 are used to encode 5 bits of data (2**5 == 32). The 16-sector state machine was redesigned to allow it to accurately discriminate slightly longer delays between transitions, allowing the use of two consecutive zero bits, but never three or more. There are 81 valid nybbles with that constraint. This is not enough to encode seven bits, but it is enough for six (2**6 == 64). Even with the extra two codes used for marks, only 66 nybble values are required, so 17 of the 81 have to be chosen as invalid. By adding the somewhat arbitrary prohibition of two double-zeros in a nybble, the number of valid nybbles is reduced to 72, which is still 6 more than necessary. A few more are also eliminated; I don't recall whether there was an easily stated criterion that was used to eliminate the remaining 6. When I say "somewhat arbitrary", that's not completely true. One of the criterion for choosing which of the 81 possible nybbles should be used for the actual data encoding is that it had to be possible in a very small amount of 6502 code to generate the decode table, because that had to be done in the 256-byte boot PROM along with the rest of the code needed to enable drive 0, recalibrate (seek to track 0), find sector 0, read it in, and jump to it. Both the 13-sector and 16-sector disk boot PROMs are amazing examples of virtuoso 6502 programming. But then, basically everything about the Disk II controller design and software is incredibly amazing. Eric

In message <Pine.LNX.4.33.0503210915440.3943-100000 at siconic.com> Vintage Computer Festival <vcf at siconic.com> wrote: I did a massive search a few years ago for exactly what you're looking for. IIRC I ended up with about 20MB of stuff, which is all on backup CDs at the moment. I don't think I ever found a detailed circuit description or a copy of the PROMs though. I could fish out the CD and put the stuff I've got online if you wanted to take a look at it. Speaking of PROMs, I still haven't found a source of bipolar PROMs.. those things seem to be like rocking horse manure. I even tried a couple of "obsolete component specialists" - all turned a blank. I think TI still make them, but only for the military, and they were about $40 each last time I checked. Later. -- Phil. | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB, philpem at philpem.me.uk | ViewFinder, 10BaseT Ethernet, 2-slice, http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI ... He's alive, Jim. Should I shoot him again?