| Author: | JimDrew (registered user: 23 posts ) |
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| Date: | Wed, Nov 27th, 2013 @ 17:41 ( . ) |
On your "protection info" area, you have the following text: "We mentioned before that if the drive gets more than two '0' bits in a row, it clocks in a random '1' bit occasionally." I can confirm with 100% absolutely certainty that this is not correct. It was long believed that having three 0 bits in a row caused the data to float (weakbits). That is not the case. I completely reverse engineered the 325572-01 ASIC used in the 1541 (the 1541-II and 1571 used different ASIC's so those are next). I found that the state machine allows for a 5th bit cell time zone. This was likely not deliberate (or it would have been documented). When the bit cell value exceeds 4.250s x 3, the encoding shifts and becomes 0001. This *is* a valid encoding value. You can put a lot more data on a track at the same clock rate using this method, and protections from Mindscape did just that. A good example is V-Max! track 20 looks for 5A/5A/5A/55, and then looks for 5A/5A/5A/FF, and then looks for 5A/5A/5A/37. If you decode 5A37 that is '101101000110111'. Notice the 3 zero bits in a row... that's valid and it never changes no matter how many times you re-read that data. This was definitely not documented anywhere at Commodore or any other source as far as I know. So, for weak bits to occur on a CBM drive, you must have 4 or more 0 bits in a row, not 3 as we all once thought. So, if the entire track is written with a single bit cell time that decodes as 0001, you are looking at a very long track. There are many games I have seen already that use 0001 in their custom format encoding. The 1541-II and 1571 also have special properties handling data that exceeds the clocking window for a bit cell time, returning at least one nibble with a 0 and most often the other bits are 0's as well. When I get those ASICs reverse engineered, I can give a full description of how that clocking mechanism works. |
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