From: Ethan Dicks (ethan.dicks_at_gmail.com)
Date: 2007-03-22 21:18:57
On 3/22/07, Anders Carlsson <email@example.com> wrote: > Ethan Dicks wrote: > > > back in the day, it was more than just C/H/S issues > > A Microsoft (!) KB article on MFM/RLL drives: > > http://support.microsoft.com/kb/36948 > > Based on that document, we have: > > Tandon 602S: 4 heads, 153 cyls, RWC 128, WP 153 > Tandon 603S: 6 heads, 153 cyls, RWC 128, WP 153 > > Seagate ST-225: 4 heads, 615 cyls, RWC 615, WP 300 > Seagate ST-251: 6 heads, 820 cyls, RWC 820, WP 820 > Seagate ST-506: 4 heads, 153 cyls, RWC 128, WP 128 That all looks familiar. > Generally, does a disk need to have a "minimum" RWC and WP in order > to theoretically work as a replacement? The ST-506 above has a lower > Write Precomp number than the Tandon disks, while both the ST-2XX > disks surpass all specs by far. The RWC and WP refer to cylinder numbers where these parameters kick in. In the case of the ST-251, there is no Write Precomp (820 == 820), and neither is there with the TM602S and TM603S (153 == 153). The problem is that starting at cylinder 128, the ST506 expects the controller to do write precompensation. I'm a big foggy on the specific mechanism, but it has to do with timing and bit density on crowded or looser tracks. In any case, you won't have reliable operation if the drive expects it and the controller doesn't provide it. In a desktop PC of the era, you told the controller what to do (within limits) and it did it. In the case of the D9060/D9090 drives, there's no mechanism to turn that on or off at a certain point. There _is_ a head-count jumper (4 vs 6), but it only changes the geometry table. I do not know how one tells a SASI<->MFM controller how to engage or disengage WP, but since the 6502 ROM code presently does not manipulate that parameter, if it _is_ possible to twiddle it from the 6502, the twiddling certainly won't fit as a patch amidst the present firmware. If one had compilable source, it might be possible to cut a new ROM rev, but I haven't seen a treatment of the D90x0 firmware at that level of detail. In practice, the thing to do is to find any drive that does not require write precomp below cyl. 153. Newer drives than the ST506 and the TM602S don't get hung up on reduced write current, which is why an ST-225 or ST-251 still work. They are internally smarter than an ST506. Different drives than that might or might not do well. Sorry there isn't an easy answer for this, but the ST506 is sort of in a class by itself. -ethan Message was sent through the cbm-hackers mailing list
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