On Thu, Sep 14, 2017 at 2:47 PM, Warner Losh <i...@bsdimp.com> wrote:
> > > On Thu, Sep 14, 2017 at 2:44 PM, Paul Koning <paulkon...@comcast.net> > wrote: > >> >> > On Sep 14, 2017, at 4:39 PM, Warner Losh <i...@bsdimp.com> wrote: >> > >> > ... >> > I don't see where you read the first 2 tracks uninterlaced, and the >> other tracks interlaced? For DOS formatted disks, that's what's required. >> While screwing up the first two tracks won't affect your ability to read >> DOS floppies (since they were reserved for the boot blocks), other formats >> aren't so forgiving and it makes the disk unbootable... >> >> The question was about RX50 disks, which are uniformly interlaced and >> skewed. > > > No. They are not. CP/M MS-DOS on the Rainbow didn't do it uniformly. Venix > on the Rainbow did it a different way. They are not uniform, and the first > two tracks were not skewed. The DECMATE did it differently (not skewing > tracks 78 and 79). I'm unsure how other OSes handled things, but it was > anything but uniform. > > I wrote IMPDRIVE back in the day to read 3.5" floppies and had to cope > with this. I have the BIOS listing for MS-DOS that shows clearly that the > first two tracks weren't skewed. I also have the CP/M listing, but don't > have it as handy. I'm quite certain of the non-uniformity. > I also have 4 real DEC Rainbows 100's in my basement (two 100As and two 100Bs), as well as 300 imaged disks (and the physical disks as well) that needed exactly the kind of skewing I talked about to be readable by other utilities. Warner
