IDE and LBA support... Non-PC

On 23 Mar 2014, at 20:00, allison <ajp166 at verizon.net> wrote: [...] I concur that LBA is the way forwards when it comes to the data structures in your operating system, but sadly CHS is very much more common in small drives and you?re just going to have to put up with it if you need to support small disks. I recently went through all of my old disks and did wonder why I was having trouble reading some of them. It turned out that my USB-PATA bridge couldn?t cope with disks that didn?t support LBA, which turned out to be all of my tiny disks. I concluded that I don?t want to bother with small disks any more. At a guess, you want to avoid CHS because divmod operations are extremely expensive on the Z80 and also a chore to implement from scratch. However, in the specific cases of division by 255 and 63, it?s not quite as horrific as it sounds, as you can do fixed-point multiplications by 1/255 and 1/63. The binary value of 1/255 is a particularly pleasing 0x0.010101... and thus quite cheap. However disks presenting as 63 sectors and 255 heads will be relatively large and probably also support LBA, making such an approach moot. Why are you discounting large drives? If you?ve got a design limitation in your code, just ignore sectors beyond that point. Your tools that probe the drive?s capacity (i.e. the partitioner and/or mkfs) need to be aware that they may see a size beyond the limit and thus need to truncate it, but other than that you can ignore the extra space. 4.3GB is also a curious figure: it?s roughly 2**32 bytes which is how the Amiga ended up with that limitation. It?s also the number of sectors that would fit into a signed 24 bit int, but if you?re writing this from scratch, you can easily make that unsigned or extend to 32 bits to let you address drives up to 8MiB and 2TiB respectively. (Although you can?t get PATA drives anything like as large as 2TiB, and if you?re using LBA28 commands, you?ll not see space beyond 128GiB anyway.) [...] If you are trying to avoid both CHS and large disks because it?s too hard to implement, you are going to get some unpleasant surprises from other hardware as well. Were you planning to drive a printer or serial device? ;)