On Sat, Feb 27, 2010 at 8:53 AM, walt <w41...@gmail.com> wrote: > On 02/26/2010 06:23 PM, BRM wrote: >> >> ----- Original Message ---- >> >>> From: Mark Knecht<markkne...@gmail.com> >>> On Fri, Feb 26, 2010 at 4:09 PM, walt wrote: >>>> >>>> Is there really any need for the "cylinder" these days? > >>> Who cares what cylinder it's on, and >>> who cares which head is getting the data? It doesn't matter to us >>> users... > >> ...Boot Loader writers (e.g. grub) need to care about it since LBA > >> is not quite available right away - you have to focus on other things >> until you can load the rest of the boot loader. > > Ah, this may be a big part of what's confusing me because I've done a > lot of playing around with grub. > > At what point *does* LBA become available, and who makes it available? > Is this one of those stupid BIOS things? >
I don't think that it's specifically a BIOS thing, but BIOS is involved at least on older machines. LBA is always 'available' (as far as I know) because it's in the hard drive. It's the way the drive expects the system to talk to it. The issue is how do the software layers talk with the drive. In the case of very old PC hardware, and I think it still exists for compatibility reasons, programs used to use a IBM BIOS call - Int13 I think - to talk to the drive at all. I'm not a software guy, and certainly not an IBM BIOS assembly language guy, but it may be that the Int13 call to BIOS required CHS. I don't know. Basically you have to make some sort of call to find the drive and get things moving. If BIOS enables LBA under the hood, which I expect it does these days, then the Int13 call using CHS gets converted to LBA and sent across the cable. The drive is however (I think) ALWAYS responding to LBA. There's not a lot of reason for them to support anything else excpet for internal testing. Note that since CHS to LBA conversion is just a bunch of integer multiples and adds if the system BIOS takes an Int13 call with CHS as input it's simple for it to convert it to LBA, and since the important part is sector alignment which is the least significant part it's the one that most closely matches with the numbers in the LBA. I'm pretty sure that once your system starts booting the consistent use of LBA through the whole stack happens as the kernel gets itself and appropriate drivers loaded, but again, this is all supposition on my part. Once those are up and running it's probably LBA completely and CHS isn't important. Note that for big drives it seems that fdisk will __always__ tell me the drive has 63 sectors/track and 255 heads but I sincerely doubt that's true physically. Hope this helps, and I hope I'm not too far off base. Read more here and help me understand better: http://en.wikipedia.org/wiki/Logical_block_addressing http://en.wikipedia.org/wiki/INT13 Cheers, Mark
