Hi Jack, thanks for the explanations - adding some "history": > Second, file I-O done by other DOS programs uses either 24-bit CHS > requests (up through V6.22 MS-DOS) or 48-bit LBA requests (all new > DOS variants including FreeDOS).
Looking in the far past, CHS was once about the real number of cylinders, heads and sectors-per-track of your harddisk. Later, harddisks started to have this "geometry" configurable and just mapped to whatever they had in hardware. The geometry would go up to 1024x256x63 or 8 GB. The number of heads in actual IDE is limited to less than 256 and some software had overflows if you had more than 254 or 255 "heads". Actual IDE has, as far as I remember, at most 16 heads. However, it has a few bits free and some implementations used them to have e.g. up to 4096 cylinders. Later, LBA came which just numbers sectors from 0 to N without any "geometry". You can have 28 bit or 48 bit LBA, but the BIOS interface is 32 bit or 64 bit. Most versions of DOS, because of the partitioning scheme being limited to 32 bit, can only use a 32 bit sector number. Also, as Jack said, it would be quite hard and incompatible to let DOS support more then 2^32 sectors per drive letter so even with modern post-32-bit partition schemes DOS would not allow your C: drive to be more than 2 TB but you could just add a D: of 1 TB to fully use a 3 TB harddisk then. A related issue is that some BIOSes know only 28 bit LBA or do know 48 bit LBA but only process the low 32 bit of those 48 bit. In the last 20 years, there were also BIOSes with problems above 0.5 GB, 2 GB, 4 GB, 8 GB and 32 GB - in addition to the obvious limits of physical CHS 504 MB, CHS interface 8 GB, LBA28 128 GB or 134*10^9 byte). So the current issue of 2 TB is not very new, it is just a bit unusual because this time the partitioning and several operating systems reach some limit even if you happen to have a BIOS which would support disks above 2 TB in size. Regards, Eric PS: As Jack also mentioned, there is a general problem in knowing what the current sector size is, so many pieces of software which can include the BIOS itself can get confused with 4 kB/sect disks. ------------------------------------------------------------------------------ Xperia(TM) PLAY It's a major breakthrough. An authentic gaming smartphone on the nation's most reliable network. And it wants your games. http://p.sf.net/sfu/verizon-sfdev _______________________________________________ Freedos-user mailing list Freedos-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-user
