I have a new harddisk(raw, without any formatting, 80GB)

I am going to connect it to hdb. When I run fdisk, it said the disk has:
255 heads, 63 sectors, 10011 cylinders

I didn't format it at this moment, but I try an experiment to connect it hdc

I run fdisk again, this time it said the disk has:
16 heads, 63 sectors, 159560 cylinders


Why fdisk gives me different setting?
If there any difference for me to format in these two ways?

Strange, look at /proc/ide/hd?/capacity to see what it says ...

I have two harddisks, they are exactly the same.
Then I connect them to be hda and hdc.

I looked at the /proc/ide/hda/ and /proc/ide/hdc
in the "capacity", they are the same.

But in the "geometry"
hda shows:

physical 159560/16/63
logical 10011/255/63

but hdc shows:

physical 159560/16/63
logical 159560/16/63

I guess that fdisk just read the data from this file, so fdisk give me different reading on the same harddisk.
But why the geometry file have different reading? or said "why Linux regards the same disk with different settings?"

Thanks

The physical is the value that kernel uses, and the logical is a value from the BIOS. Maybe something related to LBA use, but there's a situation that the BIOS is ignored:

<from: linux/drivers/ide/ide-geometry.c:74>
if ((cmos_disks & (0xf0 >> (unit*4)))
&& !drive->present && !drive->nobios) {
unsigned short cyl = *(unsigned short *)BIOS;
unsigned char head = *(BIOS+2);
unsigned char sect = *(BIOS+14);
if (cyl > 0 && head > 0 && sect > 0 && sect < 64) {
drive->cyl = drive->bios_cyl = cyl;
drive->head = drive->bios_head = head;
drive->sect = drive->bios_sect = sect;
drive->ctl = *(BIOS+8);
} else {
printk("hd%c: C/H/S=%d/%d/%d from BIOS ignored\n",
unit+'a', cyl, head, sect);
}
</from: linux/drivers/ide/ide-geometry.c>

But, from the code you quoted, my harddisk will use the bios setting. (not in ignored case since the sector is 63 only).