5 Oct
2009
5 Oct
'09
3 p.m.
Dave McGuire wrote:
I don't know where any supposed 520MB limit
comes from; if someone can
enlighten me there, I'd appreciate it.
IIRC it's from the fact that most BIOSes can only deal with a 6 bit
sector number thus :
AH = 02h
AL = number of sectors to read (must be nonzero)
CH = low eight bits of cylinder number
CL = sector number 1-63 (bits 0-5)
high two bits of cylinder (bits 6-7, hard disk only)
DH = head number
DL = drive number (bit 7 set for hard disk)
ES:BX -> data buffer
Copied from Ralf Brown's Interrupt List
So 1024 cylinders x 16 Heads x 64 sectors is 512MB Later bioses I
believe used the top bits of the head number to get greater capacity.
Cheers.
Phill.
--
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
"You can twist perceptions, but reality won't budge" -- Rush.