6 Oct
2009
6 Oct
'09
12:07 a.m.
On Oct 5, 2009, at 3:00 PM, Phill Harvey-Smith wrote:
Dave McGuire wrote:
There were 3 of us (I was one...I can go I'd have to think about who
the others were) at IBM that defined the mapping to ~512MB. We knew
at the time that this was a limit but we figured PCs would switch to
an LBA format before we hit the limit...we were wrong. :-)
TTFN - Guy
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