XTIDE controller now available

Phill Harvey-Smith afra at aurigae.demon.co.uk
Mon Oct 5 17:00:33 CDT 2009

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.



Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.

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