XTIDE controller now available

[Guy Sotomayor]

On Oct 5, 2009, at 2:10 PM, Dave McGuire wrote:

> On Oct 5, 2009, at 3:39 PM, Geoffrey Reed wrote:
>>> What am I not understanding about the need for this product?  I  
>>> have about
>>> a half-dozen 8-bit ISA IDE controllers, mostly Seagate branded.   
>>> Didn't
>>> think they were particularly rare.
>>
>> From what this sounds like it is able to talk to large capacity IDE  
>> drives,
>> presumedly 16-bit IDE drives.  The older XT ide controllers by  
>> seagate (that
>> I had) were limited to a handful of drives that could do 8-bit wide  
>> data
>> transfers and were very limited capacity wise.
>
>  The big difference here is LBA vs. C/H/S addressing.  Recall that  
> the IDE interface is a clone of the register-level programming  
> interface of the ancient WD1010 MFM hard disk controller chip  
> family, whose registers are 8 bits wide.  There's a register for the  
> starting sector number (256 sectors), a register for the low byte of  
> the cylinder address and two bits of another register for the high  
> part of the cylinder address (meaning it tops out at 1024  
> cylinders), and half of a register for the head number (meaning 16  
> heads).
>
>  1024 cylinders * 16 heads * 256 sectors * 512 bytes/sector = 2GB
>
>  I don't know where any supposed 520MB limit comes from; if someone  
> can enlighten me there, I'd appreciate it.

It was originally 63 sectors.  I was one of the 3 people at IBM who  
came up with this scheme (because the controller we had did LBA  
addressing so we needed a way to map C/H/S to LBA in a simple way).   
We *knew* at the time that this was constraining but we figured that  
LBA would be in place before we hit the limit.  We were wrong.

TTFN - Guy