[G4] 120 GB Barrier Question

[Alex]

On Tuesday, May 4, 2004, at 08:54 Canada/Eastern, Richard Kriss wrote:

> [...] this G4-450 AGP has ATA-4's. [...]  can I go up to 120 GB and 
> 120 GB?

Yes.

> [...] The 120 barrier is confusing.

Not really.

First of all, is it 120 (128 actually) or 137GB? Both. The IT industry 
got in the habit of defining the kilobyte as 1KB = 2^5 = 1024 bytes, 
and the Finder still reports sizes like this. However, strictly 
speaking, it's incorrect. The SI prefixes (kilo, mega, etc.) should 
only be used for powers of ten, so 1KB = 10^3 = 1000 bytes. Hard drives 
manufacturers have been using the correct terminology for some time now 
(whether out of respect for SI, or for marketing reasons is up to you 
to guess), and IEC has recommended that the term -binary, with the 
abbreviation -i, should be added for powers of two. So, to be 
pedantically accurate, 1KB = 1000 bytes, and 1KiB (kilobinary byte) = 
1024 bytes. If you do the math, you'll see that 128GiB = 137GB.

Next, why the barrier? Older ATA controllers used 28-bit LBA (logical 
block addressing). To use space on a hard disk, the controller must 
know how to find each chunk of space (logical block), and it finds it 
by assigning an address to it. The highest number of blocks it can use 
is determined by how many unique addresses can be created out of 28 
bits; which, it turns out, eventually amounts to 128GiB of disk space. 
So, with 28-bit LBA, the controller can access a maximum of 128GiB on 
each device; even if the device has more space available, the 
controller can't use it. And it's 128GiB on each device, because each 
device has its own separate set of addresses.

Newer controllers use 48-bit LBA, which means they theoretically can 
address up to 144 PB of disk space on each device. Jim's post tells you 
how to determine in Panther (not Jaguar, tho') if your Mac supports the 
new specification. I believe all Macs since the MDD G4s do.

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