[Coco] hard drive questions

[L. Curtis Boyle]

On Mon, 12 Jun 2006 20:17:32 -0600, Mark McDougall <msmcdoug at iinet.net.au>  
wrote:

> Bob Devries wrote:
>
>> Some SCSI and IDE drives are quoted as using RLL encoding
>> *internally*. RLL is a compression technique which is also used in
>> some picture storage methods. The "2,7" refers to the amount of
>> compression IIRC.
>
> I think you're getting confused between RLE and RLL.
>
> RLL (run length limited) is, as you've correctly pointed out, a  
> "compression" method used internally in old disk drives to increase the  
> amount of storage on a disk. However, rather than compressing the actual  
> data, it allows the controller to eliminate certain clock bits when  
> writing to the media, so that more room is left on the track for data.  
> The numbers, eg. "2,7" refer to the minimum and maximum number of 0  
> symbols which may occur between 1 symbols (inclusive of clock pulses).
>
> RLE (run length encoding) is used in image compression. Here the data is  
> compressed by replacing a sequence of identical pixel values with the  
> token value and a count of how many pixels of that value should follow.  
> This compression scheme is quite basic and works well only for images  
> with fewer colours and/or low complexity.

     Absolutely correct. There is a special version of the FORMAT command  
for OS-9/NitrOS-9 that uses this technique (eliminating some of the extra  
information on each physical sector that is redundant and not really  
necessary), that allows up to 800K on a 80 track double side, double  
density disk. In a way, it is simiiliar to RLE in that it is the "padding"  
of extra, redundant bytes on the physical sector that is being shrunk  
down, leaving the "data" area of the sector the same as a non-RLL drive.

-- 
L. Curtis Boyle