Re: Fast GCR decoding?

From: Gerrit Heitsch <>
Date: Sun, 13 Jan 2013 17:47:10 +0100
Message-ID: <>
On 01/13/2013 05:27 PM, wrote:
> On 2013-01-13, at 07:30, Groepaz wrote:
>>> Is there anything known that would be still faster? AFAIU ProfessionalDOS
>>> does it faster but with help from its hardware. Anyone heard of faster
>>> than 1571 but pure software based decoding?
>> thats basically just an eprom with an huge lookup table though (the cbm dual
>> drives do it in a similar way)
> Like putting bits on address lines and reading the outcome from data lines? It could be an option, depending on how "huge" the table has to be... It can't be 40 bits so some bit fiddling has to be done anyway.
> Also I found a post by Jim Drew on Lemon64, where he admitted to have decoded GCR in real time thanks to extra RAM on the SC+ card and "creative tables". Since he's got "only" 8K of extra RAM, he had a hard limit on the size of the table. If I fit into 8K and have GCR decoded in RT I can't have any more wishes ;-) How is it done in dual drives?

I thought GCR is a 10/8 encoding, meaning you get 10 Bits off the disk 
which translate to 8 Bits. That would mean you need a 1 KByte table to 
do decoding full bytes by lookup table.

If you do it by nibble, you need a table with 32 entries.


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Received on 2013-01-13 17:00:35

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