Re: reading data from G64 track

On 2013-05-30, at 22:48, Greg King wrote:

>>> Yes, each track can be considered a closed loop of data, just like a
>>> real disk.  It is usually good practice to have the track splice occur at
>>> this loop as it was written on the floppy; but, that is not always possible
>>> to determine.
>> 
>> Now, that makes it even more complicated (just when you thought it was safe).
>> So, I have always to wrap around when I potentially miss some data at the end of the track.
> 
> "Mirror" the track buffer.  (Think about Commodore's I/O chips and their "shadow" registers.)  Double the length of the buffer; and, store each byte twice (starting at the beginning of the buffer, and at the halfway spot).
> 
> Then, your program won't need to care where the track seam happens
> to be -- there is no wrap-around!

That's a quick shot indeed. Just - one shouldn't use "shadow" registers as we all know ;-) I mean in the modern machine a few KiBs here or there doesn't make any real difference but for a CBM almost 16KiB for the buffer alone is a high price. So far I did this for the SYNC

byte0 = gcrBytes + offset;
switch (trackDataLength - offset)	// we always need two 2 or 3 bytes to get 10 consecutive bits so checking the last single byte doesn't make sense unless we "wrap around" the buffer
{
	case 1:
		byte1 = gcrBytes;
		byte2 = gcrBytes + 1;
		break;
	case 2:
		byte1 = gcrBytes + offset + 1;
		byte2 = gcrBytes;
		break;
	default:
		byte1 = gcrBytes + offset + 1;
		byte2 = gcrBytes + offset + 2;
		break;
}

which I believe takes care of the wrapping in a relatively effective way.

-- 
SD!


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