Re: drive-side routine
Date: 2005-10-06 01:11:02

On 2005-10-05, at 09:28, Greg King wrote:

> From: silverdr; on Monday, October 03, 2005 at 07:51 PM -0400
>> The weak point for me here is the LDA $1801. It happens immediately
>> after sending the handshake, for which the 64 waits. What is then
>> being read? Because 1541 is faster than the 64, chances are that the
>> port is being read before 64 puts the next value on the port, am I
>> right?
> It looks to me that the "LDA $1801" gets the same data that the  
> previous
> "EOR $1801" got.  So, the C64 is sending each byte exclusive-orred  
> with all
> of the previous bytes, while the drive is exclusive-orring each of its
> bytes with only the most recent previous byte.

Hm, actually it looks so.

> That suggests to me that the computer "massages" the data somehow.   
> And,
> those EORs are the last two operations in that data-conversion.   
> So, I ask
> these questions:
> 1. Does the C64 code convert binary bytes into GCR codes?

Not to my understanding. While reading the data from the 1541 (with  
regular LDA:STA) it stores untouched GCR in the track buffers. On  
writing back the EORing occurs on both sides but that's the point I  
don't uderstand.

> 2. Does that nibbler remove some kind of copy-protection?

Not that I would be aware of. It's actually part of one of the  
versions of 15sec copy I have.

LOAD "SIG",8,1

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