Re: drive-side routine

From: Greg King (
Date: 2005-10-05 09:28:53

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.

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

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