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Query re C64 interrupt routine [message #104470] Tue, 01 March 2005 19:01 Go to previous message
Ian Kenny is currently offline  Ian Kenny
Messages: 21
Registered: January 2005
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Junior Member
I have a question about the interrupt code in the C64 rom that starts at
FF48. Specifically from FF4D to the branch at FF53. I can understand it as a
piece of assembly code but what is being tested here?

Here's the code:

FF4D TSX
FF4E LDA, $104,X
FF51 AND #$10
FF53 BEQ $FF58

The code then either branches to the 'normal interrupt routine' or to the
'BRK routine'. I just can't see how that particular test makes the decision.

Cheers

Ian
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