Subject: Re: [stella] BRK instruction: Request for clarification! From: "B. Watson" <atari@xxxxxxxxxxxxxx> Date: Tue, 26 Jul 2005 10:21:05 -0400 |
> The B flag is wired directly to the interrupt input pin on the > 6502. In the 6507 this pin is internally set high, so on the VCS > the B flag should always be set. ... > > None of the instructions in the 6502 affects the B flag. > It's hardwired to the interrupt input pin, so you can't change > the bit yourself. You sure about that? If you're right, someone needs to fix this tutorial: http://www.6502.org/tutorials/register_preservation.html It claims that: The B (Break) flag, bit 4 of the P (Processor status) register, is a frequent source of confusion on the 6502. The sole purpose of this flag is to distinguish a BRK from a IRQ. ...which is what I always thought it was for, too... but I'm none too confident that I know what I'm talking about, since I haven't actually used any of this interrupt-related stuff in 15+ years. -- B. Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://stella.biglist.com
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