Re: [stella] Question about opcode ANE ($8B)

Subject: Re: [stella] Question about opcode ANE ($8B)
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Thu, 30 Jun 2005 15:53:01 -0400
> I was reading up on opcode ANE (also known as XAA). From what I read it's
> the same as...
> AND #<imm value>
> Does anyone know of any limitations on this opcode? Is this what I would
> see on the real hardware and the 6502/7?

Most 6502 emulators handle ANE like this:

reg_a = (reg_a OR $ee) AND reg_x AND #<imm value>

This behaviour has been confirmed on the C64. The instability
that you mention in your other post affects the $ee value,
which might be different when video DMA occurs between the
opcode fetch and the parameter fetch. Since there is no DMA
on the 2600, there shouldn't be any instability. But then
again it hasn't been confirmed that the $ee isn't a C64
specific value. So it would be better to do some testing
on a real 2600 before using ANE in any of your games.

Ciao, Eckhard Stolberg

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