Re: C64 Expansion Port IO1 and IO2 and PEEK and POKE

From: Segher Boessenkool <>
Date: Mon, 9 May 2016 19:43:30 -0500
Message-ID: <>
On Mon, May 09, 2016 at 10:22:09PM +0200, A. Fachat wrote:
> A dummy read occurs when the processor still needs to calculate some 
> addresses or data before it can be written.

The 6502 does a bus access every cycle; sometimes it doesn't have any
useful access to do, e.g. sometimes it doesn't have the address it wants
to access handy yet, or perhaps this is a one-byte insn (that takes
two cycles), etc.

> For example, if the addressing 
> mode abs,x crosses a page boundary, it first does a dummy read on an 
> address where the low address byte is added to, but the high address byte 
> is not yet incremented. So if you read from $00f0,x with x equals $12, 
> there first is a dummy read from $0002, and then the actual valid transfer 
> (read or write) on $0102.

And if there is no carry, the 6502 does not do that last cycle at all
(it starts fetching the next insn instead), unless this is a write
(because the first access, without the page # incremented, is done as
a read -- doing a write to perhaps the wrong address isn't the smartest
thing in the world).

> With the indirect indexed addressing mode (zp),y it is similar. The dummy 
> read goes to the address stored in zp with the low byte increased by the 
> value of Y, but the high byte not yet incremented.

Yeah, it's exactly the same for NNNN,x and NNNN,y and (NN),y .


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Received on 2016-05-10 02:00:02

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