I use this help when programming a 65816 simulator.
http://www.6502.org/tutorials/65c816opcodes.html
It is very helpful, but I have a problem understanding how the PEI instruction works.

The thing is that there is no clear explanation of where the HIGH byte is taken from in the case of e=1 D=$FF00 for PEI $FF?
If the low byte comes from $FFFF, then the high byte comes from $0000 or maybe $10000?

Also, the author has obscured the issue with the "!" character. Besides, if the LOW byte is $00, adding an 8-bit value will never change the HIGH byte, which is logical.

Then why include it in the description at all?

And for example, if e=1 D=$0100 PEI $FF, Low byte is taken from $01FF and where will the High Byte be taken from?
$0200 or $0100 ?

The first post in A (WDC) 65816 calling convention might be helpful. The WDC datasheet doesn't have any notes about e=1 so don't know why the above author has specified it with !

I must still finish the second post but there's an off by one error in my stack pictures and I've lost the context to fix it.

Actually the WDC datasheet does specify:but - and I could be misreading - it just seems like a complicated way of saying the '816 in emulation mode executing PEI will behave the same as the '816 in native mode.

Bruce's document has a Notation section which explains the ! in the diagrams: it indicates that what happens to the right cannot affect what happens on the left.

So, PEI is a bank-wrapping instruction. (For old instructions in emulation mode and a page-aligned direct page, the direct mode is page-wrapping.)

Right! I was misreading Bruce's doc. Old instructions are page wrapping but the page wrapping ! does not apply to new instructions so the 'Otherwise' block always applies to the new instruction PEI.

Not to muddy the water, but there's another side effect with PEI worth noting:

In emulation mode, if the stack pointer is $0100, then PEI will push the high value to $0100 and the low value to $00FF, not $01FF. The stack pointer will reflect $01FE after the instruction is executed.

Bruce's tutorial mentions this for the PEA opcode but it also is true for PEI. This was confirmed when I validated the 65816 Test Suite.

Daryl

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Thanks for pointing that out, turns out that's a bug in my simulator.