16-bit/32-bit operations and Direct Page boundaries

[The-Beesh-Spweesh]

If a 16-bit operation (LDD, STD, ADDD, etc.) is attempted on a direct page address of $FF, what happens? Will the high byte of the address be incremented to point to the start of the next page, or will the processor wrap to the beginning of the same page?

This issue also applies to the 32-bit LDQ and STQ instructions on the 6309, in the range of $FD to $FF, along with all the 16-bit operations. Also, check both emulation and native mode, because I think it may have an effect on the behavior.

[hoglet67]

My understanding is on the 6809 or 6309 a double (or quad) byte access never wraps within a page.

In LDD, even with the direct page addressing mode, the second byte is fetched from EA+1, where the addition is a full 16-bit addition. So LDD $FF will fetch from $FF and $100.

That's how the 6809 Decoder is currently implemented.

If you can provide any evidence (or references) to the contrary I would be very interested.

Dave