32-bit access on 16-bit bus?

[NorthWay]

When you do a long write on a 16-bit bus, what are the rules for which part (upper or lower word) are written first?

Question stands for all 680x0. (This in relation to chip register writes.)

[pandy71]

http://en.wikipedia.org/wiki/Endiann...s_and_hardware

"The Motorola 6800 and 68k series of processors use the big-endian format, and for this reason, the big-endian format is also known as the Motorola convention. Other well-known processors that use the big-endian format include the Xilinx Microblaze, SuperH, IBM POWER, and System/360 and its successors such as System/370, ESA/390, and z/Architecture. The PDP-10 also used big-endian addressing for byte-oriented instructions."

http://en.wikipedia.org/wiki/Endiann...mory_locations

[NorthWay]

The answer to my question is either "upper then lower" or "lower then upper".
Endianness does not come into the equation as far as I can see?

[Toni Wilen]

AFAIK 68020+ always writes upper word first. 68000/010 has some exceptions, for example CLR.L <memory>, MOVE x,-(An).

[NorthWay]

Originally Posted by Toni Wilen

AFAIK 68020+ always writes upper word first. 68000/010 has some exceptions, for example CLR.L <memory>, MOVE x,-(An).

But 68000/010 is upper first as a rule?

[Toni Wilen]

Originally Posted by NorthWay

But 68000/010 is upper first as a rule?

Yes. Most instructions reads/writes upper word first.

[pandy71]

Originally Posted by NorthWay

The answer to my question is either "upper then lower" or "lower then upper".
Endianness does not come into the equation as far as I can see?

Hmmm i think that article in Wikipedia describe order of operation for 32 bit on 16 bit bus but perhaps i misunderstood your question.

[NorthWay]

Originally Posted by Toni Wilen

Yes. Most instructions reads/writes upper word first.

Thanks.