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Old 03 September 2013, 02:40   #1
NorthWay
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32-bit access on 16-bit bus?

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.)
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Old 03 September 2013, 11:14   #2
pandy71
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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
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Old 03 September 2013, 12:02   #3
NorthWay
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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?
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Old 03 September 2013, 12:08   #4
Toni Wilen
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AFAIK 68020+ always writes upper word first. 68000/010 has some exceptions, for example CLR.L <memory>, MOVE x,-(An).
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Old 03 September 2013, 13:14   #5
NorthWay
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Quote:
Originally Posted by Toni Wilen View Post
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?
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Old 03 September 2013, 15:39   #6
Toni Wilen
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Quote:
Originally Posted by NorthWay View Post
But 68000/010 is upper first as a rule?
Yes. Most instructions reads/writes upper word first.
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Old 03 September 2013, 18:31   #7
pandy71
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Quote:
Originally Posted by NorthWay View Post
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.
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Old 04 September 2013, 00:46   #8
NorthWay
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Quote:
Originally Posted by Toni Wilen View Post
Yes. Most instructions reads/writes upper word first.
Thanks.
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