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This Technical Note discusses a feature of the current Macintosh hardware which
will not be supported in the future. Macintosh hardware developers and driver
writers should be aware of this limitation as it affects current and future
products.
Current Macintosh hardware supports a feature of MC68020 and MC68030 processors
which is herein referred to as "byte smearing." Future Macintosh platforms may
not support this feature, and if you have hardware or software dependent on
this feature you need to revise it. Likewise you should be aware of this
limitation if you are developing new hardware or software.
[Aug 01 1990]
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Spreading the Bytes Around
MC68020 and MC68030 processors have a "feature" which causes the data for byte
and word transfers to be replicated (smeared) across all 32 data lines. An
example illustrates the problem.
Consider the following code:
MOVE.L #$12345678,D0 ; Stuff some data
MOVE.B D0,$102 ; Write a byte of data
The data actually placed on the data bus, with and without byte smearing, is
shown in Figure 1.
Figure 1 - Effect of Byte Smearing
With byte smearing, the byte of data is replicated across all the byte lanes;
without smearing, the other bytes are undefined. A similar replication of data
can occur with word transfers.
As an example where this can cause trouble, suppose you have a NuBus(TM) card
with a device register which expects to be byte addressed at byte $102. With
byte smearing it is actually possible to get away with writing a byte to any
address from $100 through $103; without byte smearing, the card only sees the
correct data when addressed at the correct byte $102.
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Summary
The lack of "byte smearing" as a feature should not be a problem for most
developers; after all, why would anyone write to byte $100 when they really
meant to write to byte $102? Well, sad to say, at least one case of this
happening has been uncovered, so if you have, either inadvertently or by
design, relied on this feature, you should revise your products to run on
future Macintosh platforms.
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References
MC68020 User's Manual, pp. 7-9
MC68030 User's Manual, pp. 7-9
NuBus is a trademark of Texas Instruments.
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