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4.16 Relationship between Data Bus Widths and Control Signals
4.16.2 Bus Access with Little Endians

When external bus access is performed for areas se t by the little endian register (LER),

those areas are handled as little endians.

Outline of Little Endians
Little endian bus access by the MB91F109 uses the bus access operation fo r big endians . The
address output sequence and control signal outp ut for big endians are basi cally the same as
those for little endians, which are implemented by swapping data bus byte locatio ns according
to the bus width.
When the devices are connected, exercise extreme caution, as the big and l ittle endian areas
must be physically separated.
The add ress output sequence is the same for both big and little endians.
Word acc ess: The MS B-side byte data, which c orresponds to address 0 0 of big en dians, is
the LSB-side data for little endians.
In word access, all 4 bytes in the word are binary inverted.
"00" --> "11", "01" --> "10", "10" --> "01", "11" --> "00"
Half-word access: The MSB-side byte data, which corresponds to address 00 of big
endians, is the LSB-side byte data for little endians.
In half-word access, all locations of 2 bytes in a half-word are exc han ged f or those h av ing an
opposite value.
"0" --> "1", "1" --> "0"
Byte ac cess: Same for both big and little endians
The data bu s control signals used in a 16/8 bit bus width are the same for both big and little
endians.
Data Format
The following shows the relationship between the internal regi s ter an d e xte rn al dat a bu s in eac h
data format:
Word access (during execution of LD and ST instructions)
Figure 4.16-11 Relationship between Internal Register and External Data Bus for Word Access
AA DD BB
BB CC AA
CC
DD
D31
D07
D15
D23
D31
D23
Internal register Exter nal bus