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CHAPTER 4 External Bus Interface
4.6.3 Bus Access of Little Endian
For areas in which little endian (LER) is set, external bus access is little endian.
Bus access of the MB91191/MB91192 series is realized by swapping the byte position of
the data bus in accordance with the bus width while the big endian output address
order and control signal output are basically the same, using the bus access operation
for big endian.
When connected, the big endian and little endian areas must be physically separated,
so special care must be taken.
■ Differences between Little Endian and Big Endian
Differences between little endian and big endian are mentioned as follows.
The order of addresses to be output is the same for big endian and little endian.
Data bus control signal to be used for the 32-, 16-, and 8-bit bus width is the same for big endian and little
endian.
●
Word access
The byte data at the MSB side supporting the big endian address "00" is the little endian byte data at the
LSB side. In the case of word access, all of the four-byte positions within the word will be reversed.
"00" → "11", "01" → "10", "10" → "01", "11" → "00"
●
Half-word access
The byte data at the MSB side supporting the big endian address "0" is the little endian byte data at the LSB
side. In the case of half-word access, the two-byte positions within the half-word will be reversed.
"0" → "1", "1" → "0"
●
Byte access
Both big endian and little endian are the same.
■ Data Format
The relationships between the internal register and external data bus is shown per data format.
●
Half word access (LDUB, STB instruction execution)
Figure 4.6-9 Relationship between internal register and external data bus of half-word access
a) Output address Lower "00"
D31
D15
D23
D7
D0
AA
BB
Internal register
D31
D15
D23
D7
D0
AA
BB
External bus
b) Output address Lower "10"
D31
D15
D23
D7
D0
AA
BB
Internal register
D31
D15
D23
D7
D0
AA
BB
External bus