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Chapter 31 External Bus
4.Endian and Bus Access
● Halfword access
The byte data on the MSB side for the big endian address 0 becomes byte data on the LSB side when the little
endian method is used.
For halfword access, the byte locations of two bytes are reversed.
0 -> 1, 1 -> 0
● Byte access
There is no difference between little endian and big endian.
■ Restrictions on the Little Endian Area
• If prefetch is enabled for a little endian area, always use word access to access the area. If data written to the
prefetch buffer is accessed with any length other than word length, the correct endian conversion is not
performed and the wrong data will be read. The reason is hardware restrictions related to the endian
conversion mechanism.
• Do not place any instruction code in a little endian area.
■ Data Format
The relationship between the internal register and external data bus is as follows:
Figure 4-14 Relationship between the Internal Register and External Data Bus for Word Access
Figure 4-15 Relationship between Internal Register and External Data Bus for Halfword Access
D31
D15
D23
D7
D0
AA
BB
CC
DD
Internal register
D31
D15
D23
D7
D0
AA
BB
CC
DD
External bus
(1) Word access (when executing the LD/ST instructions)
D31
D15
D23
D7
D0
AA
BB
D31
D15
D23
D7
D0
AA
BB
D31
D15
D23
D7
D0
AA
BB
D31
D15
D23
D7
D0
AA
BB
(2) Halfword access (when executing the LDUH/STH instructions)
a) Output address low-order digits "00" b) Output address low-order digits "10"
Internal register External bus Internal register External bus