Fujitsu MB91460 SERIES FR60 User Manual page 566

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.
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
a) Output address low-order digits "00"
Internal register External bus
550
(1) Word access (when executing the LD/ST instructions)
Internal register
D31
D23
D15
D7
D0
(2) Halfword access (when executing the LDUH/STH instructions)
D31
BB
D23
AA
D15
AA
D7
BB
D0
The reason is hardware restrictions related to the endian
External bus
D31
AA DD
D23
CC
BB
D15
BB
CC
D7
DD AA
D0
b) Output address low-order digits "10"
Internal register External bus
D31
D31
D23
D23
D15
D15
D7
D7
D0
D0
D31
D23
D15
AA BB
D7
BB AA
D0