Little Endian Bus Access - Fujitsu FR60 Hardware Manual

4.4.3

Little Endian Bus Access

Except for the CS0 area, the FR family can switch between the big endian method and
little endian method for each chip select area. When the LEND bit of the ACR register is
set to "1", the chip select area is treated as little endian.
■ Overview of Little Endian Method
Little endian bus access of the FR family is implemented by using the bus access operation used for the big
endian method. Basically, the order of output addresses and control signal output are the same as those for
the big endian method and the locations of the bytes on the data bus are swapped in accordance with the
bus width.
Note that, when a connection is made, the big endian area and the little endian area must be kept physically
separate.
• The order of addresses that are output is the same for little endian and big endian.
• Word access
The byte data on the MSB side for big endian address A1-0=00 becomes byte data on the LSB side
when the little endian method is used.
For a word access, the locations of all four bytes in the word are reversed.
• Halfword access
The byte data on the MSB side for the big endian address A0 becomes byte data on the LSB side when
the little endian method is used.
For halfword access, the byte locations of 2 bytes in the halfword are reversed.
• Byte access
There is no difference between little endian and big endian.
• There is no difference between the little and big endian methods regarding the data bus and control
signals used for bus widths of 16 and 8 bits.
[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 read 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.
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