Little Endian Bus Access - Fujitsu FR60 Hardware Manual

4.4.2

Little Endian Bus Access

On the FR family, each chip can switch between the big endian and little endian
methods independently from others, except the CS0 area. When the LEND bit in the
area configuration register (ACR) is set to "1", the corresponding area is handled in
the little endian method.
Little endian bus access on 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 for the big endian method and the byte locations
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.
■ Differences between Little Endian and Big Endian
The following explains the differences between little endian and big endian.
•
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 and A0 "00
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 two bytes in the halfword are reversed.
•
Byte access
There is no difference between little endian and big endian.
• The data bus/control signal used for 32/16/8-bit bus width is the same for 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.
CHAPTER 4 EXTERNAL BUS INTERFACE
" becomes byte data on
B
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