beautypg.com

2 little endian bus access, Word access – FUJITSU MB91460 SERIES FR60 User Manual

Page 565

background image

549

Chapter 31 External Bus

4.Endian and Bus Access

Figure 4-13 Example of Connecting the MB91460 Series to External Devices

4.2 Little Endian Bus Access

Little endian (LER) external bus access is performed for an area for which the little endian meth-
od is set.

Little endian bus access on the MB91460 series is implemented by using the bus access oper-
ation 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.

The data bus control signal used for 32/16/8-bit bus width is the same for little endian and big endian.

Word access

The byte data on the MSB side for big endian address 00 becomes byte data on the LSB side when the little
endian method is used.

For a word address, the locations of all four bytes in the word are reversed:

00 -> 11, 01 -> 10, 10 -> 01, 11 -> 00

D31

D07

D00

D00

D15

D00

D16 D15

D08 D07

D08 D07

D24 D23

This LSI

D31
to
D24

D23
to
D16

D15
to
D08

D07
to
D00

00

01

10

11

0

1

0

* For 16/8-bit devices, use the data bus on

the MSB side of this LSI.

32-bit device

(low-order 2 bits of

the address 00 to 11)

*16-bit device

(low-order 1 bit of

the address 0/1)

*8-bit device

WR0

WR1

WR2

WR3