User Manual

6-4 EP7309/11/12 User’s Manual - DS508UM4
Copyright Cirrus Logic, Inc. 2003
Processor Support
6
Endianess
The EP73xx uses little endian configuration for th e internal registers. H owever, it is
possible to connect to a big en dian external memory device. Th e big-endian/little-
endian bit in the internal registers sets whether the EP73xx treats words in memory as
being stored in big endian or little endian format.
• Big endian - least significant byte (LSB) read as most s ignificant byte (M SB)
• Little endian - LSB read as LSB
Note: dc = don’t care
Bold indicates active byte lane.
Table 6-4: Effect of Endianess on Read Operations
Address
(W/B)
Data in
Memory
(as seen
by the
EP73xx)
Byte Lanes to Memory/Ports/Registers
R0 Contents
Big Endian Memory Little Endian Memory
7:0 15:8 23:16 31:24 7:0 15:8 23: 16 31: 24 Big Endian
Little
Endian
Word + 0(W) 11223344 44 33 22 11 44 33 22 11 11223344 11223344
Word + 1(W) 11223344 44 33 22 11 44 33 22 11 44112233 44112233
Word + 2(W) 11223344 44 33 22 11 44 33 22 11 33441122 33441122
Word + 3(W) 11223344 44 33 22 11 44 33 22 11 22334411 22334411
Word + 0 (H) 11223344 44 33 22 11 44 33 22 11 00001122 00003344
Word + 1(H) 11223344 44 33 22 11 44 33 22 11 220000011 44000033
Word + 2(H) 11223344 44 33 22 11 44 33 22 11 00003344 00001122
Word + 3(H) 11223344 44 33 22 11 44 33 22 11 44000033 22000011
Word + 0 (B) 11223344 dc dc dc 11 44 dc dc dc 00000011 00000044
Word + 1 (B) 11223344 dc dc 22 dc dc 33 dc dc 00000022 00000033
Word + 2 (B) 11223344 dc 33 dc dc dc dc 22 dc 00000033 00000022
Word + 3 (B) 11223344 44 dc dc dc dc dc dc 11 00000044 00000011
Table 6-5: Effect on Endianess on Write Operations
Address
(W/B)
Register
Contents
Byte Lanes to Memory / Ports / Registers
Big Endian Memory Little Endian Memory
7:0 15:8 23:16 31:24 7:0 15:8 23:16 31:24
Word + 0 (W) 11223344 44 33 22 11 44 33 22 11
Word + 1 (W) 11223344 44 33 22 11 44 33 22 11
Word + 2 (W) 11223344 44 33 22 11 44 33 22 11
Word + 3 (W) 11223344 44 33 22 11 44 33 22 11
Word + 0 (H) 11223344 44 33 44 33 44 33 44 33
Word + 1 (H) 11223344 44 33 44 33 44 33 44 33