Big-endian, Little-endian – NEC Network Controller uPD98502 User Manual
Page 227
CHAPTER 3 SYSTEM CONTROLLER
Preliminary User’s Manual S15543EJ1V0UM
227
Figure 3-1. Bit and Byte Order of Endian Modes
B Y T E 0
B Y T E 1
B Y T E 2
B Y T E 3
B Y T E 4
B Y T E 5
B Y T E 6
B Y T E 7
4
0
M S B
L S B
31
0
MSB = Most Significant Byte
LSB = Least Significant Byte
Big-Endian
Big End
Little End
B Y T E 3
B Y T E 2
B Y T E 1
B Y T E 0
B Y T E 7
B Y T E 6
B Y T E 5
B Y T E 4
4
0
M S B
L S B
31
0
Little-Endian
Big End
Little End
If the access type matches the data item type, no swapping of data sub-items is necessary. Thus, when making
half-word accesses into a data array consisting of half-word data, no byte swapping takes place. In this case, data
item bit order is retained between the two endian modes. The code that sequentially accesses the half-word data
array would be identical, regardless of the endian mode of its V
R
4120A. The code would be endian-independent.
Figure 3-2. Half-word Data Array Example
A
H W 3
H a l f w o r d
Data Array
Big-Endian
Data extraction using sequential halfword access
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
H W 2
H W 1
H W 0
L S H W
M S H W
L S H W
M S H W
A
H W 3
Little-Endian
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
H W 2
H W 1
H W 0
L S H W
M S H W
L S H W
M S H W
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Order Retained
Data extraction using sequential halfword access
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Order Lost