Nec Network Controller uPD98502 Manual de usuario Pagina 227

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CHAPTER 3 SYSTEM CONTROLLER
Preliminary User’s Manual S15543EJ1V0UM
227
Figure 3-1. Bit and Byte Order of Endian Modes
BYTE0 BYTE1 BYTE2 BYTE3
BYTE4 BYTE5 BYTE6 BYTE7
4
0
MSB LSB
31 0
MSB = Most Significant Byte
LSB = Least Significant Byte
Big-Endian
Big End Little End
BYTE3 BYTE2 BYTE1 BYTE0
BYTE7 BYTE6 BYTE5 BYTE4
4
0
MSB LSB
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 VR
4120A. The code would be endian-independent.
Figure 3-2. Half-word Data Array Example
A
HW3
Halfword
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
HW2
HW1
HW0
LSHW
MSHW
LSHW
MSHW
A
HW3
Little-Endian
B C D
E F G H
I J K L
M N O P
HW2
HW1
HW0
LSHW
MSHW
LSHW
MSHW
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 DE F G H
I J K LM N O P
Order Lost
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