NEC Network Controller uPD98502 - Manual (Page 61)

CHAPTER 2 V R 4120A
Preliminary User’s Manual S15543EJ1V0UM 61
2.1.4 Data formats and addressing
The V R 4120A uses following four data formats:
✧ Doubleword (64 bits)
✧ Word (32 bits)
✧ Halfword (16 bits)
✧ Byte (8 bits)
For the µPD98502, byte ordering within all of the larger data formats - halfword, word, doubleword - can be
configured in either big-endian or little-endian order.
Endianness refers to the location of byte 0 within the multi-byte data structure.
When configured as a little-endian system, byte 0 is always the least-significant (rightmost) byte, which is
compatible with iAPX™ and DEC VAX™ conventions. Figures 2-4 and 2-5 show this configuration.
Figure 2-4. Little-Endian Byte Ordering in Word Data
12 12 13 14 15
0 15 16 8 23 24 7 31
8 8 9 10 11
4 4 5 6 7
0 0 1 2 3
Low-order
address
High-order
address
Word
address
Bit No.
Remarks 1. The lowest byte is the lowest address.
2. The address of word data is specified by the lowest byte’s address.
Figure 2-5. Little-Endian Byte Ordering in Double Word Data
16 23
0 31 32 8 7 63
8
0
Low-order
address
High-order
address
Double word
address
Half word
22
15 14
7 6
21 20
13 12
5 4
19 18
11 10
3 2
17 16
9 8
1 0
16 15
Word Byte
48 47
Remarks 1. The lowest byte is the lowest address.
2. The address of word data is specified by the lowest byte’s address.
The CPU core uses the following byte boundaries for halfword, word, and doubleword accesses:
— Halfword: An even byte boundary (0, 2, 4...)
— Word: A byte boundary divisible by four (0, 4, 8...)
— Doubleword: A byte boundary divisible by eight (0, 8, 16...)