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124 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
Intel XScale
®
Core
Word Write (16-Bits Write)
When the Intel XScale
®
core writes a 16-bit word to external memory, it puts the bytes in the byte
lanes where it intends to write them along with the byte enables for those bytes turned ON based on
the endian setting of the system. The Intel XScale
®
core does not allow a word write on an
odd-byte address. The Intel XScale
®
core register bits [15:0] always contain the word to be written
regardless of the B-bit setting.
For example, if the Intel XScale
®
core wants to write one word to a little-endian system at address
0x0002, it will copy byte 0 to byte lane 2 and byte 1 to byte lane 3 along with X_BE[2] and
X_BE[3] turned ON. If the Intel XScale
®
core wants to write one word to a big-endian system at
address 0x0002, it will copy byte 0 to byte lane 0 and byte 1 to byte lane 1 along with X_BE[0] and
X_BE[1] turned ON. Table 48 shows other possible combinations of byte lanes and byte enables.
Byte lanes other than those currently driven by the Intel XScale
®
core contain undefined data.
Longword (32-Bits) Write
The longword to be written is put on the Intel XScale
®
core’s data bus with byte 0 on X[7:0],
byte 1 on X[15:8], byte 2 on X[23:16], and byte 4 on X[31:24] (see Figure 25). All of the byte
enables are turned ON. A 32-bit longword write (0x12345678) by the Intel XScale
®
core to address
0x0000 regardless of endianness, causes byte 0 (0x78) to be written to address 0x0000, byte 1
(0x56) to address 0x0001, byte 2 (0x34) to address 0x0002, and byte 3 (0x12) to address 0x0003.
Table 48. Byte-Enable Generation by the Intel XScale
®
Core for Word Writes in Little- and
Big-Endian Systems
Word
to be
Written
Byte-Enables for Little-Endian Systems Byte-Enables for Big-Endian Systems
X_BE[0] X_BE[1] X_BE[2] X_BE[3] X_BE[0] X_BE[1] X_BE[2] X_BE[3]
Byte 0,
Byte 1
11000011
Byte 2,
Byte 3
00111100
Figure 25. Intel XScale
®
Core-Initiated Write to the IXP2800 Network Processor
A9695-03
Byte
Write
M[7:0]
M[15:8]
M[23:16]
M[31:24]
X [7:0]
X_BE [0]
X [15:8]
X [23:18]
X [31:24]
Byte Write by Intel XScale
®
Core
Intel
®
IXP2800
Core Gasket
X_BE [1]
X_BE [2]
X_BE [3]