Intel NETWORK PROCESSOR IXP2800 - Manual (Page 123)

Hardware Reference Manual 123
Intel
®
IXP2800 Network Processor
Intel XScale ® Core
32-Bit (Longword) Read
32-bit (longword) reads are independent of endianness. Byte lane 0 from the Intel XScale ® core’s
data bus gets into the byte 0 location of the read register inside the Intel XScale
®
core, byte lane 1
from the Intel XScale
®
core’s data bus gets into the byte 1 location of the read register inside the
Intel XScale
®
core, etc. The software determines byte location, based on the endian setting.
3.10.1.1.2 The Intel XScale ® Core Writing to the IXP2800
Network Processor
Writes by the Intel XScale ® core can also be divided into the following three categories:
• Byte Write
• Word Write (16 bits)
• Longword write (32 bits)
Byte Write
When the Intel XScale ® core writes a single byte to external memory, it puts the byte in the byte
lane where it intends to write it, along with the byte enable for that byte turned ON, based on the
endian setting of the system. Intel XScale
®
core register bits [7:0] always contain the byte to be
written, regardless of the B-bit setting.
For example, if the Intel XScale
®
core wants to write to byte 0 in the little-endian system, it puts
the byte in byte lane 0 and turns X_BE[0] to ON. If the system is big-endian, the Intel XScale
®
core puts byte 0 in byte lane 3 and turns X_BE[3] to ON. Other possible combinations of byte lanes
and byte enables are shown in the
Table 47 . Byte lanes other than the one currently being driven by
the Intel XScale ® core, contain undefined data.
Table 46. Byte-Enable Generation by the Intel XScale
®
Core for 16-Bit Data Transfers in Little-
and Big-Endian Systems
Word to
be Read
Byte-Enables for Little-Endian System Byte-Enables for Big-Endian System
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
1 1 0 0 0 0 1 1
Byte 2,
Byte 3
0 0 1 1 1 1 0 0
Table 47. Byte-Enable Generation by the Intel XScale
®
Core for Byte Writes in Little- and
Big-Endian Systems
Byte Number
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 1 0 0 0 0 0 0 1
Byte 1 0 1 0 0 0 0 1 0
Byte 2 0 0 1 0 0 1 0 0
Byte 3 0 0 0 1 1 0 0 0