2/24/2008 9T6WP
Preliminary Hardware Data Module
BCM7405
06/29/07
Functional Description
Bro a d c o m Co rp o r a ti o n
Document
7405-1HDM00-R
MIPS4380 Processor Core Page 1-69
L
EVEL
-T
WO
C
ACHE
The CPU has an on-core Level-Two (L2) cache, which is physically indexed and tagged. The L2 cache is shared by all the
TPs and can be viewed as the extension of all the L1 caches. When a miss of one of the L1 caches hits the L2 cache, the
L2 line and the line being replaced from the L1 cache are swapped between the L1 and L2 caches. When there is a L1 cache
miss also misses the L2 cache, the request is sent to look up the RAC. When refilling the missing line into the L1 cache, the
line being replaced is kept in the L2 cache. As such, this is generally called the exclusive L2 cache.
R
EADAHEAD
C
ACHE
The CPU has an on-core set associative readahead cache (RAC). The RAC is physically indexed and tagged and can
prefetch and stage a memory block ahead of the instruction and/or data cache misses. The replacement algorithm of the
RAC is LRU (least recently used). The RAC is here are two RAC control registers in the core register space for an application
to set up and control the RAC operations.
There are replicated RAC control fields for each TP to set up the prefetching options independently.
L
ITTLE
AND
B
IG
E
NDIANNESS
OF
B
YTE
O
RDERING
The CPU allows byte ordering of operands in either big or little endian configuration. Through the configuration register, a
user can specify the order of placement of bytes in the memory within halfword, word, or doubleword boundary.
An example is provided below for the case of word boundary. In the big endian configuration, byte 0 is the most significant
byte (MSB) and a word is addressed beginning with the MSB. In the little endian configuration, byte 0 is the least significant
byte (LSB) and a word is address beginning with the LSB. An illustration can be found in
. Since all instructions
must always fall on a word boundary, this endianness option has no effect on the instruction addressing.
Figure 1-17: Little and Big Endian Byte Ordering
Higher
Addresses
Big Endian
Little Endian
Word
Address
8
4
0
31
24 23
16 15
8 7
0
31
24 23
16 15
8 7
0
3
2
1
0
4
5
6
7
11
10
9
8
MSB
LSB
MSB: Most significant byte.
LSB: Least significant byte.
MSB
LSB
Word
Address
8
4
0
Higher
Addresses
10
6
2
9
11
4
5
7
0
1
3
8