68
SAM4CP [DATASHEET]
43051E–ATPL–08/14
Figure 12-4.
Bit-band Mapping
Directly Accessing an Alias Region
Writing to a word in the alias region updates a single bit in the bit-band region.
Bit[0] of the value written to a word in the alias region determines the value written to the targeted bit in the bit-band
region. Writing a value with bit[0] set to 1 writes a 1 to the bit-band bit, and writing a value with bit[0] set to 0 writes a 0 to
the bit-band bit.
Bits[31:1] of the alias word have no effect on the bit-band bit. Writing
0x01
has the same effect as writing
0xFF
. Writing
0x00
has the same effect as writing
0x0E
.
Reading a word in the alias region:
0x00000000 indicates that the targeted bit in the bit-band region is set to 0.
0x00000001 indicates that the targeted bit in the bit-band region is set to 1.
Directly Accessing a Bit-band Region
“Behavior of Memory Accesses”
describes the behavior of direct byte, halfword, or word accesses to the bit-band
regions.
12.4.2.6 Memory Endianness
The processor views memory as a linear collection of bytes numbered in ascending order from zero. For example, bytes
0-3 hold the first stored word, and bytes 4-7 hold the second stored word.
“Little-endian Format”
describes how words of
data are stored in memory.
Little-endian Format
In little-endian format, the processor stores the least significant byte of a word at the lowest-numbered byte, and the most
significant byte at the highest-numbered byte. For example:
Figure 12-5.
Little-endian Format
0x23FFFFE4
0x22000004
0x23FFFFE0
0x23FFFFE8
0x23FFFFEC
0x23FFFFF0
0x23FFFFF4
0x23FFFFF8
0x23FFFFFC
0x22000000
0x22000014
0x22000018
0x2200001C
0x22000008
0x22000010
0x2200000C
32 MB alias region
0
7
0
0
7
0x20000000
0x20000001
0x20000002
0x20000003
6
5
4
3
2
1
0
7
6
5
4
3
2
1
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0x200FFFFC
0x200FFFFD
0x200FFFFE
0x200FFFFF
1 MB SRAM bit-band region
Memory
Register
Address
A
A+1
lsbyte
msbyte
A+2
A+3
0
7
B0
B1
B3
B2
31
24 23
16 15
8 7
0
B0
B1
B2
B3