ARM Cortex-M4 - 32 MB Alias Region; 1 MB SRAM Bit-Band Region; Directly Accessing an Alias Region; Directly Accessing a Bit-Band Region

The Cortex-M4 Processor
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 2-17
ID121610 Non-Confidential
• Bit_number is the bit position, 0-7, of the targeted bit.
Figure 2-1 shows examples of bit-band mapping between the SRAM bit-band alias region and
the SRAM bit-band region:
• the alias word at
0x23FFFFE0 maps to bit[0] of the bit-band byte at
0x200FFFFF : 0x23FFFFE0 = 0x22000000 + ( 0xFFFFF *32) + (0*4)
• the alias word at
0x23FFFFFC maps to bit[7] of the bit-band byte at 0x200FFFFF :
0x23FFFFFC = 0x22000000 + ( 0xFFFFF *32) + (7*4)
• the alias word at
0x22000000 maps to bit[0] of the bit-band byte at 0x20000000 :
0x22000000 = 0x22000000 + (0*32) + (0 *4)
• the alias word at
0x2200001C maps to bit[7] of the bit-band byte at 0x20000000 :
0x2200001C = 0x22000000 + (0*32) + (7*4).
Figure 2-1 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 zero
•
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 on page 2-14 describes the behavior of direct byte, halfword, or
word accesses to the bit-band regions.
0x23FFFFE4
0x22000004
0x23FFFFE00x23FFFFE80x23FFFFEC0x23FFFFF00x23FFFFF40x23FFFFF80x23FFFFFC
0x220000000x220000140x220000180x2200001C 0x220000080x22000010 0x2200000C
32MB alias region
0
7 0
07
0x200000000x200000010x200000020x20000003
6 5 4 3 2 1 07 6 5 4 3 2 1 7 6 5 4 3 2 1 07 6 5 4 3 2 1
07 6 5 4 3 2 1 6 5 4 3 2 107 6 5 4 3 2 1 07 6 5 4 3 2 1
0x200FFFFC0x200FFFFD0x200FFFFE0x200FFFFF
1MB SRAM bit-band region