LM3S2939 Datasheet, PDF (71/749 Pages) Texas Instruments – Stellaris® LM3S2939 Microcontroller

Stellaris^® LM3S2939 Microcontroller
Register 21: Control Register (CONTROL)
The CONTROL register controls the stack used and the privilege level for software execution when
the processor is in Thread mode. This register is only accessible in privileged mode.
Handler mode always uses MSP, so the processor ignores explicit writes to the ASP bit of the
CONTROL register when in Handler mode. The exception entry and return mechanisms automatically
update the CONTROL register based on the EXC_RETURN value (see Table 2-10 on page 88).
In an OS environment, threads running in Thread mode should use the process stack and the kernel
and exception handlers should use the main stack. By default, Thread mode uses MSP. To switch
the stack pointer used in Thread mode to PSP, either use the MSR instruction to set the ASP bit, as
detailed in the Cortex™-M3/M4 Instruction Set Technical User's Manual, or perform an exception
return to Thread mode with the appropriate EXC_RETURN value, as shown in Table 2-10 on page 88.
Note: When changing the stack pointer, software must use an ISB instruction immediately after
the MSR instruction, ensuring that instructions after the ISB execute use the new stack
pointer. See the Cortex™-M3/M4 Instruction Set Technical User's Manual.
Control Register (CONTROL)
Type R/W, reset 0x0000.0000
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
reserved
Type RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
reserved
ASP
TMPL
Type RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
R/W
R/W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Bit/Field
31:2
1
0
Name
reserved
ASP
TMPL
Type
Reset Description
RO 0x0000.000 Software should not rely on the value of a reserved bit. To provide
compatibility with future products, the value of a reserved bit should be
preserved across a read-modify-write operation.
R/W
0
Active Stack Pointer
Value Description
1 PSP is the current stack pointer.
0 MSP is the current stack pointer
In Handler mode, this bit reads as zero and ignores writes. The
Cortex-M3 updates this bit automatically on exception return.
R/W
0
Thread Mode Privilege Level
Value Description
1 Unprivileged software can be executed in Thread mode.
0 Only privileged software can be executed in Thread mode.
June 18, 2012
71
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