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2: Programmer's Model
2.7
Program status registers
The ARM720T processor contains a CPSR, and five SPSRs for use by exception handlers.
These registers:
•
hold information about the most recently performed ALU operation
•
control the enabling and disabling of interrupts
•
set the processor operating mode.
The arrangement of bits is shown in Figure 2-6.
Condition
code flags
31 30 29 28 27
2.7.1
The condition code flags
The N, Z, C, and V bits are the condition code flags. These can be changed as a result of
arithmetic and logical operations, and tested to determine if an instruction must execute or
not.
In ARM state, all instructions can be executed conditionally. In Thumb state, only the Branch
instruction is capable of conditional execution. See the
for details.
2.7.2
The control bits
The bottom eight bits of a PSR (incorporating I, F, T, and M[4:0]) are known collectively as the
control bits. These change when an exception arises. If the processor is operating in a
privileged mode, they can also be manipulated by software:
I and F bits
The T bit
M[4:0] bits
If you program any illegal value into the mode bits, M[4:0], then the processor
Note:
enters an unrecoverable state. If this occurs, apply reset.
2-8
Reserved
Overflow (V)
Carry or borrow or extend (C)
Zero (Z)
Negative or less than (N)
Figure 2-6 Program status register format
These are the interrupt disable bits. When set, these disable the
IRQ and FIQ interrupts respectively.
This reflects the operating state. When this bit is set, the processor
is executing in Thumb state, otherwise it is executing in ARM
state. This is reflected on the CPTBIT external signal. Software
must never change the state of the CPTBIT in the CPSR. If this
happens, the processor enters an Unpredictable state.
These are the mode bits. These determine the processor operating
mode, as shown in Table 2-2 on page 2-9. Not all combinations of
the mode bits define a valid processor mode. Only those explicitly
described can be used.
EPSON
Control bits
8
7
6 5
4
3
2
1
0
ARM Architecture Reference Manual
ARM720T CORE CPU MANUAL
Mode bits (M[4:0])
State bit (T)
FIQ disable (F)
IRQ disable (I)
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