The Program Status Registers; Figure 2-6 Program Status Register Format - ARM ARM7TDMI Technical Reference Manual

2.7

The program status registers

2.7.1 Condition code flags
ARM DDI 0210C
The ARM7TDMI processor contains a CPSR and five SPSRs for exception handlers to
use. The program status 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 26 25 24 23
N Z C V
Note
To maintain compatibility with future ARM processors, you must not alter any of the
reserved bits. One method of preserving these bits is to use a read-write-modify strategy
when changing the CPSR.
The remainder of this section describes:
• Condition code flags
• Control bits on page 2-14
• Reserved bits on page 2-15.
The N, Z, C, and V bits are the condition code flags, you can set them by arithmetic and
logical operations. They can also be set by MSR and LDM instructions. The
ARM7TDMI processor tests these flags to determine whether to execute an instruction.
Copyright © 2001, 2004 ARM Limited. All rights reserved.
Reserved
· · · · ·
Overflow
Carry or borrow or extend
Zero
Negative or less than

Figure 2-6 Program status register format

Programmer's Model
Control bits
8 7 6 5 4 3 2
·
I F T M4 M3 M2 M1 M0
1 0
Mode bits
State bit
FIQ disable
IRQ disable
2-13