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There are banked registers SPs, LRs and SPSRs for each privileged mode (for more details see
the ARM9EJ-S Technical Reference Manual, revision r1p2 page 2-12).
9.4.7.1
Status Registers
The ARM9EJ-S core contains one 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 operation mode
Figure 9-2.
Status Register Format
shows the status register format, where:
• N: Negative, Z: Zero, C: Carry, and V: Overflow are the four ALU flags
• The Sticky Overflow (Q) flag can be set by certain multiply and fractional arithmetic
instructions like QADD, QDADD, QSUB, QDSUB, SMLAxy, and SMLAWy needed to achieve
DSP operations.
The Q flag is sticky in that, when set by an instruction, it remains set until explicitly cleared by
an MSR instruction writing to the CPSR. Instructions cannot execute conditionally on the
status of the Q flag.
• The J bit in the CPSR indicates when the ARM9EJ-S core is in Jazelle state, where:
– J = 0: The processor is in ARM or Thumb state, depending on the T bit
– J = 1: The processor is in Jazelle state.
• Mode: five bits to encode the current processor mode
9.4.7.2
Exceptions
Exception Types and Priorities
The
ARM9EJ-S supports five types of exceptions. Each type drives the ARM9EJ-S in a privi-
leged mode. The types of exceptions are:
• Fast interrupt (FIQ)
• Normal interrupt (IRQ)
• Data and Prefetched aborts (Abort)
• Undefined instruction (Undefined)
• Software interrupt and Reset (Supervisor)
When an exception occurs, the banked version of R14 and the SPSR for the exception mode
are used to save the state.
N Z C V Q
J
I
F T
Mode
Reserved
Mode bits
Thumb state bit
FIQ disable
IRQ disable
Jazelle state bit
Reserved
Sticky Overflow
Overflow
Carry/Borrow/Extend
Zero
Negative/Less than
31 30 29 28 27
24
7 6 5
0
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