Laptop User Manual

PROGRAMMER'S MODEL S3C2440A RISC MICROPROCESSOR
2-8
The Condition Code Flags
The N, Z, C and V bits are the condition code flags. These may be changed as a result of arithmetic and logical
operations, and may be tested to determine whether an instruction should be executed.
In ARM state, all instructions may be executed conditionally: see Table 3-2 for details.
In THUMB state, only the Branch instruction is capable of conditional execution: see Figure 3-46 for details.
The Control Bits
The bottom 8 bits of a PSR (incorporating I, F, T and M[4:0]) are known collectively as the control bits. These will
be changed when an exception arises. If the processor is operating in a privileged mode, they can also be
manipulated by software.
The T bit This reflects the operating state. When this bit is set, the processor is executed in
THUMB state, or otherwise it is executing in ARM state. This is reflected on the TBIT
external signal.
Note that the software must never change the state of the TBIT in the CPSR. If this
happens, the processor will enter an unpredictable state.
Interrupt disable bits I and F bits are the interrupt disable bits. When set, these disable the IRQ and FIQ
interrupts respectively.
The mode bits The M4, M3, M2, M1 and M0 bits (M[4:0]) are the mode bits. These determine the
processor's operating mode, as shown in Table 2-1. Not all combinations of the mode
bits define a valid processor mode. Only those explicitly described shall be used. The
user should be aware that if any illegal value is programmed into the mode bits, M[4:0],
then the processor will enter an unrecoverable state. If this occurs, reset should be
applied.
Reserved bits
The remaining bits in the PSRs are reserved. When changing a PSR's flag or control
bits, you must ensure that these unused bits are not altered. Also, your program should
not rely on them containing specific values, since in future processors they may read as
one or zero.