
CHAPTER 9 CPU CORE INTERRUPTS
232
9.3 SOFTWARE INTERRUPTS GENERATED IN CPU CORE
Software interrupts generated in the CPU core are acknowledged by setting bits 1 and 0 of the IP (interrupt
pending) field in the Cause register. These may be written by software, but there is no hardware mechanism to set
or clear these bits.
After the processing of a software interrupt exception, corresponding bit of the IP field in the Cause register must
be cleared before returning to ordinary routine or enabling multiple interrupts.
These interrupts are maskable through the IM(1:0), IE, and EXL fields of the Status register.
9.4 TIMER INTERRUPT
The timer interrupt uses bit 15 of the Cause register, which is bit 7 of the IP (interrupt pending) field. This bit is
automatically set whenever the value of the Count register equals the value of the Compare register, to acknowledge
an interrupt request. This interrupt is maskable by setting IM7 of the Status register.
9.5 ASSERTING INTERRUPTS
9.5.1 Detecting Hardware Interrupts
Figure 9-2 shows how the hardware interrupt request is detected through the Cause register.
—
The timer interrupt signal, IP7, is directly readable as bit 15 of the Cause register.
—
Bits 4:0 of the Interrupt register are bit-wise ORed with the current value of the Int(4:0) signals and the result is
directly readable as bits 14:10 of the Cause register.
IP(1:0)of the Cause register, which are described in Chapter 6, are software interrupts. There is no hardware
mechanism for setting or clearing the software interrupts.