Supplement for Exception

CPU within Exception

• execute a new function when an exception occurs
• CPU don't need to save time for unexpected events
• bne, beq will cause a diffferent operation in PC
• but sometimes the hardware will urge the CPU to do something unexpected
• within the processor: Expection
• from outside the processor: Interrupt
• What must the processor do:

  A predefined process routine

RISC-V Privileged

• CPU: user operation, many regs cannot be accessed, only access to ports provided by operating system
• Monitoring: Operating System (S)
• Machine Mode: highest privilege, having access to everything

Control and Status Registers

• Environment call(ecall) MEPC->&ecall(exception, interrupt is PC+4)
• Breakpoint (ebreak) MEPC->&ebreak