Supplement to the hw4 main assignment.

Exception Handling

Description

In this part of the assignment, "all you have to do" is implement the hardware-side of trap handling. In particular, you need to:
• implement a new instruction, syscall, that forces a trap. When the trap occurs, the hardware needs to do all of the following in the current cycle:
  • EPC <- PC
  • Cause <- a unique id number indicating the event that occurred
  • Status <- 1
  • PC <- trap handler address
• implement a new instruction, rfe, that returns from an interrupt
  • PC <- EPC
  • Status <- 2
• test your implementation. Make sure syscall and rfe do the desired things by writing tiny test programs and observing that the machine jumps to the correct address and that the Status register is updated correctly.

(The formats of the new instructions are in the Cebollita documentation.)

The SMOK container provides a lot of support for these functions -- you should not have to understand the internals of that component to get this working. More information is provided at the end of this document.

Cause Codes

Exception Cause ID	Meaning
1	Char device needs attention
2	Block device needs attention
4	Addressing violation
8	System call
16	Integer overflow

Note that of these you need to implement only the syscall and addressing violation exceptions. You can have your trap handler just halt the machine.

Building Applications

All the previous rules apply. However, when you load your program you're going to need to set the trap handler register in your SMOK machine to the address of your trap handler in memory. Also, you will want to make sure that the Status register has interrupts enabled.