Supplement to the hw5 main assignment.

Exception Handling

Description

In this part of the assignment, "all you have to do" is implement the hardware-side of trap handling. You'll do this by adding additional components to the model. Because detecting and dealing with exceptions can be a bit tedious using raw hardware, we'll take a shortcut: we'll implement a new SMOK component type in software and insert it into the model. (More on that shortly.)

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
• detect some other exceptions, and take the steps listed for syscall. (This is the part we'll do in software.)
• 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. It is much easier to debug on small programs than on the OS.

(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.

Cause Codes

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

Cause ID	Meaning
1	Char device needs attention
2	Block device needs attention
4	Addressing violation
6	Bus error (no such address)
8	System call
9	Breakpoint
10	Privileged instruction
16	Integer overflow
32	Illegal instruction

Build a New Component Type

You'll be writing in C++, using a development environment you're not accustomed to, but:

• The code is only a couple dozen lines, in file ExceptionComponent.cpp.
• All of those lines would be the same in Java (even though this is C++).

See this page. (Some of the images are a version or so out of date, but shouldn't cause any confusion.)

You don't need the component you build to do very much. Its outputs are simply whether or not an exception has occurred (a single, one-bit output) and a cause (a single output a few bits wide). It takes as inputs whatever you find is needed to compute those outputs. (Note that the cause output is a "don't care" if there is no exception.) You code should not be very long.

One last thing: if you REALLY want to you can avoid C++ and Visual Studio and just build this entire step in logic. I think it will be easier to take he approach suggested, though, even if you don't know C++ and Visual Studio.

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.