Exceptions and Interrupts

Exceptions and Interrupts
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Notation:
- The sequence of addresses are denoted by \(a_0, a_1, \dots, a_{n-1}\)
- \(a_k\) denotes the address of instruction \(I_k\)
Control Transfer:
- A transition from \(a_k\) to \(a_{k+1}\) is called a control transfer and denoted by \(t_k\)
The Flow of Control:
- A sequence of control transfers is called the flow of control (or control flow) of the processor

Smooth and Non-Smooth Control Flow

Smooth Flow:
- \(I_{k+1}\) is adjacent to \(I_{k}\) (i.e., \(a_{k+1} = a_k + S(I_k)\), where \(S(I_k)\) denotes the size of instruction \(I_k\))
Some "Normal" Causes of Abrupt Changes:
- jumps, calls, returns
Some "Exceptional" Abrupt Changes:
- Abrupt changes that are not captured by internal program variables (e.g., dealing with a hardware timer)
- Abrupt changes that are not related to the execution of the program (e.g., data arriving at a network adapter)

Exceptions and Interrupts

Defined:
- An abrupt change in the control flow as a result of a change in the processor's state (called an event)
What Happens (assuming \(I_k\) is executing):
1. The processor detects that an event has occurred
2. A jump table is used to transfer control to a handler (which runs in kernel mode)
3. The program is aborted OR control is returned to \(I_k\) OR control is returned to \(I_{k+1}\) (in user mode)
Exceptions vs. Interrupts:
- Exceptions (including traps, faults, and aborts) are said to be synchronous (i.e., due to the execution of an instruction)
- Interrupts are said to be asynchronous (i.e., not due to the execution of any instruction)

Exceptions and Interrupts (cont.)

At "Design Time":
- Each exception/interrupt is assigned a unique nonnegative integer
At Boot Time:
- A jump table is created that associates exception/interrupt \(i\) with the address of the appropriate handler
- The starting address of the jump table is stored in the exception/interrupt table base register

Interrupts

Cause:
- A signal (not caused by the execution of any particular instruction) from an I/O device that is external to the processor
Return Behavior:
- Return to \(I_{k+1}\)

Traps

Faults

Cause:
- A potentially recoverable error
An Example - Page Faults:
- An instruction that references a virtual address whose corresponding page is not resident in memory (and must be retrieved from disk)
Return Behavior:
- Return to \(I_{k}\) (if the fault can be corrected)

Aborts

Cause:
- Unrecoverable error
Examples:
- Parity errors resulting from corrupted bits in DRAM or SRAM
- Divide errors
- Segmentation "faults" (i.e., a reference to an underfined area of virtual memory)
Return Behavior:
- Does not return