⭐ Exceptions in Instruction Sets

Definition

An exception is an unexpected or special event that occurs during the execution of an instruction and requires the CPU to temporarily interrupt normal execution to handle the event.

Exceptions alter the normal flow of a program due to internal CPU or instruction-related conditions.

They are handled by special routines called exception handlers or trap handlers.

Exceptions are part of the ISA (Instruction Set Architecture) because:

• The ISA defines how exceptions occur,
• What types exist,
• How the CPU should respond,
• Where the handler is located.

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⭐ Interrupts vs. Exceptions (Quick Difference)

Feature	Interrupt	Exception
Cause	External device	Internal CPU/instruction error
Timing	After current instruction	Usually during the instruction
Example	Keyboard press	Divide by zero

Exceptions are caused inside the CPU. Interrupts come from outside.

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⭐ Types of Exceptions

Exceptions can be classified into several categories. These categories commonly appear in exams.

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1. Faults

Definition

A fault is an exception that can be repaired, allowing the program to resume execution from the same instruction that caused the exception.

Examples

• Page fault (memory page not in RAM)
• Invalid address
• Segment fault
• Missing translation in virtual memory

Behavior

• Hardware restarts the faulting instruction after the handler fixes the problem.

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2. Traps

Definition

A trap is an exception that occurs intentionally to request a service from the operating system. They represent system calls.

Examples

• System call (e.g., Linux syscall)
• Breakpoint trap (for debugging)
• Software interrupt

Behavior

• Return to the next instruction after the trap.

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3. Aborts

Definition

An abort is a severe exception that cannot be corrected; the program cannot resume.

Examples

• Hardware failure
• Corrupted instruction
• Memory parity error
• Invalid machine instruction format

Behavior

• Program is terminated
• Often triggers OS-level error message

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4. Asynchronous Exceptions

Definition

These occur asynchronously with program execution (rarely considered exceptions, more like interrupts).

Examples include:

• Power failure
• Reset signal

These are handled similar to interrupts.

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⭐ Causes of Exceptions in Instruction Sets

Exceptions can be triggered during instruction execution due to:

1. Arithmetic Errors

• Divide by zero
• Overflow
• Underflow
• Invalid floating-point operation

2. Memory Access Errors

• Page fault
• Accessing invalid memory
• Misaligned memory access

3. Illegal Instructions

• Executing undefined opcodes
• Privileged instruction executed in user-mode

4. System Calls (Software Requests)

• Using syscall or trap instructions

5. Protection Violations

• Write to read-only memory
• Accessing kernel memory from user mode

6. Hardware Failures

• Parity/ECC errors
• Cache failure

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⭐ Exception Mechanism (How CPU Handles Exceptions)

When an exception occurs:

Step 1: Save Context

The CPU saves critical information:

• Program Counter (PC)
• Cause of exception
• Register state

Step 2: Transfer Control

The CPU jumps to exception handler located at a specific memory address (defined by the ISA).

Step 3: Execute Handler

The OS or hardware:

• Fixes the error (e.g., loads missing page)
• Or stops the program if fatal

Step 4: Resume or Terminate

• Faults → resume faulty instruction
• Traps → resume next instruction
• Aborts → terminate program

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⭐ Exceptions in RISC Architectures (e.g., MIPS)

Common exceptions in MIPS:

• Overflow
• Address error (misaligned access)
• Page fault
• System call (syscall)
• Breakpoint
• Reserved instruction

MIPS uses:

• Exception Program Counter (EPC) to store the address of the instruction
• Cause register to store the reason

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⭐ Exceptions in CISC Architectures (e.g., x86)

x86 has many exceptions:

• Divide error
• Debug trap
• Breakpoint
• Overflow trap
• Invalid opcode
• Page fault
• General protection fault
• Double fault

It uses the Interrupt Descriptor Table (IDT) to define handlers.

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⭐ Why Exceptions Are Important in ISA

1. Support for virtual memory
2. Protection and security
3. Error handling
4. Support for system calls
5. Smooth interaction between user programs and operating system
6. Hardware-level fault recovery

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⭐ Exam-Friendly Summary

• Exception: An internal, unexpected event during instruction execution.
• Types: Faults (restart), Traps (next instruction), Aborts (terminate).
• Causes: Arithmetic errors, illegal opcodes, page faults, system calls.
• Handling: Save PC, jump to handler, fix or terminate, resume if possible.
• ISA defines how exceptions are signaled and handled.