Handling Interrupts in Assembly Language
Interrupts are signals sent by hardware devices or software to the CPU, indicating an event that requires attention. In assembly language programming, interrupts play a crucial role in multitasking, asynchronous I/O operations, and error handling.
Types of interrupts
Interrupts can be classified into two main types:
- Hardware interrupts: These interrupts originate from hardware devices, such as timers, keyboards, or network interfaces, to signal events like timer ticks, key presses, or incoming data packets.
- Software interrupts: These interrupts are generated by software instructions, typically used for error handling or system calls.
Interrupt Vector Table (IVT)
The Interrupt Vector Table is a data structure that contains addresses of interrupt service routines for different interrupt types.
The sti (Set Interrupts) instruction enables interrupts, allowing the CPU to respond to interrupt requests.
Generate Software Interrupt
Software interrupts (also called system calls) can be triggered using the int instruction with a specified interrupt number.
Hardware devices, such as timers or keyboards, can generate interrupts. The CPU responds by executing the corresponding ISR.
Interrupt Service Routine (ISR)
An ISR is a subroutine that handles the interrupt. It must save the CPU's state, perform the required task, and restore the state before returning.
The cli (Clear Interrupts) instruction disables interrupts, preventing the CPU from responding to further interrupt requests.
Interrupt Descriptor Table (IDT)
On x86 architectures, the IDT is a more sophisticated structure that replaces the simpler IVT and provides a mapping between interrupt numbers and their corresponding ISR addresses.
Interrupts are often prioritized to ensure that critical events are handled promptly. Hardware devices can have different interrupt priorities, with higher-priority interrupts being served before lower-priority ones.
Interrupts are essential for asynchronous I/O operations, allowing the CPU to perform other tasks while waiting for I/O events. For instance, when a key is pressed, the keyboard sends an interrupt to the CPU, signaling the key press event. The CPU can then handle the key press without continuously polling the keyboard for input.
Interrupt Handling in Assembly Language
Interrupt handling is the process of responding to interrupts in assembly language programming. An interrupt is a signal sent by a hardware device or software to the CPU, indicating an event that requires attention. When an interrupt occurs, the CPU temporarily halts its current task and jumps to an interrupt handling routine, also known as an interrupt service routine (ISR). The ISR is a specific piece of code designed to handle the specific interrupt event.
Interrupt handling process
The interrupt handling process involves the following steps:
- Interrupt acknowledgment: The CPU sends an acknowledgment signal to the interrupting device, indicating that the interrupt has been received.
- Save processor state: The CPU saves its current state, including register values and stack pointers, on the stack to preserve the program's context.
- Jump to ISR: The CPU jumps to the ISR corresponding to the interrupt event.
- Execute ISR: The ISR executes the necessary code to handle the interrupt, such as processing input data, responding to hardware events, or performing error handling.
- Restore processor state: The CPU restores its saved state from the stack, allowing the interrupted program to resume execution from where it left off.
Below are detailed steps along with examples:
To allow the CPU to respond to interrupts, enable interrupts using the sti (Set Interrupts) instruction.
Define the Interrupt Vector Table (IVT)
The IVT contains addresses of interrupt service routines (ISRs) for various interrupt types.
Define individual ISRs for each interrupt type. ISRs handle specific interrupt events.
Hardware devices generate interrupts. ISRs for hardware interrupts must be defined and registered in the IVT.
Software interrupts (system calls) are triggered using the int instruction with a specified interrupt number.
Handle Interrupts in ISRs
Inside each ISR, save the CPU state, perform the required task, and restore the state before returning.
Interrupt Descriptor Table (IDT)
On x86 architectures, use the Interrupt Descriptor Table (IDT) to map interrupt numbers to ISR addresses.
Load IDT Address
Load the base address of the IDT into the IDTR (Interrupt Descriptor Table Register).
After handling an interrupt, acknowledge it to the interrupt controller (e.g., Programmable Interrupt Controller - PIC).
Use the cli (Clear Interrupts) instruction to disable interrupts temporarily.
Handle Interrupt Context Switching
If multitasking is implemented, handle context switching when switching between tasks during interrupts.
Example (for task-switching on x86):
Below is a simplified example of interrupt handling in assembly language, specifically for x86 architecture on Linux. This example uses software interrupts for simplicity:
Interrupt handling techniques
Several techniques are commonly used for interrupt handling in assembly language programming:
- Nested interrupts: Nested interrupts allow multiple interrupts to occur while an ISR is executing. The CPU maintains a stack of interrupt vectors, allowing it to handle interrupts in a nested fashion.
- Priority levels: Interrupts can be assigned priority levels to ensure that critical events are handled promptly. Higher-priority interrupts are served before lower-priority interrupts.
- Interrupt disable/enable flags: Interrupts can be disabled using special instructions or flags to prevent them from interrupting the CPU during critical code sections.
- Polling: Polling involves periodically checking the status of a hardware device to detect events instead of relying on interrupts. This approach is less efficient than interrupt-driven I/O but may be necessary for some devices.
Interrupt handling is an essential aspect of assembly language programming, enabling efficient multitasking, asynchronous I/O operations, and reliable error handling. By understanding and utilizing interrupt handling techniques effectively, assembly language programmers can write robust and responsive programs.