Interrupts and Instruction set of 8086

Introduction

Microprocessors are the heart of modern computing systems, and they rely on interrupts and instruction sets to perform various tasks. In this topic, we will explore the importance of interrupts in microprocessors and delve into the instruction set of the 8086 microprocessor.

Importance of interrupts in microprocessors

Interrupts play a crucial role in microprocessors as they allow the processor to respond to external events or internal conditions that require immediate attention. By interrupting the normal execution of a program, interrupts enable the processor to handle time-critical tasks efficiently.

Overview of the 8086 microprocessor and its instruction set

The 8086 microprocessor, developed by Intel, is a 16-bit microprocessor that revolutionized the world of computing. It introduced a new instruction set architecture (ISA) that offered enhanced capabilities and improved performance.

Interrupts

Interrupts are signals that temporarily suspend the execution of a program and transfer control to a specific interrupt handler or interrupt service routine (ISR). The 8086 microprocessor supports two types of interrupts: hardware interrupts and software interrupts.

Hardware interrupts

Hardware interrupts are generated by external devices to request the attention of the microprocessor. These interrupts can be triggered by events such as keyboard input, timer expiration, or I/O device status changes. The 8086 microprocessor has a dedicated hardware interrupt pin, INTR, which is used to signal the occurrence of a hardware interrupt.

Software interrupts

Software interrupts, also known as software traps or software exceptions, are generated by software instructions to invoke specific routines or services. These interrupts are typically used for system calls, error handling, or other software-related tasks. The 8086 microprocessor provides a software interrupt instruction, INT, which is used to generate a software interrupt.

Interrupt vector table

The interrupt vector table is a data structure that contains the addresses of the interrupt service routines for different interrupts. It is stored in memory and is accessed by the microprocessor to determine the appropriate ISR to execute when an interrupt occurs.

Structure and organization

The interrupt vector table in the 8086 microprocessor is organized as a sequential list of 256 entries, each corresponding to a specific interrupt number. Each entry in the table contains a 4-byte address that points to the starting address of the ISR for that interrupt.

Role in handling interrupts

When an interrupt occurs, the microprocessor uses the interrupt number to index into the interrupt vector table and fetches the corresponding ISR address. It then transfers control to the ISR by jumping to that address. Once the ISR completes its execution, the microprocessor resumes the interrupted program by returning to the instruction following the interrupt.

Interrupt service routine (ISR)

The interrupt service routine, also known as the interrupt handler, is a subroutine that is executed when an interrupt occurs. Its main function is to handle the interrupt by performing the necessary tasks and then returning control to the interrupted program.

Definition and function

An ISR is a piece of code that is specifically written to handle a particular interrupt. It is responsible for saving the state of the interrupted program, executing the required tasks, and restoring the saved state before returning control to the interrupted program.

Steps involved in handling an interrupt

The execution of an ISR typically involves the following steps:

1. Saving the state of the interrupted program, including the values of registers and flags.
2. Disabling further interrupts to prevent nested interrupts.
3. Executing the necessary tasks or routines to handle the interrupt.
4. Restoring the saved state of the interrupted program.
5. Enabling interrupts to allow the occurrence of further interrupts.

Interrupt priority and masking

Interrupt priority determines the order in which interrupts are serviced when multiple interrupts occur simultaneously. The 8086 microprocessor supports three levels of interrupt priority: high, medium, and low. Higher priority interrupts are serviced before lower priority interrupts.

Priority levels of interrupts

The interrupt priority levels in the 8086 microprocessor are assigned as follows:

• High priority: Non-maskable interrupt (NMI)
• Medium priority: Maskable hardware interrupts (INT0-INT7)
• Low priority: Software interrupts (INT 8-255)

Masking and unmasking interrupts

Interrupts can be masked or disabled to prevent their occurrence or to temporarily suspend their servicing. The 8086 microprocessor provides instructions to enable or disable interrupts by setting or clearing the interrupt flag (IF) in the flags register.

Instruction Set of 8086

The instruction set of the 8086 microprocessor consists of a wide range of instructions that perform various operations, including data movement, arithmetic and logical operations, control transfers, string operations, and input/output operations.

Overview of the instruction set architecture (ISA)

The instruction set architecture (ISA) of a microprocessor defines the set of instructions that the processor can execute. The ISA of the 8086 microprocessor is based on the x86 architecture and is known for its rich set of instructions and addressing modes.

Data movement instructions

Data movement instructions are used to transfer data between registers, memory locations, and immediate values.

Register-to-register data transfer

Register-to-register data transfer instructions move data between general-purpose registers or between a general-purpose register and a segment register.

Immediate data transfer

Immediate data transfer instructions load immediate values into registers or memory locations.

Memory-to-register and register-to-memory data transfer

Memory-to-register instructions transfer data from memory locations to registers, while register-to-memory instructions transfer data from registers to memory locations.

Arithmetic and logical instructions

Arithmetic and logical instructions perform mathematical and logical operations on data stored in registers or memory locations.

Addition, subtraction, multiplication, and division

The 8086 microprocessor provides instructions for addition, subtraction, multiplication, and division of binary numbers.

Bitwise operations (AND, OR, XOR, NOT)

Bitwise operations manipulate individual bits of data using logical operators such as AND, OR, XOR, and NOT.

Control transfer instructions

Control transfer instructions alter the flow of program execution by changing the sequence of instructions.

Unconditional and conditional jumps

Unconditional jump instructions transfer control to a specified memory address unconditionally. Conditional jump instructions transfer control based on the values of specific flags or registers.

Looping instructions

Looping instructions are used to repeat a sequence of instructions a specific number of times.

Procedure calls and returns

Procedure call instructions transfer control to a subroutine or procedure, while return instructions transfer control back to the calling program.

String instructions

String instructions are used to perform operations on strings of data, such as moving, comparing, and manipulating strings.

String movement and comparison instructions

String movement instructions copy or move blocks of data from one memory location to another. String comparison instructions compare two strings and set the appropriate flags based on the comparison result.

String manipulation instructions

String manipulation instructions perform operations on strings, such as filling a string with a specific character or searching for a substring within a string.

Input/output instructions

Input/output instructions are used to communicate with external devices, such as keyboards, displays, and storage devices.

Port input and output

Port input instructions read data from a specified input port, while port output instructions write data to a specified output port.

Memory-mapped input and output

Memory-mapped input and output instructions transfer data between memory locations and I/O devices that are mapped to specific memory addresses.

Examples and Applications

The knowledge of interrupts and the instruction set of the 8086 microprocessor can be applied in various scenarios, such as:

Writing an assembly program to handle a specific interrupt

By understanding interrupts and ISRs, you can write assembly programs that respond to specific events or conditions. For example, you can write a program that handles keyboard interrupts to perform certain actions when specific keys are pressed.

Using the instruction set to perform mathematical calculations

The rich instruction set of the 8086 microprocessor allows you to perform complex mathematical calculations efficiently. You can write assembly programs that leverage the arithmetic and logical instructions to solve mathematical problems.

Controlling external devices through input/output instructions

The input/output instructions of the 8086 microprocessor enable you to communicate with external devices. You can write assembly programs that control devices such as displays, printers, or storage devices by using the appropriate input/output instructions.

Advantages and Disadvantages

Interrupts and the instruction set of the 8086 microprocessor offer several advantages and disadvantages.

Advantages of interrupts in microprocessors

1. Efficient handling of external events: Interrupts allow microprocessors to respond quickly to external events, such as user input or device status changes. This enables efficient multitasking and real-time processing.
2. Improved system responsiveness: By using interrupts, microprocessors can prioritize time-critical tasks and respond to them immediately. This improves the overall responsiveness and performance of the system.

Disadvantages of interrupts

1. Increased complexity in programming and debugging: Interrupt-driven programming requires careful handling of interrupts and synchronization between different interrupt service routines. This complexity can make programming and debugging more challenging.
2. Potential for interrupt conflicts and priority issues: When multiple interrupts occur simultaneously or in quick succession, conflicts can arise, leading to unpredictable behavior or priority issues. Proper interrupt handling and priority management are essential to avoid such problems.

Summary

Microprocessors rely on interrupts and instruction sets to perform various tasks. Interrupts are signals that temporarily suspend the execution of a program and transfer control to a specific interrupt handler or interrupt service routine (ISR). The 8086 microprocessor supports two types of interrupts: hardware interrupts and software interrupts. Hardware interrupts are generated by external devices, while software interrupts are generated by software instructions. The interrupt vector table is a data structure that contains the addresses of the ISRs for different interrupts. The ISR is a subroutine that handles the interrupt by performing the necessary tasks. The 8086 microprocessor also has an instruction set that consists of various instructions for data movement, arithmetic and logical operations, control transfers, string operations, and input/output operations. Understanding interrupts and the instruction set of the 8086 microprocessor is crucial for efficient programming and utilization of the microprocessor's capabilities.

Analogy

Imagine you are a teacher in a classroom full of students. You are delivering a lecture, and suddenly a student raises their hand, indicating that they have a question. This interruption requires you to pause your lecture, address the student's question, and then resume your lecture. Similarly, interrupts in microprocessors are signals that temporarily suspend the execution of a program, transfer control to a specific interrupt handler, and then resume the program once the interrupt is handled.