The 8088 and 8086 Microprocessors

1. Microprocessors: The Foundation of Modern Computing

A reprogrammable microcomputer is a general-purpose computer designed to run programs for a wide variety of applications, for instance, accounting, word processing, and languages such as BASIC.

Defining Microprocessors. Microprocessors are the central processing units (CPUs) of microcomputers, designed to execute instructions and perform a wide range of tasks. They are the core component that enables computers to run software applications, from simple accounting programs to complex word processors and programming languages. The evolution of microprocessors has driven the advancement of computing technology, leading to smaller, faster, and more powerful devices.

Key Components. A microcomputer system consists of several essential units: the input unit (e.g., keyboard, mouse), the output unit (e.g., monitor, printer), the microprocessing unit (MPU), and the memory unit. The MPU, or microprocessor, is responsible for fetching instructions from memory, decoding them, and executing the specified operations. Memory units store both the instructions and the data that the MPU processes.

Evolution and Impact. Microprocessors have evolved from 4-bit to 64-bit architectures, with significant increases in processing power and capabilities. Early microprocessors like the Intel 4004 paved the way for more advanced processors such as the 8086, 80386, and Pentium series. This evolution has enabled the development of personal computers, embedded systems, and a wide array of digital devices that have transformed modern life.

2. 8088/8086 Architecture: Understanding the Core Components

Bus interface unit and execution unit.

Dual Architecture. The 8088 and 8086 microprocessors feature a dual architecture comprising the Bus Interface Unit (BIU) and the Execution Unit (EU). The BIU handles all memory and I/O operations, fetching instructions and data, while the EU executes these instructions. This division of labor allows for more efficient processing, as the BIU can pre-fetch instructions while the EU is executing previous ones.

Memory Segmentation. The 8088/8086 architecture uses a segmented memory model, dividing the 1MB address space into 64KB segments. This segmentation is managed through segment registers: the Code Segment (CS), Stack Segment (SS), Data Segment (DS), and Extra Segment (ES). Each segment register holds the base address of a segment, and an offset is added to this base to access specific memory locations within the segment.

Registers and Flags. The 8088/8086 have a set of general-purpose registers (AX, BX, CX, DX) that can be used for various operations, as well as pointer registers (SP, BP) and index registers (SI, DI) for memory addressing. The flag register contains status and control flags that reflect the results of arithmetic and logical operations, such as the Carry Flag (CF), Zero Flag (ZF), and Overflow Flag (OF). These flags are crucial for conditional branching and program control.

3. Assembly Language Programming: Bridging the Gap

Mnemonic that identifies the operation to be performed by the instruction; ADD and MOV.

Low-Level Control. Assembly language provides a low-level interface to the microprocessor, allowing programmers to directly control hardware resources and optimize code for performance. Unlike high-level languages, assembly language uses mnemonic codes to represent machine instructions, making it more readable than raw machine code. Instructions like MOV (move data) and ADD (add data) are fundamental to assembly programming.

Assemblers and Compilers. An assembler is a program that translates assembly language source code into machine code, which the microprocessor can execute. Compilers, on the other hand, translate high-level languages like C or Java into machine code. Assembly language is often used for tasks that require precise control over hardware, such as device drivers, embedded systems, and real-time applications.

The Development Process. The assembly language program development process involves several steps: application specification, algorithm design, source code creation, assembly, linking, and debugging. The source code is written using a text editor and then assembled into an object module. The linker combines multiple object modules into a single executable file. Debugging tools are used to identify and fix errors in the program.

4. Debugging and Machine Language: Getting Under the Hood

The DEBUG program allows us to enter a program into the PC's memory, execute i t under control, view its operation, and modify it to fix errors.

Understanding Machine Code. Machine language is the binary code that the microprocessor directly executes. Each instruction is represented by a sequence of 0s and 1s, which specify the operation to be performed and the operands to be used. Understanding machine code is essential for debugging and optimizing assembly language programs.

The DEBUG Program. The DEBUG program is a command-line tool that allows programmers to interact with the microprocessor at a low level. It provides commands for examining and modifying memory, registers, and flags, as well as for executing programs step-by-step. DEBUG is invaluable for understanding how assembly language instructions are translated into machine code and how they affect the state of the microprocessor.

Debugging Techniques. Debugging involves identifying and fixing errors in a program. Syntax errors are violations of the assembly language rules, while execution errors are logical errors in the program's design. DEBUG can be used to trace the execution of a program, examine the contents of registers and memory, and set breakpoints to pause execution at specific points. This allows programmers to pinpoint the source of errors and correct them.

5. Integer Instructions and Computations: The Building Blocks

Value of immediate operand 0110H is moved into AX.

Data Transfer Instructions. Integer instructions form the foundation of arithmetic and logical operations in assembly language. Data transfer instructions, such as MOV, are used to copy data between registers, memory locations, and immediate values. These instructions are essential for initializing variables, loading data for processing, and storing results.

Arithmetic Operations. Arithmetic instructions perform basic mathematical operations on integer data. Instructions like ADD, SUB, MUL, and DIV are used for addition, subtraction, multiplication, and division, respectively. The 8088/8086 also provides instructions for incrementing (INC) and decrementing (DEC) values, as well as for performing signed and unsigned arithmetic.

Logical Operations. Logical instructions perform bitwise operations on integer data. Instructions like AND, OR, XOR, and NOT are used for logical AND, OR, exclusive OR, and NOT operations, respectively. These instructions are useful for masking bits, setting or clearing flags, and performing bit-level manipulations. Shift and rotate instructions, such as SHL, SHR, ROL, and ROR, are used to shift or rotate the bits in a register or memory location.

6. Control Flow and Program Structures: Directing the Action

Executing the first instruction causes the contents of the status register to be copied into AH.

Controlling Program Execution. Control flow instructions are used to alter the sequential execution of a program, allowing for conditional branching, looping, and subroutine calls. These instructions are essential for creating complex program structures and implementing algorithms.

Conditional Jumps. Conditional jump instructions, such as JE (jump if equal), JNE (jump if not equal), JG (jump if greater), and JL (jump if less), are used to branch to different parts of the program based on the state of the flags. These instructions allow for implementing conditional logic and decision-making in assembly language.

Loops and Subroutines. Loop instructions, such as LOOP, LOOPZ, and LOOPNZ, are used to repeat a block of code a specified number of times. Subroutine calls, using the CALL instruction, allow for modular programming by breaking a program into smaller, reusable functions. The RET instruction is used to return from a subroutine. String manipulation instructions, such as MOVSB, MOVSD, CMPSB, and CMPSD, are used to perform operations on strings of data.

7. Memory and I/O Interfaces: Connecting to the World

HMOS.

Interfacing with Memory. The 8088/8086 microprocessors interact with memory and I/O devices through dedicated interfaces. The memory interface allows the MPU to read and write data to memory locations, while the I/O interface allows it to communicate with peripheral devices. Understanding these interfaces is crucial for designing and building microcomputer systems.

Memory Organization. The 8088/8086 can address up to 1MB of memory, which is organized as a linear address space. Memory is accessed through a combination of segment and offset addresses. The memory interface involves address lines, data lines, and control signals such as RD (read) and WR (write).

I/O Operations. The I/O interface allows the MPU to communicate with peripheral devices such as keyboards, displays, and printers. I/O ports are accessed using the IN (input) and OUT (output) instructions. I/O devices can be addressed using either isolated I/O or memory-mapped I/O techniques.

8. Interrupts: Handling the Unexpected

Executing the first instruction causes the contents of the status register to be copied into AH.

Responding to Events. Interrupts are a mechanism for handling asynchronous events that require immediate attention from the microprocessor. They allow external devices or internal conditions to interrupt the normal execution of a program and transfer control to a specific interrupt service routine (ISR).

Types of Interrupts. The 8088/8086 supports several types of interrupts, including external hardware interrupts, software interrupts, internal interrupts, the non-maskable interrupt (NMI), and reset. External hardware interrupts are triggered by external devices, while software interrupts are initiated by the INT instruction. Internal interrupts are generated by the MPU in response to certain conditions, such as division by zero or overflow.

Interrupt Handling. When an interrupt occurs, the MPU saves the current state of the program (CS and IP) on the stack and transfers control to the ISR. The ISR performs the necessary actions to handle the interrupt and then returns control to the interrupted program using the IRET instruction. The interrupt controller manages external interrupt requests and prioritizes them if multiple interrupts occur simultaneously.

9. The Original IBM PC: A Hardware Overview

System address bus, system data bus, and system control bus.

System Architecture. The original IBM PC, based on the Intel 8088 microprocessor, established a standard architecture that influenced the design of subsequent personal computers. The PC's system architecture includes the system address bus, system data bus, and system control bus, which facilitate communication between the MPU, memory, and peripheral devices.

Key Components. The IBM PC's system board includes several key components, such as the 8284 clock generator, the 8259A interrupt controller, the 8253/5 timer, and the 8255A parallel peripheral interface. These components provide essential functions such as clock signals, interrupt management, timing, and parallel I/O.

Expansion Slots. The IBM PC features I/O channel slots that allow for the addition of expansion cards, enabling users to customize the system with additional functionality. These slots provide access to the system address bus, data bus, and control bus, allowing expansion cards to communicate with the MPU and other system resources.

10. Advanced Processors: Evolution and Compatibility

80386DX and 80386SX.

Advancements in Technology. The Intel 80286, 80386, 80486, and Pentium processors represent significant advancements in microprocessor technology, building upon the foundation of the 8088/8086 architecture. These processors feature increased processing power, larger address spaces, and enhanced capabilities such as protected mode and virtual memory.

Protected Mode. The 80286 introduced protected mode, which provides memory protection, multitasking, and virtual memory capabilities. Protected mode allows the operating system to manage memory resources more effectively and prevent programs from interfering with each other.

Compatibility. The advanced processors maintain a degree of backward compatibility with the 8088/8086, allowing programs written for the earlier processors to run on the newer ones. This compatibility is crucial for ensuring that existing software can be used on new hardware. The 80386 introduced virtual 8086 mode, which allows multiple 8086 programs to run concurrently in a protected environment.

Last updated: April 23, 2025