Features of Intel 8086 Microprocessor: Architecture Basics for Beginners

Intel 8086 is a 16-bit microprocessor developed by Intel in the year between early 1976 and mid-1978 and released in 1978. It is an advanced version of the Intel 8080 microprocessor. The 8086 microprocessor had a significant impact on the computer industry as it was the first processor to use the x86 instruction set architecture (ISA). The x86 ISA is still used in most modern processors.

The 8086 microprocessor has a complex architecture that consists of several functional blocks. The architecture includes registers, data bus, address bus, control unit, and arithmetic and logic unit (ALU) etc. 

Registers:

The registers are used to store data and instructions during the execution of a program. The registers can be classified into three categories: general-purpose registers, segment registers, and index registers.

Data Bus:

Data bus is used to transfer data between the processor and memory or input/output (I/O) devices.

Address Bus:

Address bus is used to identify the memory location or I/O device to be accessed.

Control Unit:

The control unit is responsible for controlling the flow of data and instructions within the processor. It retrieves instructions from memory and decodes them so that the arithmetic and logic unit can execute them.

Arithmetic and Logic Unit (ALU):

The ALU is responsible for performing arithmetic and logical operations on data. It can perform operations such as addition, subtraction, multiplication, division, and logical operations like AND, OR, and XOR.

Modes of Operation:

The 8086 microprocessor has two modes of operation: minimum mode and maximum mode. In the minimum mode, the 8086 microprocessor is used in a simple system that has only one processor. The minimum mode is used in applications that do not require the use of external coprocessors. In the maximum mode, the 8086 microprocessor is used in a complex system that has multiple processors and external coprocessors. In maximum mode, an external 8288 bus controller generates the bus control signals, which lets the 8086 work with a coprocessor such as the 8087 math coprocessor. The mode is selected with the MN/MX pin: connect it to +5 V for minimum mode and to ground for maximum mode.

Features of Intel 8086 Microprocessor:

• The 8086("eighty eighty-six") is also called iAPX 86
• Most popular and successful product by Intel
• Package: 40 pin dual inline pack (DIP)
• Maximum CPU clock rates 5 MHz to 10 MHz
• Instruction set:  x86-16
• 16 bits ALU (Arithmetic Logic Unit)
• 16 bits data bus or path 
• 20 bits address bus (The address refers to a byte in memory)
• The 8086 has eight more or less general 16-bit registers (including the stack pointer but excluding the instruction pointer, flag register and segment registers). Four of them, AX, BX, CX, DX, can also be accessed as twice as many 8-bit registers )while the other four, BP, SI, DI, SP, are 16-bit only.
• 8086 has a 16-bit flags register. Nine of these condition code flags are active, and indicate the current state of the processor: Carry flag (CF), Parity flag (PF), Auxiliary carry flag (AF), Zero flag (ZF), Sign flag (SF), Trap flag (TF), Interrupt flag (IF), Direction flag (DF), and Overflow flag (OF).
• Segment registers are also 16 bit

Applications:

The 8086 microprocessor has been used in a wide range of applications, including personal computers, industrial control systems, and embedded systems. The microprocessor has also been used in scientific equipment and medical devices.

The Intel 8086 microprocessor was a significant advancement in the computer industry. It introduced the x86 instruction set architecture, which is still used in modern processors. The microprocessor's complex architecture and modes of operation have made it suitable for use in a wide range of applications.

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Internal Architecture: BIU and EU

The 8086 is divided into two independent units that work in parallel:

• Bus Interface Unit (BIU): fetches instructions and data from memory, calculates the 20-bit physical address, and contains the segment registers, the instruction pointer (IP) and a 6-byte instruction queue.
• Execution Unit (EU): decodes and executes instructions. It contains the ALU, the general-purpose registers, the pointer and index registers, and the flag register.

While the EU executes one instruction, the BIU pre-fetches the next instruction bytes into the 6-byte queue. This overlap is called pipelining and makes the 8086 faster than the 8085.

Memory Segmentation and Physical Address

The 8086 has a 20-bit address bus, so it can address 2²⁰ = 1 MB of memory. Its registers are only 16 bits, so it uses segmentation: memory is divided into segments of up to 64 KB, and the physical address is formed as:

Physical address = Segment register × 10H + Offset

Example: If CS = 2000H and IP = 1234H, the physical address = 20000H + 1234H = 21234H.

8086 Registers at a Glance

Register	Name	Main use
AX	Accumulator	Arithmetic, I/O, MUL and DIV
BX	Base register	Base address for memory access
CX	Count register	Loop counter (LOOP), shift and string count
DX	Data register	I/O port address, high word in MUL/DIV
SP / BP	Stack pointer / Base pointer	Top of stack / accessing data on the stack
SI / DI	Source / Destination index	String operations and indexed addressing
CS, DS, SS, ES	Segment registers	Code, data, stack and extra segments
IP	Instruction pointer	Offset of the next instruction in the code segment
FLAGS	Flag register	6 status flags (CF, PF, AF, ZF, SF, OF) and 3 control flags (TF, IF, DF)

Practice MCQs for Job and University Exams

1. The 8086 can address how much memory?
(a) 64 KB
(b) 256 KB
(c) 1 MB
(d) 4 GB
Answer: (c)

2. The size of the instruction queue in the 8086 is:
(a) 4 bytes
(b) 6 bytes
(c) 8 bytes
(d) 16 bytes
Answer: (b)

3. If DS = 1000H and the offset is 0020H, the physical address is:
(a) 1020H
(b) 10020H
(c) 01020H
(d) 10200H
Answer: (b)

4. Which pin selects minimum or maximum mode in the 8086?
(a) ALE
(b) MN/MX
(c) READY
(d) HOLD
Answer: (b)

5. How many flags in the 8086 are control flags?
(a) 2
(b) 3
(c) 6
(d) 9
Answer: (b)

6. Which unit of the 8086 calculates the physical address?
(a) EU
(b) ALU
(c) BIU
(d) Control unit
Answer: (c)

7. The 8086 is a:
(a) 8-bit processor
(b) 16-bit processor
(c) 32-bit processor
(d) 64-bit processor
Answer: (b)

Frequently Asked Questions (FAQs)

Q: What is the difference between the 8085 and the 8086?
A: The 8085 is an 8-bit processor with a 16-bit address bus (64 KB memory) and no pipelining. The 8086 is a 16-bit processor with a 20-bit address bus (1 MB memory), memory segmentation and a 6-byte instruction queue.

Q: Why is segmentation used in the 8086?
A: The registers are 16 bits wide, which can only address 64 KB directly. Combining a segment and an offset produces a 20-bit address, so the processor can reach the full 1 MB of memory.

Q: What is the 8088?
A: The 8088 is a version of the 8086 with an 8-bit external data bus and a 4-byte instruction queue. It was used in the original IBM PC.

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