8086 information and architecture document, Exams of Microprocessor and Interfacing

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Microprocessor - 8086 Overview

8086 Microprocessor is an enhanced version of 8085Microprocessor that was designed by Intel in 1976. It is a 16-bit Microprocessor having 20 address lines and16 data lines that provides up to 1MB storage. It consists of powerful instruction set, which provides operations like multiplication and division easily. It supports two modes of operation, i.e. Maximum mode and Minimum mode. Maximum mode is suitable for system having multiple processors and Minimum mode is suitable for system having a single processor.

Features of 8086

The most prominent features of a 8086 microprocessor are as follows −  It has an instruction queue, which is capable of storing six instruction bytes from the memory resulting in faster processing.  It was the first 16-bit processor having 16-bit ALU, 16-bit registers, internal data bus, and 16-bit external data bus resulting in faster processing.  It is available in 3 versions based on the frequency of operation − o 8086 → 5MHz o 8086 - 2 → 8MHz o (c)8086-1 → 10 MHz  It uses two stages of pipelining, i.e. Fetch Stage and Execute Stage, which improves performance.  Fetch stage can prefetch up to 6 bytes of instructions and stores them in the queue.  Execute stage executes these instructions.  It has 256 vectored interrupts.  It consists of 29,000 transistors.

Comparison between 8085 & 8086 Microprocessor

 Size − 8085 is 8 - bit microprocessor, whereas 8086 is 16-bit microprocessor.  Address Bus − 8085 has 16-bit address bus while 8086 has 20-bit address bus.  Memory − 8085 can access up to 64Kb, whereas 8086 can access up to 1 Mb of memory.

 Instruction − 8085 doesn’t have an instruction queue, whereas 8086 has an instruction queue.  Pipelining − 8085 doesn’t support a pipelined architecture while 8086 supports a pipelined architecture.  I/O − 8085 can address 2^8 = 256 I/O's, whereas 8086 can access 2^16 = 65,536 I/O's.  Cost − The cost of 8085 is low whereas that of 8086 is high.

Architecture of 8086

The following diagram depicts the architecture of a 8086 Microprocessor −

Microprocessor - 8086 Functional Units

8086 Microprocessor is divided into two functional units, i.e., EU (Execution Unit) and BIU (Bus Interface Unit).

EU (Execution Unit)

Execution unit gives instructions to BIU stating from where to fetch the data and then decode and execute those instructions. Its function is to

 Trap flag − It is used for single step control and allows the user to execute one instruction at a time for debugging. If it is set, then the program can be run in a single step mode.  Interrupt flag − It is an interrupt enable/disable flag, i.e. used to allow/prohibit the interruption of a program. It is set to 1 for interrupt enabled condition and set to 0 for interrupt disabled condition.  Direction flag − It is used in string operation. As the name suggests when it is set then string bytes are accessed from the higher memory address to the lower memory address and vice-a-versa.

General purpose register

There are 8 general purpose registers, i.e., AH, AL, BH, BL, CH, CL, DH, and DL. These registers can be used individually to store 8-bit data and can be used in pairs to store 16bit data. The valid register pairs are AH and AL, BH and BL, CH and CL, and DH and DL. It is referred to the AX, BX, CX, and DX respectively.  AX register − It is also known as accumulator register. It is used to store operands for arithmetic operations.  BX register − It is used as a base register. It is used to store the starting base address of the memory area within the data segment.  CX register − It is referred to as counter. It is used in loop instruction to store the loop counter.  DX register − This register is used to hold I/O port address for I/O instruction.

Stack pointer register

It is a 16-bit register, which holds the address from the start of the segment to the memory location, where a word was most recently stored on the stack.

BIU (Bus Interface Unit)

BIU takes care of all data and addresses transfers on the buses for the EU like sending addresses, fetching instructions from the memory, reading data from the ports and the memory as well as writing data to the ports and the memory. EU has no direction connection with System Buses so this is possible with the BIU. EU and BIU are connected with the Internal Bus. It has the following functional parts −  Instruction queue − BIU contains the instruction queue. BIU gets upto 6 bytes of next instructions and stores them in the instruction queue. When EU executes instructions

and is ready for its next instruction, then it simply reads the instruction from this instruction queue resulting in increased execution speed.

 Fetching the next instruction while the current instruction executes is called pipelining.

 Segment register − BIU has 4 segment buses, i.e. CS, DS, SS& ES. It holds the addresses of instructions and data in memory, which are used by the processor to access memory locations. It also contains 1 pointer register IP, which holds the address of the next instruction to executed by the EU. o CS − It stands for Code Segment. It is used for addressing a memory location in the code segment of the memory, where the executable program is stored. o DS − It stands for Data Segment. It consists of data used by the program andis accessed in the data segment by an offset address or the content of other register that holds the offset address. o SS − It stands for Stack Segment. It handles memory to store data and addresses during execution. o ES − It stands for Extra Segment. ES is additional data segment, which is used by the string to hold the extra destination data.

 Instruction pointer − It is a 16-bit register used to hold the address of the next instruction to be executed.