Introduction to 8086 Microprocessor

Features of 8086 It is 16 bit processor. So that it has 16 bit ALU,

16 bit registers and internal data bus and 16 bit external data bus. It make s faster processing.

It has three version based on the frequency of operation: 8086 -> 5MHz 8086-2 ->8MHz 8086-1 ->10 MHz

Features of 8086 (cont..) 8086 has 20 bit address lines to access

memory. Hence it can access                   2^20 = 1 MB memory location. 8086 has 16-bit address lines to access I/O

devices, hence it can access        2^16 = 64K I/O location

Features of 8086 (cont..) Pipelining:-8086 uses two stage of

pipelining. First is Fetch Stage and the second is Execute Stage. Fetch stage that prefetch upto 6 bytes of

instructions stores them in the queue. Execute stage that executes these instructions.

Pipelining improves the performance of the processor so that operation is faster.

Features of 8086 (cont..) Operates in two modes:-8086 operates in two

modes: Minimum Mode: A system with only one

microprocessor. Maximum Mode:-A system with multiprocessor.

8086 uses memory banksThe 8086 uses a memory banking system. It means entire data is not stored sequentially in a single memory of 1 MB but memory is divided into two banks of 512KB.

Features of 8086 (cont..) Interrupts

8086 has 256 vectored interrupts. Multiplication And Division

8086 has a powerful instruction set. So that it supports Multiply and Divide operation.

Architecture of 8086 The architecture of 8086 includes

Arithmetic Logic Unit (ALU) Flags General registers Instruction byte queue Segment registers

EU & BIU The 8086 CPU logic has been partitioned into two

functional units namely Bus Interface Unit (BIU) Execution Unit (EU)

The major reason for this separation is to increase the processing speed of the processor

The BIU has to interact with memory and input and output devices in fetching the instructions and data required by the EU

EU is responsible for executing the instructions of the programs and to carry out the required processing

EU & BIU

Architecture Diagram

Execution Unit The Execution Unit (EU) has

Control unit Instruction decoder Arithmetic and Logical Unit (ALU) General registers Flag register Pointers Index registers

Execution Unit Control unit is responsible for the co-

ordination of all other units of the processor ALU performs various arithmetic and logical

operations over the data The instruction decoder translates the

instructions fetched from the memory into a series of actions that are carried out by the EU

Execution Unit - Registers The EU has eight general purpose registers labeled

AH, AL, BH, BL, CH, CL, DH, and DL.  These registers can be used individually for temporary

storage of 8-bit data. The AL register is also called the accumulator. Certain pairs of these general purpose registers can

be used together to store 16-bit data. The valid register pairs are AH and AL, BH and BL,

CH and CL and DH and DL. These register pairs is referred to the AX, BX, CX, and

DX resp.

Execution Unit - Registers 1. AX Register: For 16-bit operations, AX is called

the accumulator register that stores operands for arithmetic operations.

2. BX Register: This register is mainly used as a base register. It holds the starting base location of a memory region within a data segment.

3. CX Register: It is defined as a counter. It is primarily used in loop instruction to store loop counter.

4. DX Register: DX register is used to contain I/O port address for I/O instruction.

Execution Unit - Registers

Execution Unit - Flags

Execution Unit - Flags Overflow Flag (OF) - set if the result is too large

positive number, or is too small negative number to fit into destination operand

Direction Flag (DF) - if set then string manipulation instructions will auto-decrement index registers. If cleared then the index registers will be auto-incremented

Interrupt-enable Flag (IF) - setting this bit enables maskable interrupts

Single-step Flag (TF) - if set then single-step interrupt will occur after the next instruction

Execution Unit - Flags Sign Flag (SF) - set if the most significant bit of the

result is set. Zero Flag (ZF) - set if the result is zero. Auxiliary carry Flag (AF) - set if there was a carry

from or borrow to bits 0-3 in the AL register. Parity Flag (PF) - set if parity (the number of "1" bits)

in the low-order byte of the result is even. Carry Flag (CF) - set if there was a carry from or

borrow to the most significant bit during last result calculation

Execution Unit - Flags

Execution Unit - Pointers Stack Pointer (SP) is a 16-bit register pointing to

program stack Base Pointer (BP) is a 16-bit register pointing to data

in stack segment. BP register is usually used for based, based indexed or register indirect addressing.

Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data addresses in string manipulation instructions.

Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data addresses in string manipulation instructions.

Execution Unit - Pointers

Bus Interface Unit The BIU has

Instruction Queue Segment registers Instruction pointer

BIU – Instruction Queue To increase the execution speed, BIU fetches as many as

six instruction bytes ahead to time from memory. The prefetched instruction bytes are held for the EU in a

first in first out group of registers called a instruction queue. 

When the EU is ready for its next instruction, it simply reads the instruction from this instruction queue.

This is much faster than sending out an address to the system memory and to send back the next instruction byte.

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

BIU – Instruction Byte Queue Execution and decoding of certain instructions do

not require the use of buses While such instructions are executed, the BIU

fetches up to six instruction bytes for the following instructions (the subsequent instructions)

The BIU store these prefetched bytes in a first-in-first out register by name instruction byte queue

When the EU is ready for its next instruction, it simply reads the instruction byte(s) for the instruction from the queue in BIU

Segment Registers The memory of 8086 is divided into 4 segments

namely Code segment (program memory) Data segment (data memory) Stack memory (stack segment) Extra memory (extra segment)

These segment registers are used to hold the upper 16 bits of the starting address for each of the segments.

The part of a segment starting address stored in a segment register is often called the segment base.

Instruction Pointer (IP)

The instruction pointer (IP) holds the 16-bit address of the next code byte within this code segment.