microprocessor 80286, 80386 , 80486, pentium

80286 microprocessor:

Basically this microprocessor is an advanced version of 8086. So before

starting with 80286 we must know something about

8086. Intel 8086 is a 16 bit microprocessor intended to be

used as a CPU in a microcomputer. The term 16 bit

means that its arithmetic logic unit, its internal registers,

its instructions are designed to work with 16 bit binary

words. It has a 20 bit address bus and 16 bit data bus. So

this means that it can address any one of the 1048576

memory locations and it can read data from or write data

to the memory and ports either 16 bit or 8 bit at a time.

Here the word is stored in two consecutive memory

locations. Also one thing to note here is that if the first

byte of the word is stored at even address, the 8086 can

read the entire word in one operation. Alternatively if the

first byte of the word is at odd address, then the 8086 will

read the first byte in one operation and the second byte

in the second operation. 8088 is just similar to 8086 but

the difference between the two is that 8088 has a 8 bit

data bus. The Intel 80186 is an improved version of 8086.

It is also a 16 bit microprocessor but it has a

programmable peripheral devices integrated in the same

package. 80186 instruction set has all the instructions of

8086 but also has certain additional instructions. A

program written in 8086 will execute properly on 80186

but the vice- versa is not true. With this brief introduction

of 8086,8088,80186 let’s explain 80286.

The Intel 80286 was introduced on

February 1, 1982 (also called Intel 286 or iAPX 286)

belongs to the family of 8086, is a high performance 16

bit microprocessor. As explained earlier it is an advanced

version of 8086 but with a different architectural

philosophy. It was widely used in IBM PC compatible

computers during the mid 1980s to early 1990s. Its initial

releases were of 6 and 8 MHZ but they were

subsequently scaled up to 12.5 MHZ (AMD and Harris

later pushed the architecture to speeds as high as 20 and

25 MHz, respectively.) On average, the 80286 had a

speed of about 0.21 Million instructions per clock. The 6

MHZ model operates at 0.9 MIPS, the 10MHZ model at 1.5

MIPS, and the 12 MHZ model at 1.8 MIPS. The 80286’s

performance was more than twice of its predecessors

(the intel 8086 and 8088) per clock cycle. Here the

complex mathematical operations took fewer clock cycles

compared to the 8086.

The 80286 eliminates the multiplexing of the buses. It

has a linear address bus with 24 address lines that can

address 16M bytes of memory directly. It also supports a

memory management unit, and through the memory

management unit it can address 1G bytes of memory,

also known as virutal memory. The processor includes

various built in mechanisms that can protect the system

software from the user programs, and restrict access to

some regions of the memory. The 80286 is specially

designed for multiuser and multitasking system. It has

four level memory protections and supports for operating

system.

There are two operating modes for 80286. The real

address mode and the protected virtual address mode. As

explained in the real address mode the processor can

address up to 1MB of the physical memory. The virtual

address mode is for multiuser and multitasking system.

In this mode of operation the memory management unit

can manage upto 1 GB of the virtual memory though the

real memory may be much less,only 16 MB. Basically in

this mode one user do not interfere with the other. Also

users cannot interfere with the operating system. These

features are called protection. THE 80286 contains four

processing units:

1.Bus unit

2.Instruction unit

3.Execution unit

4. Address unit

All memory and I/O read /write operations are performed

by BU. While the current instruction is being executed,

the BU prefetches instructions and keeps them in a queue

of six bytes. The function of IU is to decode the perfected

instructions and to maintain a queue of 3 decoded

instructions for execution. The EU executes instruction.

The address unit computes address of memory or I/O

devices, which is to be sent by BU for read and write

operation. All the four units work in parallel within the

CPU. This type of parallel operation is called pipelining. All

modern 16 bit CPU use pipelining. In pipelining several

execution units in a processor work simultaneously in

parallel.

80386 Microprocessor:

The Intel 80386 (also called

Intel386) is a microprocessor which has been used as the

CPU of many personal computers since 1986. During its

design phase the processor was code-named simply “p3”,

the third generation processor in the x 86 lines but it is

normally referred to as eitheri386 or just 386. The 80386

operated at 5 million instructions per second to 11.4MIPS

for the 33MHz model. It was the first x86 processor to

have 32 bit architecture, with a basic programming model

that has remained virtually unchanged for over 20 years.

Successively newer implementations of this same

architecture have become literally several hundred times

faster than the original i386 chip during these years.

As this is a 32 bit microprocessor it

has a circuitry of 275000 transistors. It was basically

introduced in the year 1985. It is compatible with 8086,

8088, 80186, 80286 microprocessors. It also contains a

four-level protection mechanism on the chip itself. It has

a total of 129 instructions. The 80386 is a 32 bit

microprocessor with a nonmultiplexed 32 bit address bus

housed in a 132 pin grid array package. Basically this

microprocessor has three versions: 80386SX,SL and DX.

The DX version has a 32 bit internal architecture

and a 32 bit data bus whereas the SX and the SL

version have a 32 bit internal architecture but a 16 bit

wide data bus . The SL version consumes less power

and is basically used in laptops and notebooks. These

versions operate from 20MHz to 33MHz. It is capable of

addressing 4G bytes of physical memory and through its

memory management unit it can address 64 terabytes of

the virtual memory. The processor can operate in two

modes: Real and protected. In the real mode physical

address space is 1Mbytes (20 address lines), which is

extended to 4G bytes in the protected mode (32 address

lines). The primary difference between these modes is

the availability of the memory space and the addressing

scheme. The 80386 has 32 bit registers and is upward

software compatible with the 8086. The execution of the

instructions is highly pipelined and the processor is

designed to operate in a multiuser and multitasking

environment. It has the protection mechanism for this

type of environment.

It has basically six functional units:

bus interface unit, code prefetch unit, instruction decode

unit, execution unit, segmentation unit and the paging

unit. It has the provision for both memory segmentation

and paging. A page is of fixed size 4KB each. Segment

vary in size, 4GB is the maximum size of a segment. The

80386 has 11 addressing modes: register, immediate,

direct, register indirect, based, indexed, scale indexed,

base indexed, base scale indexed, base indexed with

displacement and base scale indexed with displacement

addressing. In the scale indexed addressing the contents

of an indexed register are multiplied with a scaling factor

and the result is added to the displacement to obtain the

operand’s offset. As explained earlier it has 32 bit register

and has eight general purpose registers, six 16 bit

segment registers, also has a 32 bit instruction pointer,

six debug registers and a 32 bit status register. The

80386 has a segment descriptor register associated with

each segment register. The 80386 was widely used in

powerful PCs before the 80486 was devopled.

80486 Microprocessor:

Basically this is an

upgraded advanced version of 80386 and it was released

in the year 1989. It contains a 32 bit CPU, a floating-point

math coprocessor, unified instruction and data cache

memory and memory management unit in a single IC. It

contains an electronic circuitry of 1.2 million transistors.

Its operating frequency for its different versions is 25, 33,

66 and 100MHz. It is 3 to 5 times faster than 80386.

Basically this is available in two versions: DX and SX. The

DX type version is a 32 bit processor housed in a 168 pin

grid array package and can operate with the clock

frequencies from 25 to 66 MHz as explained earlier. The

important additional features of the 486 processor in

comparison with the 386 processor are as follows. The

486 processor includes:

· Built in math coprocessor. In the 386 system, a math

coprocessor is an external device. Therefore, the

math instructions in 486 systems are executed three

times faster than in 386 systems.

· 8K byte of code and data cache memory on the chip.

· Highly pipelined execution unit. Therefore the

execution time for many instructions is one clock

period.

Basically we do not use 80486 but instead of that we use

i486 because of a court ruling that prohibited

trademarking numbers. Intel dropped number-based

naming altogether with the successor to the i486-the

Pentium processor. The 486 contains the following

functional units:

· Execution unit

· Control unit

· Bus interface unit

· Code prefetch unit

· Instruction decode unit

· Segmentation unit

· Paging unit

· Cache unit

· Floating point unit.

The code prefetch unit contains a 32 byte queue to

store fetched instruction codes. The control unit also

contains a control ROM to store microcodes. The

segmentation unit calculates linear address (the

starting address of the segment plus the offset) from

the logical address. The address given in the

program is called the logical address. It also provides

4-level of protection for isolating and protecting tasks

and the operating system from each other. The

paging unit provides the paging facility within a

segment. It translates the linear address into the

physical address. The actual capacity of RAM and

ROM existing in a computer is known as physical

memory. The segmentation and the paging unit

constitute memory management unit.

In summary, the 486 is a high speed, high performance 32

bit microprocessor. It executes many of its instructions in

one clock cycle by using highly pipelined execution units.

It is designed to facilitate the execution of high level

languages and suited for multiprocessing and

multitasking systems. In the early 1990s, 486 was

generally used in high end microcomputers and network

environments.