Download Document

University Of Hail
Community College
Electrical Engineering Department
Electronics Engineering and Instrumentation Program
Digital Circuits II (EEI 200)
Dr. Fawzy Hashem
Date:
INTRODUCTION TO MICROPROCESSORS
The Basic Computer
Special-purpose computers control various functions in automobiles, consumer
appliances, control manufacturing processes in industry, and are used in navigation
systems such as GPS (global positioning system), and many other applications.
However the most familiar type of computer is the general-purpose computer that
can be programmed to do many different tasks.
All computers consist of basic functional blocks that include a central processing unit
(CPU), memory, and input/output ports. These functional blocks are connected
together with internal buses, as shown below:
The three buses are the data bus, the address bus, and the control bus. Input and
output devices (peripherals) are connected through the input/output ports. A port is a
physical interface on computer through which data are passed to and from peripherals.
Instruction and data are stored in memory in specific locations determined by the
program, a list of instructions designed to solve a specific problem. Each location has
unique address associated with it. Instructions are obtained by the CPU by placing an
address on the address bus. Instructions are transferred via the data bus as they are
requested by the CPU. The CPU executes the instructions sequentially; frequently,
the instructions modify data stored in memory or obtained from an input device.
Processed data may be stored back in memory or sent to an output device via the data
bus. Signals on control bus are generated by the CPU to coordinate all of these
operations.
1
Central Processing Unit (CPU)
The CPU is the “brain” of the computer; it consists of the microprocessor with
associated circuits that control the running of the computer software programs.
Basically, the CPU obtains (fetches) each program instruction from memory and
carry out (executes) the instruction. After completing one instruction, the CPU moves
on the next one and in most cases can operate on more than one instruction at the
same time. This “fetch” and “execute” process is repeated until all of the instructions
in a specific program have been executed.
The Microprocessor
The microprocessor is a digital integrated circuit that can be programmed with series
of instructions to perform various operations on data. It can do arithmetic and logic
operations, move data from one place to another, and make decisions based on certain
instructions.
A microprocessor consists of several units, each designed for a specific job. Four
basic units that are common to all microprocessors are: the arithmetic and logic unit
(ALU), the instruction decoder, the register array, and the control unit, as shown
below:
Arithmetic Logic Unit: the ALU is the key processing elements of the
microprocessor. It is directed by the control unit to perform arithmetic operations
(addition, subtraction, multiplication, and division) and logic operation (NOT, AND,
OR, and exclusive-OR), as well as many other types of operations. Data for the ALU
are obtained from the register array.
Instruction decoder: the instruction decoder takes each binary instruction in order in
which it appears in memory and decodes (translates) it.
Register array: the register array is a collection of registers that are contained within
the microprocessor. During the execution of a program, data and memory addresses
are temporarily stored in registers that make up this array. The ALU can access the
registers very quickly, making the program run more efficiently. Some registers are
classed as general-purpose, meaning they can be used for any purpose dictated by the
program.
2
Other registers have a specific capabilities and functions and cannot be used as
general-purpose registers. Still others are called program invisible registers, used only
by the microprocessor and not available to the program.
Control unit: the control unit is “in charge” of the processing of instructions once
they are decoded. It provides the timing and control signals for getting data into and
out of microprocessor and for synchronizing the execution of instructions.
Microprocessor Buses
The three busses mentioned earlier are the connections for microprocessor to allow
data, addresses, and instructions to be moved. They are:
The address bus: the address bus is a “one-way-street” over which the
microprocessor sends an address code to memory or other external device. The size or
width of the address bus is specified by the number of conductive paths or bits. The
more bits lines there are in the address, the higher the number of memory locations
that can be accessed. The number of address bits has advanced to the point where
microprocessor have up to 64 address bits and can access over 264 memory locations.
The data bus: the data bus is “two-way-street” on which data or instructions codes
are transferred into the microprocessor or the result of an operation or computation is
sent out. The original microprocessors had 8-bit data busses. Today’s microprocessors
have up to 64-bit data buses.
The control bus: the control bus is used by the microprocessor to coordinates its
operation and to communicate with external devices. The control bus has signals that
enable a memory or an input/output operation at the proper time to read or write data.
Control lines are also used to insert special wait states for slower devices and prevent
bus contention, a condition that occur if two or more devices try to communicate at
the same time.
Single-Core and Multi-Core Processors
A single-core processor has one microprocessor in a chip. When a single-core
processor, such as the Pentium, runs multiple programs (multitasking), it has to divide
its time between all of the programs by assigning different “time slices” to each one.
This process increases the time it takes to complete a given program.
A multi-core processor has two or more microprocessor (cores) each with its own
memory cash on a single chip, as shown below:
3
The cores operate in parallel and can run programs much faster than a single-core chip
with a comparable processor. Most manufactures, such as Intel, offer dual-core and
quad-core processors. As technology advances, the number of cores will continue to
increase.
Pipelining
A technique where the microprocessor begins executing the next instructions in a
program before the previous instruction has been completed is called pipelining. That
is, several instructions are in the pipeline simultaneously, each in different processing
stage.
Typically, a pipeline is divided into stages or segments, and each stage can execute its
operation concurrently with other stages. When a segment completes an operation, it
passes the result to the next segment in the pipeline and fetches the next operation
from the preceding segment. Thus, pipelining results in much shorter overall
execution times. Once the pipeline is “full”, there are no idle processing stages.
Multitasking
Many computers operating systems, including OS/2 and Windows, are capable of
running many tasks (programs) at the same time with a technique called
multitasking. In multitasking, only one processor is involved, but it switches from
one program to another so quickly that it gives the appearance of executing all of the
programs simultaneously (at the same time).
Multithreading
Multithreading is the process of executing different parts of a single program, called
threads, simultaneously. Multithreading is an extension of multitasking concept.
Instead of multiple programs, Multithreading involves multiple threads within a single
program. A thread is a single sequence of execution within a program.
The operating system of a computer not only can run multiple programs but it also
can run multiple threads within each program.
4
Basic Microprocessor Operation
The original Intel microprocessor family has undergone through a tremendous change
over the years from the 8086/8088 to the Pentium family and to the multi-core
processors, both in speed and in complexity. However, the concept of the basic
register set and other features of the 8086/8088 have been retained (and expanded)
throughout the evolutionary process so that all of the newer processors respond to the
same instructions as the original devices.
A microprocessor executes programs by repeatedly cycling through the following
three basic steps:
1. Fetch an instruction from memory and place it in the CPU.
2. Decode the instruction. In decode step, the program counter is updated to
point to the next instruction.
3. Execute the instruction. Results are returned to registers and memory during
this step.
The architecture of the 8086/8088 microprocessor has two separate internal units: the
execution unit (EU), which executes instructions, and the bus interface unit (BIU),
which interfaces with the system buses and fetches instructions, reads operands, and
write results. These units are as shown below:
While the EU is executing instructions, the BIU is fetching the next instruction from
the memory, and storing the next instruction in a high speed memory called the cache.
In the Pentium processors, two execution units (EUs) allow instructions that are
independent of each other to execute at the same time.
Basic 8086/8088 Architecture
The following figure is a block diagram of the architecture (internal organization) of
an 8088 microprocessor:
5
The Bus Interface Unit (BIU)
The major parts of the BIU are: the 4-byte instruction queue, the segment registers
(CS, DS, SS and ES), the instruction pointer (IP), and the address summing block (∑).
The 16-bit data bus and the Q bus interconnect the BIU and the EU.
Instruction Queue
The instruction queue increases the average speed with which a program is executed
(called throughput) by storing up to four bytes. This technique allows the 8088 to
fetch and execute at the same time (fetch the next instruction to be executed while
executing the present instruction).
Segment Registers
The 8086/8088 processors had four segment registers: code segment (CS), data
segment (DS), stack segment (CS), and the extra segment (ES). They are all 16-bit
registers used in the process of forming a 20-bit address. A segment is a 64 kB block
of memory and can begin at any point in the 1 MB of memory space.
Addressing
The physical address is formed by combining a 16-bit address in a segment register
(CS) with a 16-bit address in the instruction pointer (IP) register. The addresses
“overlap” as shown, with an implied 00002 on the right side of the segment register,
as shown below:
6
Example
Assume CS=A00016 and IP = A0B016, what physical address is formed?
Answer
When the CS register is shifted and added to the IP register, we get:
A00016 + A0B016 = AA0B0 for the physical address, as illustrated below:
The Execution Unit
The EU decodes instructions fetched by the BIU, generates appropriate control
signals, and executes the instructions. The main parts of EU are the arithmetic logic
unit (ALU), the general registers, and the flag registers.
The ALU
This unit does all the arithmetic and logic operations, working with either 8-bit or 16bit operands.
The General Registers
This set of 16-bit registers is divided into two sets of four register each, as shown
below:
One set consists of the data registers, and the other set consists of the pointer and
index registers. The pointer and index registers are generally used to keep offset
addresses. The data registers can be used in most arithmetic and logic operations; also
some of the registers are used specifically by certain program instructions.
7
The flag registers
The flag registers contain nine independent status and control bits (flags), as indicated
below:
A status flag is one-bit indicator used to reflect a certain condition after arithmetic or
logic operation by the ALU, such as a carry (CF), a zero result (ZF), or the sign of a
result (SF), among others. The control flags are used to alter processor operations in
certain situations.
Microprocessor Programming
All microprocessors work with an instruction set that implements the basic operations.
Each instruction consists of a group of bits (1s and 0s) that is decoded by the
microprocessor before being executed. These binary code instructions are called
machine language and are all that the microprocessor recognizes.
The first computers were programmed by actually writing instructions in binary
codes, which was a tedious job and prone to error. This primitive method of
programming in binary code has evolved to higher form where instructions are
represented by English-like words to form what is known as assembly language.
Assembly Language
To avoid having to write out a long string of 1s and 0s to represent microprocessor
instructions, English-like terms called mnemonics or op-codes are used. Each type of
microprocessor has its own mnemonics instructions that represent binary codes for the
instructions. All of the mnemonic instructions for a given microprocessor are called
the instruction set. Assembly language uses the instruction set to create programs for
the microprocessor.
Like the compiler in high-level programming languages, an assembler must be used
to convert the assembly program instructions into machine language, in order
for the microprocessor to “recognize” these instruction, as illustrated below:
8
Assembly language is rarely used to create large application programs. However,
assembly language is often used in subroutine (a small program within a larger
program) that can be called from a high-level language program and runs faster.
Assembly language Program
For a simple assembly language program, let’s say that we want the computer to add a
list of numbers from the memory and place the sum of the numbers back to the
memory. A zero is used as the last number in the list to indicate the end of the list of
numbers.
The steps required to accomplish this task is as follows:
1. Clear a register (in the microprocessor) for the total or sum of the numbers.
2. Point to the first number in the memory (RAM).
3. Check to see if the number is zero. If it is a zero, then all the numbers have
been already added.
4. If the number is not zero, add the number in the memory to the total in the
register.
5. Point to the next number in the memory.
6. Repeat steps 3, 4, and 5.
A flowchart that represents the sequence of executing the above steps is shown
below:
9
Using two of the microprocessor’s internal registers ax and bx, the assembly code for
the addition subroutine (program) will be as follows:
mov ax, 0
; Replace the content of the ax register with zero.
; Register ax will store the total of the addition.
mov bx, 50H
; Place memory address hexadecimal 50 into the bx register.
next: cmp word ptr [bx], 0
; Compare the number in the memory location pointed to by the
bx register to zero.
Jz done
; If the number in the memory location is zero, jump to “done”.
add ax, [bx]
; Add the number in the memory location pointed to by the bx
register to the number in the ax register and place the sum
into the ax register.
Jmp next
; Loop back to “next” and repeat the process.
done: mov [bx], ax
; Replace the zero last number in the memory location pointed
to by the bx register with the total in the ax register.
nop
;No operation, this indicates the end of the program.
10