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MICROCOMPUTER
CONCEPTS
The first step in understanding microcomputers is under standing computer terminology, concepts, and methods. This
chapter lays a basic foundation of computer fundamentals so
that the student will be prepared for more advanced concepts in
other courses.
12.0 INTRODUCTION
Upon completion of this chapter you should be able to:
12.1 OBJECTIVES
• Define microcomputer in relation to mainframes and
minicomputers.
• Understand the basic organization of a microcomputer.
• Explain how microcomputers are programmed.
• Understand the basics of microcomputer interfacing.
12.2 WHAT IS A
MICROCOMPUTER?
A microcomputer is a small, versatile computer that is generally
used in process control, robotic, military, and specific industrial
applications. Microcomputers usually have limited data entry
and display capabilities. Some microcomputers use very little
power, which makes them ideal for battery operation.
To understand where the microcomputer fits into the
computer family tree, see Figure 12-1.
As shown in Figure 12-1, the mainframe is the largest and
most powerful member of the computer family. Mainframes are
used in applications where speed and precision are very
important and size and power consumption are of much lesser
concern. Mainframes are typically used by such organizations as
NASA, universities, and the government to store and process
large amounts of data. The miniframe is a scaled-down version
of the mainframe. The miniframe is generally used in smaller
businesses and at multi-user workstations that do not require
extensive speed or memory capabilities. The miniframe is
slightly less powerful than the mainframe and much smaller.
The miniframe is generally used in applications where storage
space, speed, and cost are important.
The microcomputer is the next lower step in performance
from the miniframe. One might wonder why the micro computer has become so popular over the years in light of the
superior speed and storage capabilities of the mainframe and
miniframe. One reason is the performance/cost ratio. Microcomputers are very inexpensive when compared to the
mainframe or miniframe. It is possible to have many single -
user microcomputer systems, rather than a single minicomputer
with multiple users for the same cost. Another reason for the
popularity of microcomputers is inherent flexibility. Microcomputers are, for the most part, constructed on a single circuitboard, whereas main and miniframes are big and usually consist
of a chassis with several highly-specialized circuit boards to
perform computer functions. It is easily understood why
microprocessors have found their way into small, portable
computer systems and many military, and industrial ap plications.
The microcontroller is a step lower in performance than
the microcomputer. The microcontroller is designed for a
specific control-oriented application. Microcontrollers are typically found in modern appliances and cars where semi intelligent, adaptable control is desired. Microcontrollers usually
have just enough speed and storage capabilites to meet a specific
requirement in order to keep costs down.
Figure 12-1 shows the computer heirarchy. The student
should realize that, as with most definitions, gray areas exist.
There are some miniframes that approach performance levels of
mainframes and so forth. Advancements in technology are
largely responsible for producing gray areas of performance
standards.
Now that the microcomputer has been located in the
computer family, the basic workings of the microcomputer can
be examined. This section introduces major subsystems that are
common to virtually every microcomputer system. Figure 12-2
shows the organization of a generalized microcomputer.
Until approximately the mid-sixties computers did not
use bus structures to transfer information. Prior to the midsixties computers had no defined data, address, or control paths
and often these signals shared common lines. Today's com -
12.2.0
Organization of the
Microcomputer
puters, particularly microcomputers, use what is called a bus
structure to transfer data, address, and control information. As
shown in Figure 12-2, three buses (address, control and data) are
common between all devices of the microcomputer. Buses are predefined electrical paths of communications between devices in
the microcomputer.
The address originates from the microprocessor to select a
particular device, via the address bus, to read information from
or to write information to. The data bus transfers information
between the microprocessor and other devices on the data bus.
The control bus qualifies a data transfer as either to (read) or
from (write) the microprocessor.
If, for example, data is to be a read from the RAM (random
access or read/write memory) to the microprocessor, the address
bus supplies the RAM address where the data is stored. The
control bus "tells" the RAM with specialized logic-enable signals
that data, specified by the address bus, is to be read by the
microprocessor. The RAM responds by placing the appropriate
data on the data bus. The data is then read and utilized by the
microprocessor.
The microprocessor is the heart of the microcomputer.
Microprocessors continually read, process, and output data
under program control. The microprocessor systematically
fetches instructions from memory, then interprets and executes
the instructions accordingly. Some instructions require multiple
fetches from memory. An add instruction, for example, requires
the microprocessor to first read the add instruction, fetch the two
arguments to be added, add them, and then store the result. The
add instruction, therefore, requires a total of three fetches and
therefore is called a three-byte instruction.
As shown in Figure 12-2, the microprocessor can be divided
into four primary functional areas: timing, control, arithmetic-logic
unit (ALU), and registers.
The timing portion of the microprocessor serves to
coordinate all activities of the microprocessor and the microcomputer system. A stable clock oscillator (usually in the low
megahertz range) drives sequencing circuitry inside the
microprocessor that causes address generation, instruction,
fetching, etc. Every operation that takes place inside the
microprocessor, and even the entire microcomputer, is
synchronized with the clock.
The control portion of the microprocessor uses timing
signals to coordinate microcomputer activities. When the
microprocessor encounters an instruction in program memory,
the control circuitry directs data to the ALU, causes another
memory read, or writes data to a port, for example, to complete
the instruction encountered as specified. The control portion of
a microcomputer generally consists of registers and random logic
that run a program internal to the microcomputer called
microcode. Microcode is the microprocessors program that is
continually running to read and execute commands in external
program memory. For a simple instruction, like a port write, the
microcode performs the following generalized sequence of
events:
1.
Fetch the instruction.
2.
Perform a series of comparisons to determine that the
instruction is a port write.
3.
Obtain the value to be written to the port from the ALU.
4.
Generate control and address signals for the port write
opera tionn.
5.
Write the value to the designated port.
6.
Fetch the next instruction.
It is easy to see that the microcode is a very specific closed
loop program that directs the activity of the microprocessor
under timing control.
The ALU performs all arithmetic oriented and logic
oriented operations of the microprocessor, as specified by timing
and control. Some examples of arithmetic operations performed
by the ALU are add and subtract (and, in some microprocessors,
multiply and divide). An add command executed by the
microprocessor involves reading the two values to be added by
the control section of the microprocessor. The two values read
are sent to the ALU where they are added. The sum is then
written to either the ALU's designated holding register (the
accumulator) for multiple addition requirements, or to a
memory location. The ALU also performs logic related tasks as
specified by the microprocessor's control and timing sections.
Some examples of logic operations are: inverting the contents of
the accumulator, ANDing and ORing two arguments, and other
logical comparisons.
Special registers inside the microprocessor hold important
memory locations, storage locations, and are also used for
temporary storage in calculation during program execution.
Listed below are specialized registers that are common to
virtually every microcomputer.
PROGRAM COUNTER (PC): The PC register holds the address of
the next instruction line to be executed.
STACK POINTER (SP): The SP register holds the location of the
'top of the stack.' The stack is a portion of RAM memory set
aside for temporary storage of registers and other important
information. The stack area is used when the microprocessor is
interrupted from regular program operation to service a
subroutine. After the subroutine has been serviced the values
on the stack are 'popped off and returned to their original
location, regular program operation then continues.
ACCUMULATOR (A): The A register is a register that is used in
virtually all arithmatic and data manipulation instructions.
Together, timing, control, ALU, and registers make up the
microprocessor which is the central point of activity in the
microcomputer.
As shown in Figure 12-2, there are two types of memory
used in the microcomputer. Random access memory (RAM) is a
non- permanent storage medium in which "random" memory
locations in the RAM can be read or written b y the
microprocessor at any time. RAM is a temporary form of
memory and the contents of the RAM are lost when power is
removed from the chip. Figure 12-2 shows that the address,
control, and data buses interface to the RAM. The address bus
supplies the location of the memory location that is accessed.
The control bus directs the RAM to supply, from a location
specified by the address bus, either data to the data bus (memory
read), or accept data from the data bus (memory write). Note
that data flow is bidirectional on the data bus. Read only
me mo r y ( R OM ) i s s i mi la r t o R A M ex ce pt t ha t t h e
microprocessor can only perform memory read operations on
the ROM, and the ROM is a permanent memory.
ROM contains programs and information that doesn't
change, like equation constants or specialized subroutines that
read the keyboard and update the display, etc. The control bus to
the ROM supplies signals to distinguish between memory and
input/output (I/O) access. When the control and address buses
have specified a valid ROM location, data is output to the data
bus and is read by the microprocessor.
RAM and ROM are vital elements of any microcomputer.
RAM provides the user with a changeable storage medium in
which programs are developed. ROM ,on the other hand, is
transparent to the user and provides the necessary instructions
to the microprocessor to operate and interface with the user.
Input/output ports allow the microcomputer to interface
12.2.1 Interfacing
to the outside world through keyboards, disk storage, modems,
displays and other devices. Figure 12-2 shows how the I/O port
provides an interface between the microcomputer and the
outside world. There are two fundamental types of I/O ports:
serial and parallel. Both serial and parallel I/O ports utilize the
three microcomputer buses shown in Figure 12 -2. Most I/O
ports are semi-intelligent and therefore require minimal
interraction with the microcomputer.
An I/O write, for example, usually requires only that the
microcomputer write the appropriate data word to the I/O
interface chip(s). The I/O interface chip(s) then performs any
control operations necessary to transfer the data to the external
device while the microcomputer is free to continue normal
operation. An I/O read, on the other hand, usually involves the
I/O interface interrupting the microprocessors normal
operation. The I/O port is then read in much the same way as a
memory device except that the control bus signals an I/O read
rather than a memory read.
Figure 12-2 shows some typical peripheral devices
connected to the microcomputer's I/O port with data flow
direction indicated by the arrows. Peripheral devices provide an
interface between the microcomputer and the user. The
keyboard allows the user to enter data and key-in programs. The
display provides the user with visual information on the status
of the microcomputer. Printers provide a hard copy of program
or data, etc. Modems allow the user to communicate with other
computers. It is easy to see that peripherals are essential to the
microcomputer/human interface. Peripherals use either serial
port or parallel port data transfer for interface with the outside
world. Serial port data transfer implies that data words are sent
in sequential parts. Computer data words are broken-down by
the microcomputer into individual bits and then sent, by the
serial port, over a single electrical transmission line. It is easy to
see that serial transmission is inherently slow. Parallel port data
transfer is very similar to data transfer that takes place over the
microcomputer data bus to a memory location or I/O port.
Whole computer words are sent to the parallel port and then
over a set of parallel electrical transmission lines. Although
parallel port data transfer is much faster than serial data transfer,
many more electrical wires are required, increasing both size and
cost of the transmission medium.
In order for the microcomputer to communicate with the
outside world and peripherals, either parallel or serial data
transmission must be used. This section provides a brief
introduction to the basic concepts of parallel and serial data
transmission techniques.
Parallel data transmission is very similar to 'com munication' over the data bus between the microprocessor and
memory, I/O, etc. In parallel transmission, data is either writtento or read from a port address just as if the port was a memory
location. Data written to an I/O port is buff erred and output in
whole byte form to an external device. Similarly, data read from
an I/O port is buff erred, input to the data bus, and read by the
microprocessor. Interface control lines, referred to as handshake
lines, originate from the microprocessor's control bus and are
used to coordinate the data transfer. Figure 12-3 shows a typical
parallel port interface functional diagram. Parallel ports are
useful in short transmission paths because of their high data
transfer rates but suffer in longer transmission paths because of
noise problems and physically large and expensive cables.
12.2.1.0 Parallel
and Serial Data
Transmission
Serial data transmission is much more complex and
slower than parallel data transmission, but it has some
advantages over parallel transmission. The main advantage of
serial transmission is that transmitted data over a single
electrical line, rather than man y lines as in parallel
trans missi on. T his means that t he cabling f or seria l
transmission is much less expensive and bulky than that of
parallel transmission. Also, serial transmission is not as easily
affected by noise as is parallel transmission. Serial data transmission requires much more microprocessor interaction than
does parallel transmission although there are some chips
available that relieve the microprocessor of the transmission
burden. Figure 12-4 shows a functional diagram of a typical
serial I/O interface.
Figure 12-4 shows that data is either read from or written
to the serial port in a parallel manner. When data is written to
an external device, the address bus selects the serial port and data
is written to the parallel-to-serial (PISO) shift register. Data is
sequentially shifted through the buffer and out over the single
data line to the external device. The buffer portion of the serial
port modulates or level shifts the incoming data bits for better
signal integrity over the data line. The data is shifted under
control of the shift clock and the direction control bit. In a
similar manner, data is read over the serial interface. Data is
shifted in over the single data line, bufferred, and shifted into
the shift register. When a complete byte has been received, the
microprocessor performs a data read of the serial port. For either
the serial data write or serial data read, handshake lines provide
information of the status of both the transmitting and receiving
devices. On the transmitting end, the sending device uses
handshake lines to signal that data is ready and waiting to be
transmitted. On the receiving end, the receiving device uses
handshake lines to signal that the receiver is ready to begin
accepting data.
Computers use binary data and instruction words to
perform all computational and data manipulation functions. A
binary data word in a computer is defined in size by the 'width'
of the data bus. If, for instance, the computer data bus consists of
eight electrical connections between the microprocessor,
memory, and input/output (I/O), then the computer is said to
have a word size of eight 'bits.' A data bus consisting of eight
data bit lines is called a byte wide data bus. Other data bus bit
widths are four bits (called a nibble), sixteen bits (called a word),
and even thirty-two bits and beyond. Figure 12-5 lists industrystandard definitions for the computer data bus.
12.2.2 Programming
Computers continually process binary numbers. They
input binary numbers from input devices and output binary
numbers to output devices. For the computer to know what to
do with the binary data words there must be processing
instructions. Computer processing instructions are called binary
instruction words.
Computers perform program instructions sequentially
and each program step is called an instruction code. During an
instruction code cycle, a complete computer instruction is
initiated and completed.
The first part of any instruction cycle is the fetch cycle.
The computer fetches a binary instruction word from memory
so that it knows what to do next. A memory fetch is initiated by
the microprocessor at the start of a new program line or when
power is first applied to the microprocessor. The data read by the
microprocessor at the start of a new instruction is always a binary
instruction, and is sometimes followed by one or more bytes of
information that are part of the instruction. The microprocessor
always expects the first binary word read from memory to be an
instruction. The microprocessor reads the binary data into an
internal register and decodes the data as an instruction by
performing a series of comparisons to identify the instruction.
Once the instruction is identified, the microprocessor acts
accordingly by performing other memory read operations (called
multiple-byte instructions), or by performing the designated
instruction without any additional modifier bytes (called singlebyte instructions). Computers continually fetch and process data,
from when power is applied to them to when they are turned
off. Even if a computer appears to be idle, it is busy updating the
display, performing a computation, or simply scanning the
keyboard waiting for an input.
Topics covered up to this point have introduced basic
computer architecture, computer words, and interfacing
techniques. This section provides fundamental insight into
different levels of computer programming from basic to
complex.
12.2.2.0 Machine
Code
Software, in any computer, is simply a series of bit
patterns representing specific instruction that control the
operation of the computer. Machine code is that bit pattern of
1 's and O's that ultimately controls all activity of the computer.
Machine code instructions are sequentially read from
memory and decoded by the microprocessor to perform the
desired activity. Different machine code bit patterns represent
different microprocessor instructions. Additionally, different
microprocessors use unique machine-code bit patterns for
similar instructions. The set of instructions that direct the
activity of the microprocessor is called the instruction set of the
microprocessor. The instruction set of the MOS Technology 6502
8-bit microprocessor is significantly different than the Zilog Z80 8bit microprocessor. The binary machine code representation of
the add-with-carry (ADC) instruction of the 6502, for example, is
01101001 while the Z80's ADC instruction has the binary
representation 10001110. Therefore, it can be seen that though
different microprocessors may share similar instructions, the
difference in machine code can prohibit program compatibility
between microprocessors.
There are, of course, some exceptions. The Z80 has what
is called a superset of the 8080 8-bit microprocessor instruction
set. This means that all 8080 machine codes are included in the
Z80 instruction set and all 8080 programs will run on the Z80.
Because the Z80's instruction set is larger than the 8080's,
programs written on the Z80 are not gauranteed to run on the
8080.
Machine code programming is the most basic and direct
form of programming. Machine code programming is rarely
used because of the high probability of programming error when
entering the codes into computer memory, the difficulty of
entering the codes, and the difficulty of the programmer
following program flow. Shown below is a sample machine
code program that loads the Z80 register pair BC with the binary
value 00000000 00000011, adds the contents of the C register
(00000011) to the Z80 accumulator (A register), and retains the
sum in A.
It is apparent that machine code programming is very
limited. Machine code, also referred to as object code, must be
used by the microprocessor because the microprocessor requires
binary instructions. Different programming techniques allow
the programmer to use a more symbolic, less confusing
programming language than machine code.
12.2.2.1 Assembly
Language
Assembly language, like machine code, is used to directly
program the desired activity of the microprocessor and
microcomputer system. Assembly language is the next level of
programming above machine code programming. Alphabetic
abbreviations called mnemonics are used in place of bits to
represent instructions and even the numeric quantities being
processed. Shown below is an assembly language program,
written in Z80 assembly mnemonics, that performs the same
function as the sample machine code program introduced in the
previous section.
Notice how easy it is to read the assembly language
program and determine the function of the program. Also, the
program is more easily entered into the microcomputer and
with much less chance of error than the equivalent machine
code program. In the assembly language program above, the
load command is abbreviated by the mnemonic 'LD' and the addwith-carry command is abbreviated by 'ADC. 1 Also, 'START'
and 'END' are called labels and they represent memory address
locations. In the example program the START label represents
address 0000000000000000 and the end label represents address
0000000000000011. This should be verified by the reader. A label
can also represent constants in assembly programs too. The label
'THREE' can be used in place of the 03 quantity in the first line of
the program above. The first line will then read:
START:
LD BC/THREE
;Load BC with THREE.
Where the label THREE has the binary representation
0000000000000011.
A natural question that arises is: how does the
microprocessor interpret the assembly language program? After
all, the microprocessor requires binary machine code to function.
Assembly language programming is a two-step process:
first, the programmer develops the assembly language program
using mnemonics and labels, etc. Next, the program is
'assembled,' either by hand or by another program, to create
binary machine code that the microprocessor understands.
When the mnemonics and labels of an assembly language
program are translated into machine code directly by a
programmer, the process of converting the assembly language
program to machine code is referred to as hand assembly. If the
assembly languagee program is assembled by another program to
automatically generate executable machine code, the program is
called an assembler program.
The assembler program examines each line of the
assembly language program (called the source code), performs a
series of comparisons to determine what the instruction or label
represents, and creates the corresponding executable binary
object code. Regardless of how the assembly language program is
assembled, it is apparent that the use of mnemonics and labels to
represent binary program instructions and constant values
greatly assists the programmer in creating easily understood,
error-free programs.
High level language is the g eneral category of
programming that uses english-like program statements to
accomplish program goals. Languages like BASIC, FORTRAN,
PASCAL, and COBOL use program instruction statements that
somewhat remove the programmer from direct control of
microprocessor activity. The instruction set used in high level
languages allows the programmer to perform scientific
calculations, interract with the display, perform character
12.2.2.2 High Level
Language
manipulation, and so forth without direct co ncern with what
the microprocessor is doing.
The machine is virtually transparent to the user, that is,
the user has no idea what the microprocessor is doing to
perform the desired function or what type of processor is used.
High level language programs are primarily used in business,
scientific and industrial applications where ease of programming
is important. High level program statements usually resemble
common english words. Shown below is an example of a typical
high level language program statement that adds two numbers
and stores the resulting sum in 'SUM.'
10
SUM = A+B
It is easy to see that the main advantage of high level
language over lower-level languages, like machine code and
assembly language, is the ease of programming and
understanding of program flow and operation. The main
disadvantage of high level language is that the programmer has
little or no control over the microprocessor and computer
system. As an example of a situation where high level
programming is limited is in system interfacing.
If a printer is to be interfaced to a computer it may require
a series of control characters before it can accept regular data (as
in printing an alternate set of characters). High level languages
usually do not support bit-level commands (as required by the
printer), therefore a small assembly or machine code program
must be written to interface with the printer. High level
language programs must be converted into the binary object code
format that the microprocessor understands. High level
programs are 'passed' through a compiler or program to create
the binary object code. The compiler is analogous to the
assembler in assembly language programming. The compiler
assigns microprocessor registers to variables used in the high
level program and generates standardized binary subroutines in
place of high-level commands. A command like
100
Y = M»X + B
involves many computational steps.
The compiler must set
aside memory space for the variables Y, M, X, and B. The
variables M and X are assigned to a multiplication subroutine
that may be many machine code lines long. The result of the
multiplication of M and X is then added to the variable B to
obtain Y. In short, a seemingly simple high level language
program line, when compiled, can become many machine code
lines long.
This chapter has introduced basic computer concepts with
emphasis on how they apply to the microcomputer. The
computer was analyzed on a functional level for introduction to
some basic computer components: CPU, memory, bus, ports,
and peripherals. Computer words were reviewed for insight
into how computers uses data and instruction words to process
information. Interfacing computers using parallel and serial
interface ICs was discussed. Programming methods and levels
were reviewed to demonstrate where particular language types
relate to different caliber machines. This chapter has provided
valuable background information for further building of
programming skills and understanding of microcomputers and
microprocessor control.
1.
Give an example of a situation where a mainframe
computer is used. Why?
2.
Give an example of a situation where a microcomputer
is used. Why?
12.3 SUMMARY
12.4 REVIEW
QUESTIONS
3.
Why isn't the address bus in a computer bidirectional?
4.
What is microcode?
5.
Why is digital logic used in modern computers?
6.
What is the primary difference between RAM and ROM?
7.
If a serial communications port can transmit at a rate of
eight bits per second, what is the equivalent parallel port
transmission speed?
8.
How is object code obtained from an assembly language pro
gram?
9.
Why are three bytes required for a typical machine code
'jump' instruction? Label the three bytes that make up
the typical machine code 'jump' instruction (i.e. data,
instruction, or address).