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http://blacksun.box.sk ____________________
_______________________I Topic: I_____________________ \ I Bascis of
am I / \ E-mail: I 3mp33j3w I Written by: / >
I I < / fu@ckz.org I____________________I
Ralph \ /___________________________> <_________________________\ Basics of am 3mp33j3wby
Ralph (fu@ckz.org) -AWC (http://awc.rejects.net) 1. Introduction -About this thing -Who should read this? -
Brief intro to Mircoprocessors2. Basics -ALU -Accumulator -Data Register -Address Register -Program Counter
-Stack Pointer -Instruction Register -Instruction Decoder3. Architecture -Buses -Instruction Responses
-Fetch -Decode -Execute -Microprocessor Instructions -Modes of Addressing -Inherent Mode -
Immediate Mode -Direct Mode4. Machine Cycles5. Interfacing -Digital -Analog -Robotics6. Other1.
Introduction================About this thing----------------This article will explain the basics of general MPUs. It is by
no means in-depth or anything, but it should be enough. I'm taking a fairly lowlevel approach to this thing.As in, I will
be talking about the circuits and such.Please note that while I tryed to keep this information as acurate as possible by
reading multiple views on something, I can't guarentee that this info is actually all true since I don't know anyone who
wants to and knows enough to proof-read it.Also, this thing is very general! Just by reading it you won't be able to
distinguish an Apple from an IBM or even a calculator.Finally, even though this thing only explains the very basics of
CPUs, it's not really recommended reading material for people who are new to computers. I assume you'refamiliar with
concepts such as the binary numbering system, PC hardware, circuits, etc.Who should read this?---------------------I have
honestly no idea. Maybe low level programmers, or people new to this might beable to put this to some use. This is
basicly just background information you shouldknow, but will most likely never actually use it. Kinda the way most people
inhigh school math are thinking. In addition, this information is fairly old, thus some of this shit has long since been
replaced or added on to.Brief intro to Mircoprocessors------------------------------Inside every computer, there is a
little ceramic block of purified crystaline silicon, coated with impurities that give it electronic swithing abilities, and
etched with poisonous gases. This little block was developt in 1971 by Integrated Electronics (Intel). The earliest
version of this thing was the 4004 chip used in Japanesecalculators. One year later, the founders of Intel, Robert Noyce
and Gorden Moore, designed the 8008 chip. These things sold rapidly and Motorola produced it's own chip,the 6800. I
little later more companies like Signetics (the 2650) and Rockwell (the PPS8) produced their own chips. Still later many
companies started releasing anew generations of chips with more functions build into a single chip. Those are forexample
Fairchild and Mostek's F8, Intels 8048, and my favourite, Zilog's Z80 (still used in GameBoys). The Z80 stands out because
it can handle the numbers DEAD and BEEF.Ph33r! Anyway, eventually most of these companies droped out of the race, and
onlyIntel and Motorola kept on developing new processors. Intel started to work with IBMand produced such famous chips as
the 8088 and the 8086. Later a number was addedwith each new version: 80286, 80386, 80486. The 80586 was renamed to
Pentium. Motorolaon the other hand went to Apple. I don't like Apples, so I have no idea what the current chip versions
are.Basics======A microprocessor is a chip which must perform at least all following functions:* Receive and store binary
data* Perform arithmetic operations* Make Logical decisions* Deliver processed data to output circuitsThese statements are
very general, and along the way, I will explain how each isperformed.ALU---ALU stands for Arithmetic Logic Unit, and is
just that. It performs all the arithmeticand logical operations based on binary data being fed to it. It performs
additionsviva combinational logic based on usually a Full Adder Circuit. Subtractions is doneby first negetating the
numbers, than adding them. Mulitplication and Division is doneby repeaded adding or subtracting respectivly.This k-rad
ASCII shows you how the ALU would go about adding two numbers (well it'ssupposed to anyway): +----------+
| Memory | +----------+ | | Number 2 Number 1 | |
+-----------+ +-------------+|Accumalator| |Data Register|+-----------+ +-------------+ \
/ Number 2 Number 1 \ / \ /
+---------------+ | ALU | +---------------+ | Result
| +-----------+ |Accumulator| +-----------+NOTE: The accumalator that stores the
result is the same that stored number 2, I just couldn't figure out how to express that in
ASCII.Accumulator-----------This is a multi-purpose circuit section. As you can see from my ASCII picture it willnot only
hold a number to be mulitplied, but also store the result. Usually, the resultwill than be transfered to some form of
memory.Data Register-------------Temporary storage for data comming from, and going to the bus.Address
Register----------------Conains the address of the current memory data. Used to access specific points in memory.Program
Counter---------------As the name implies, this thing keeps track of the memory location containing the nextinstruction to
be executed. Whenever a break in the flow of the program (a jump, call,interrupt, etc), the current number in the program
counter is stored in on the stack andthe new location goes into the program counter. When the break is finished and
controlis returned to the original routine, the number on the stack gets poped back into theprogram counter. On most
modern CPUs this part would be the Instruction Pointe (IP)register.Stack Pointer-------------A data structure that records
the last program step prior to a break in the routine. There are two basics systems. One is to store the last program
step in memory and usethe stack-point counter to index the address. The other is the one used on almost allsystems today,
Last In First Out, or LIFO. Think of it as a stack of books. You put one on top of it, and that one will be the first one
to come of next. Putting stuff on the stack is called Pushing, getting stuff from the stack is called Poping. For
example, say you have 5 books called A, B, C, D, and E stack on top of each other like this:ABCDENow you add (push) book F
to the stack:FABCDEIf you pop the stack, you get book F back and the stack looks like this again:ABCDEInstruction
Register--------------------The IR holds the binary data fetched from memory during the decoding and executing ofthat
instruction. Instruction Decoder-------------------The ID sets up the logic which controls the entire MPU. This includes
the control lines, the clock, and bus access. Everything happending in your processor is controlled from this section of
the chip. Sometimes you may here that everything is controled by the Control Unit, but in fact CU is just a name that
summarizes the previously discussedunits.3. Architecture================Architecture in terms of processors means the
layout of a chip. This might not seemas important, but the architecture is what has so far kept Intel alive and
dominant.Every single micrometer has to carefully layed out for best performance and efficiency.Buses-----Buses are a
conductors that transfer binary data in a system. A bus can be external, or internal. External buses are known as System
Buses, but we're only interested in theinternal buses, as they dictate the architecture of the CPU. The most basic bus
design is the Single-Bus System. In this design one one set ofconductors is used to connect all the units.31337 ASCII
warning!=======<>============================= | | | | | | | | +---+ |
| | | |ACU| | | | | +---+ | | | | | |
+---+ +---+ | +---+ |R1 | |R2 | | |ALU| +---+ +---+ | +---+
|===<==|<> - Two-way data flow< - One-way data flowSorry, didn't feel like making a detailed diagram here, but you
can kinda see what Imean. There is only a single major bus connectiong all registers and the ALU. There are other
conducting paths branching, but those aren't important. This design is not being used anymore as it is very slow. You can
see that doing a simple multiplication would already require to be done on a time-sharing basis. This of course slows the
whole thing down by quite a bit. However this design does have one advantage, size. It's very small, thus the next
generation of pens will most likely be using it (that was a joke by the way).To get around this speed problem, designers
used the Triple-Bus System. In this design3 seperate main buses are being used. Sorry, no ASCII for this one, just too
complexfor notpadActual chips today are even more detailed. They use a decendent of the triple-bus design and the bus can
be anywhere from 8-bit to 64-bit in size (that is, between 8 and 64 conducting wires). And again, no ASCII for this one,
if you're a good ASCIIartist, please contact me and I could send you a .bmp or something. Might not beeasy to draw a
detailed 8080 chip with characters though :).Instruction Responses---------------------Every instruction invloves a three-
stage sequence, Fetch, Decode and Execute.FetchIn this stage the Program Counter is switched to the address bus and the
binary sequenceis read by the Address Decoder. Then latched into the Instruction Register.DecodeWhen the fetch was
completed successfully, the decoder kicks in. The binary datarecieved represents a specific instruction, and will be
conditioned in the ProgrammableLogic Array (PLA) in the decoder. This part of the MPU is a complex network ofcombinational
logic. The sequences that occur in the decoder are controled by an internal ROM called CROM, or Control Read-Only Memory.
This thing sets up a controlsequence corosponding to the instruction code entered (OpCode). Every MPU has its own unique
CROM with a set of OpCodes (Operating Codes) called Instruction Set. Back inthe days, the 8080 has 78 different opcodes,
today a Pentium has about 700. Some (well most) of these have additional paramters called Operands following them. These
are fetched as the opcode is executed.ExecuteAfter decoding the instruction, the execute unit directs the synchronzing
signals whichcontrol the ALU, memory, and I/O circuits. This sequencing and timing is kepy in sync by the Program Counter,
which keeps track of every step in the program and control breaks as mentioned earlier.Microprocessor
Instructions---------------------------Every MPU is designed to respond to a fixed set of instructions. As mentioned
above,the old 8080 has 78 of them. However, opcodes actually consist of two parts. One partindicates the address mode,
the other the actual code. Usually, only 2 bits are usedfor the address mode part. A 8-bit computer can thus theoreticly
have 64 opcodes, butmanufactures usually push this limit a bit and include a few variations.Some opcode examples are:Binary
Hex Mnemonic Describtion01110010b 72h JC Jump short if carry flag set00001100b 0Ch OR AL Perform
logical OR on contents of register ALBinary is the raw instruction opcode being used in the Decoding process, Hex is
thesame number converted to hexidecimal, mnemonic is a more human readable form of thatinstruction. Mnemonix are what
assembly programmers code in.Modes of Addressing-------------------As stated before, a opcode usually contains two bits
telling the MPU what addressingmode to use. I will cover the three basic (and outdated) 16-bit address modes, Inherent,
immediate and Direct.Inherent ModeThis mode invlovles the MPU chip itself as data is being stored on one or more of
theonchip registers during the execution of the instruction. The opcode itself holdsall the information required to
execute the instruction. Each register is identified in the opcode. For example, say we want to add registers A and B,
where A = 000 and B = 111, the opcode for that might be 01000111 The 01 indicates Inherent Mode addressing, the 000 the
source register, and 111 the destination register. As you can see, this opcode contains all the information the decoder
needs in order to execute this instruction.Steps to execution:1. a) Fetch instruction b) Decode instruction2. Execute
instructionImmediate ModeThis mode involves the opcode PLUS the operand. The operand can contain (at the timethis was
writen) 8, 16, 32, or 64 bit information. This information can either bean address where data to be used is located, or
the data itself. The decoder is toldhow many more bytes to read in by the opcode. An example of this is a jump
instruction.First comes the opcode telling the MPU to perform a jump, the next few bytes containthe address to jump
to.Steps to execution:1. a) Fetch instruction b) Decode instruction2. a) Obtain operand b) Execute
instructionDirect ModeThis mode is used when transfering data from the MPU to memory or vice versa. Instructions based on
this mode come in three bytes, one for the opcode, two for the (16-bit) address. For example, the following statement
moves the value DEADh into the AX register:10111000 10101101 1101111010111000 is the opcode for move AX. Since AX is a 16
bit register, the decoder will assume that the next 16 bits are are the value.Steps to execution:1. a) Fetch instruction
b) Decode instruction2. a) Obtain address b) Decode address3. a) Obtain operand b) Execute instruction4. Machine
Cycles==================Timing is a very important thing in MPUs. Many circuits share the same data bus, andmust be
precisly timed. I briefly mentioned this earlier, and now I'm covering it a bitmore in-depth.MPU instructions are executed
by the control section and timed by a series of machinecycles. The cycles are subdevided into 5 parts called Clock
Periods. A instruction from memory is given 5 clock periods in which it must complete the instruction. If itfinishes in
less than 5 clock periods, the remaining periods are idled. The followingk-rad ASCII shows a instruction fetch cycle:|---
Machine Cycle---|+---+---+---+---+---+| 1 | 2 | 3 | 4 | 5 |+---+---+---+---+---+|-----------| | | | | |
Instruction | Idle Fetch | | Increment PC and decodeOn a modern computer one machine
cycle occurs ever 1193180/65536 of a second, or about18.2/sec.5. Interfacing===============Without interfacing, a MPU is
about as usefull as condom made of ice. Even the simplesttask requires some sort of way to input data, and some way to get
the output. In fact,interfacing is so fundamental to the MPU that a interfacing apparatus is usually alreadypart of the
MPU design.Digital-------When the MPU recieves data, and performs some function, the result is placed on a data bus. These
bits can then be read by any imaginable device. All that is necessary is adecoder than can read those bits.31337 ASCII
Alert!+------------+ +------------+ +------------+| | | | | || MPU |
| Interfacing| | Display || |----| Chip(s) |----| || | | | |
|+------------+ +------------+ +------------+6. Other========I cannot be held responsible for anything that
happens to you as a result of using this information.You may freely distribute this text as long as you don't change
anything. If there'ssomething you think should be changed, contact me first.Send feedback to fu@ckz.orgAnd very important!
If you noticed a mistake (speling, grammer, and especially technical!), please contact me asap. As I said before, most of
this information is fairly outdated and useless, if you know of a specific change that has happened toanything discussed
here, please let me know.Please always get the newest version of this and other tutorials at http://awc.rejects.net as they
usually contained updated information, and addons.Greetings to:cozgedal, Alf (the cat eating fury dude), Sad1stick, rpc,
moJoe, Lindex, and everyone Iforgot to mention.This space intentionally left blank.
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