The Von Neumann Model

The whole model

Four basic components:•Memory•A processing Unit•Input•Output•Control

MDR = Memory Data RegisterMAR = Memory Address RegisterPC = Program CounterIR = Instruction RegisterALU = Arithmetic and Logic UnitLED = Light Emitting Diode

Something about memory

Memory in its most general sense refers to places to put things for later retrieval. Some special memory for temporary storage

(registers) is located on the processor chip. Access to this memory is extremely fast, but it

is very expensive and in very small supply (16, 32, that sort of number).

Mostly, when we talk about memory, we mean RAM -- Random Access Memory

RAM

An array of bits Addressability refers to how many bits are associated

with each address provided.• N bit address can access 2N locations

In our 4 x 3 example, there were four addresses, each of which specified a set of three bits

Power required to store the value Gated D-latch type of circuit

• Has to be loaded after the computer is powered up

RAM continuedAmount of RAM is now measured in Gigabytes

Giga means billions• Kilo (K) is actually 1024

• Mega (M) is actually 1,048,576

• Giga (G) is actually 1,073,741,824 byte means 8 bits (almost always) How big an address would be required to access each byte in an

80 GB memory?

Word size is the number of bits accessed at one time in a particular machine’s architecture

If the computers of today contain 2 to the 28th by 8 bits, then how does the 256 megabyte memory come into play? What exactly is 256?

More RAMTo access the content of a memory location, specify the

location (its address) Location and content are two distinct, very important concepts. We will see the distinction clearly when we study the C

language. (C allows us to manipulate the address, as well as the content)

Memory can be read from or written to. Reading is non destructive -- what was there before remains Writing is destructive -- what was there before is replaced by

the new information.

Reading and Writing RAM

MAR contains the address to be used (the values on the A lines in our map of a small memory)

MDR contains the data to be written (if writing) or will receive the value read (if reading)

These are two registers (located on the processor chip) that are dedicated to these specific purposes.

Processing Unit

The location of all the circuitry related to manipulating the bits for achieving some purpose

The Arithmetic and Logic Unit performs arithmetic and logical operations on data

stored in registers, usually. (It would be very inefficient to have these circuits try to access RAM directly)

Registers local storage for data in immediate use

Control

We have memory to store instructions and data, and the processing unit to carry out operations.

The control unit determines what happens next and keeps the processing unit supplied with the next instruction.

Good question asked: How are these instructions which allow the computer to operate defined. Meaning were do they originate from (RAM, bios, etc.) and how do they function in the processor.

From the beginning…

MARMDR

IR R0R1R2 …

PC

Memory - empty

The bare machine without power. Nothing is happening.

The control unit controls the rest of the computer’s functions. How does the control unit figure out what it has to do first and where it looks for the components it needs to run.

To get started

MARMDR

IR R0R1R2 …

PC

Memory -ROM

Something else is needed.ROM stores the instructions needed to get started. When power is available, the circuitry in the computer loads the contents of ROM into a predetermined location in memory.

First steps

MARMDR

IR R0R1R2 …

PC

Memory -

00..0 00010100…00..100..200..3Etc….

Now, these few instructions control the computer. They have one purpose – to load the operating system core into memory and turn over control of the computer to the operating system.

Under control of the OS

MARMDR

IR R0R1R2 …

PC

Memory - OS loaded

Now the OS is in control. The user commands to the operating system asks for the appropriate application program to be loaded. In turn, the application program calls for its data as needed.

What operations?That’s what makes machines differentThe list of things that a given machine can do is called its

Instruction Set The Instruction Set Architecture (ISA) is the description of what

a given machine can do, including the instructions it can execute and the amount of local memory it has available to work with.

An instruction is a combination of bits with a specific meaning.

Think of an instruction as a set of switch settings

Instructions

Can you wash your clothes in your microwave oven? Of course not, because that is not in the ISA of a

microwave.

Are all microwave ovens alike? No. They vary in power and in conveniences such as

rotating trays. Some microwave ovens have an “instruction” (a setting)

for baking a potato. Others do not have that instruction, but can do the same thing if you give it the right steps to follow.

CPUThe central processing unit of the computer is where the

work gets done. The instruction set of a particular computer consists of the

recognized instructions -- ADD, AND, NOT, OR, Subtract, Multiply, Divide, Move, Compare, etc.

The list varies from one machine to another.

All general purpose computers are equivalent. Given enough memory and enough time they can do the same things. (They are all Turing machines.)

Input and OutputA computer is not very useful if we are unable

to give it commands and see what it has doneInput is a way of getting instructions and data

into the computer. Mouse, Keyboard, sensors, scanners,

Output is a way of getting results out of the computer Monitor, printer, plotter, etc.

Some devices are used for both input and output CD-RWs, various disks, network connections, etc.

The instruction cycleBasic version: fetch and executeMore detailed version:

fetch, decode, evaluate address, fetch operands, execute, store result

Underlying theme: Instructions and data are stored in RAM until

needed. To execute an instruction, the CPU has to find the

instruction, move it into the IR, find the data, move it into registers, carry out the instruction and store the result back in memory

The LC-2In Fig 4.3, what are the 3 arrows marked DR, SR1, and SR2 that go out of the control logic box? Where do they output to?

DR = Destination registerSR = Source RegisterAny others?

the LC-2 drawing on pg 80 confuses me, because the PC seems not to connect to the MAR or main memory or anything, and i don’t know what the purpose ofthose two big arrows pointing to

the left are.

Finding the instruction and data

The MAR contains the address of the instruction or the data to be fetched. How the MAR gets its value is an important part of how the computer works. In the normal cycle, the PC is incremented to

point to the next physical location. (Not all computers require that the PC contents be moved to the MAR. The

PC can serve as an MAR for fetching instructions.) To fetch data, the MAR must be loaded

explicitly from information provided in the instruction.

Example instructions

In each type of computer, there is a specific format for each type of instruction. (Refer to the IBM 360/370 ISA distributed earlier.) Each bit position has meaning A bit is part of the operation to be performed or

part of the specification of the location of data or the location where the result will be stored.

There are a number of formats for specifying the location of data. More about that soon.

ADD in the LC-2

The LC-2 ADD instruction

Bit pattern 0001 means ADDThe source and destination addresses are registers Bit positions [5:3] are not usedWhat does instruction

0001010101000111 mean?

Op Code Result Source 1 Source 2

15 0

How do you know that something is in an ADD instruction format and not an LDR instruction format? Does it have to be labeled? And how would you do an instruction format?

The cycle

How do you know that something is in an ADD instruction format and not an LDR instruction format? Does it have to be labeled? And how would you do an instruction format?

This is why we have several steps to the fetch execute cycle. After fetching the instruction, we then decode it. Once we know which operation we have, we know what format the addresses will be in. Then we can evaluate the addresses and then we can fetch the operands.

Only then can we execute the instruction and store the results.

Other formatsThe LDR example

15 0

0 1 1 0

LDR DR

0 1 0

Base register + offset amount

0 1 1 0 0 1 1 1 0

This is just a sample. We will see other addressing formats in the next chapter.

Changing the flow

JMPR The JMP is a Jump instruction and will specify

exactly where the next instruction is to come from.

The R at the end says that the addressing format used is base + displacement

The address obtained will be written into the PC to control where the next fetch cycle will go to find an instruction.

The clock

As we saw with the simulator, a clock in a computer is just a pulse generator. It cycles at regular intervals and controls the events occurring. When the clock goes faster, everything in the machine goes faster. If the clock goes too fast, the operations will not be finished and the state of the machine will be confused. If the clock goes too slow, the machine will run slower than it needs to.