Internal computer arvhitectures

Question 1

summarise Von neumann architecture

Answer 1

CPU, arrow, system bus, RAM, Instructions, Data

• shared memory space for instructions and data
• instructions and data stored in the same format
• a single control unit it processor follows a linear fetch, decode, execute cycle
• one instruction at a time
• registers are used as fast access to instructions and data

Question 2

summarise the Harvard architecture

Answer 2

• instructions and data stored in separate memory units
• each has its own bus
• reading and writing data can be done at the same time as fetching an instruction
• used by RISC processors

Question 3

disadvantages of harvard architecture

Answer 3

• no adaptability
• not appropriate for general or multi purpose machines as you can’t install and uninstall programs

Question 4

what is the stored program concept

Answer 4

the idea that instructions are fetched one at a time from main memory and executed immediately by the processor in the sequential order
- program can be replaced by loading another program into memory
- contents in memory location can be interpreted as instruction or data

Question 5

What happens during the Fetch stage of the Fetch-Execute cycle

Answer 5

• contents of PC transferred to MAR
• Address bus is used to transfer this address to main memory
• main memory content is transferred using the data bus
• contents of memory address location is loaded into MBR
• contents of MBR copied into CIR

Question 6

What happens during the Decode stage of the Fetch-Execute cycle?

Answer 6

• The Control Unit decodes the instruction held in the CIR.
• : The instruction is split into the opcode and operand

Question 7

What happens during the execute stage of fetch-decode-execute

Answer 7

• if necessary, data is fetched
• opcode identifies type of operation to be performed
• ALU is used to carry out the operation
• result stored in main memory

Question 8

what does an instruction contain

Answer 8

an opcode and an operand

Question 9

what does the opcode do

Answer 9

Tells the CPU what to do

Question 10

what does an operand do

Answer 10

is a value or is data that will be used in an operation

Question 11

what can the operand be

Answer 11

• the actual data that you’re performing the operation on
• a memory location where the data is found

Question 12

whats the difference between direct and immediate addressing

Answer 12

• addressing mode 01 specifies that the data is an address, not a value (direct)
• addressing mode 00 specifies that data is a value, not an address (immediate), represented with a hashtag before the value

Question 13

Why are registers used as a fast access to instructions and data in the von Neumann architecture?

Answer 13

Because it’s quicker to work with data in the CPU than it is to work with directly in memory

Question 14

Examples of alternative architecture

Answer 14

• Parallel processing-where the processor carries out a single instruction on multiple data
• distributor computing

Question 15

What could the operand contain?

Answer 15

It could contain the actual data or an address where the data is found

Question 16

How to read an instruction

Answer 16

If we are using eight bits:

• The first four bits are the opcode, but the first three bits are the basic machine operation (ADD)
• the last bit of the opcode is the addressing mode
• the last four bits are the operand

Question 17

How many different opcode would be available in an 8-bit instruction

Answer 17

2^4 (four bits in opcode)

Question 18

when is the harvard architecture used

Answer 18

• in microcontrollers
• microprocessors
• embedded devices, eg embedded digital signal processing, traffic lights, navigation systems

Question 19

differences between von numann and harvard

Answer 19

• Harvard uses separate buses, Neumann uses combined buses

Question 20

why might Harvard architecture be more preferred over Von

Answer 20

• instruction and data can be accessed simultaneously
• avoids possibility of data being executed as code
• program cannot be accidentally overwritten
• different technologies can be used to implement instruction and data memory
• instruction and data memory can have different word lengths

Question 21

difference between direct and intermediate addressing

Answer 21

direct addressing means that the operand is the address number whereas intermediate addressing means the operand is the datum

Question 22

Explain why the data bus in a computer system must be bidirectional.

Answer 22

• When data is fetched they have to be transferred from memory to
  the processor
• data may need to be transferred back to memory

Question 23

if the data bus is 8 bits wide, how long is each instruction

Answer 23

16

Question 24

if the address bus is 8 characters long, what is the max address it can transmit

Answer 24

255 (2^8)-1

Question 25

what is an instruction set

Answer 25

all the instructions which are supported by a processors hardware

Question 26

why might compiles code not run on a computer with a certain processor

Answer 26

• different processors have different instruction sets
• and the program makes use of the instructions

Question 27

what does the addressing mode do

Answer 27

indicates how the value in the operand should be interpreted
- indicates if the value in the operand is a memory address or a data value

Question 28

Describe what an interrupt is and explain the purpose of interrupts

Answer 28

• A signal sent to the processor
• So that a device that needs the (immediate) attention of the processor can be serviced

Question 29

where is the harvard architecture used

Answer 29

medical body signal monitoring

Question 30

explain why desktop computers usually have secondary storage devices

Answer 30

• to store programs whilst computer is turned off
• as the contents of RAM are lost when computer is turned off