Computer Architecture

Question 1

What is an instruction set?

Answer 1

The interface between the hardware and the software

Question 2

List four different types of ISAs

Answer 2

1. Accumulator based ISAs
2. General purpose register ISAs
3. Reduced instruction set computers
4. Stack based ISAs

Question 3

Describe the architecture of a reduced instruction set computer

Answer 3

It has a small number of instructions which all have a common format and all of these instructions apart from LOAD and STORE operate on the registers. LOAD and STORE transfer information to and from memory.

Question 4

How many clock ticks does an instruction take to execute in a RISC?

Answer 4

Typically 1 clock cycle

Question 5

WHat is the opposite to a RISC computer?

Answer 5

A Complex Instruction Set Computer

Question 6

What is the principle behind a stack based ISA?

Answer 6

Instructions are stored in a stack and only the first element in the stack can be manipulated using push or pop

Question 7

What is MIPS 1 ISA?

Answer 7

An influential instruction set

Question 8

What are the three categories of instructions in a MIPS 1 ISA?

Answer 8

Arithmetic and logic instructions, data transfer instructions, and control transfer instructions

Question 9

What type of form is a MIPS 1 ISA?

Answer 9

A RISC form ISA

Question 10

What does MIPS32 mean?

Answer 10

There are 32 registers each of which has 32 bits

Question 11

What is 1 downsides to MIPS1 ISA?

Answer 11

There is no floating point capability

Question 12

How are instructions stored in MIPS1 ISA?

Answer 12

6 bits for the operation class
5 bits for the first source register
5 bits for the second source register
5 bits for the destination register
5 bits for a shift operation
6 bits for the functions in class

Question 13

What is the default ‘mode’ of the PC?

Answer 13

Sequential execution

Question 14

Describe the concept behind a multi stage fill adder with ripple carry

Answer 14

We can connect multiple adders together to allow the carry from the original adder to flow to the next one but this is too slow

Question 15

What is a carry-select adder?

Answer 15

Carry select adders split multi stage ripple carry adders into two so that the addition of lower value and upper value bits are executed simultaneously

Question 16

What are status flags?

Answer 16

Contained in a flags register

Question 17

What are status flags for the ALU?

Answer 17

1. Zero flag checks whether all of the outputs are 0
2. The negative flag checks the value of the MSB to determine if a number if negative
3. The overflow flag checks for overflow by using negative flags

Question 18

What are the two types of volatile memory?

Answer 18

Static and dynamic

Question 19

Difference cost and efficiency of static and dynamic memory

Answer 19

Dynamic memory is slower and cheaper
Static memory is faster but more expensive

Question 20

Uses of static memory

Answer 20

Registers and cache, fast but small so not too expensive

Question 21

What does the address decoder do?

Answer 21

Maps a linear address provided by the processor to a specific location on a specific memory chip

Question 22

Difference between big endian and little endian

Answer 22

Big endian stores the MSB in the lowest memory address
Little endian stores the MSB in the highest memory address

Question 23

What type of logic does static memory use as opposed to dynamic

Answer 23

Static memory uses sequential logic where as dynamic memory uses combinatorial logic (Boolean)

Question 24

What is sequential logic?

Answer 24

The outputs from sequential logic are based on the inputs and outputs meaning that you have a feedback loop

Question 25

Give an example of sequential logic

Answer 25

Flip-flops

Question 26

Describe an S-R flip-slop

Answer 26

The flip flop has distinct SET and RESET inputs and consists of several NAND gates. But it is limited to only SET and RESET inputs

Question 27

What is the master-slave flip-flop?

Answer 27

It consists of two D-type flip-flops the first being the master and the second being the slave. The output of the slave can only be read if the output enable is high which ensures that data is read being it is overwritten

Question 28

Describe how a master-slave flip-flop works

Answer 28

The master side acts like a latch and pulses high allowing the data signal to be stored in the master side and the slave side is untouched. When the signal returns to low the data in the master flip-flop is transferred to the slave where it is then controlled by the output enable

Question 29

How can we split data busses up?

Answer 29

Internal and external

Question 30

What is the correct way to reference the MBR?

Answer 30

MDR - memory data register

Question 31

What are the two ways of executing the fetch-decode-execute cycle?

Answer 31

Using FSMs or microcode

Question 32

What does a FSM do?

Answer 32

A FSM consists of hard wired sequential logic which are high performance but expensive and hard to evolve

Question 33

What is microcode?

Answer 33

A sequence of microinstructions that are contained in micro-memory, it is much more flexible than a FSM as it is not hard-wired but it is slightly lower performance

Question 34

What is pipelining?

Answer 34

Pipelining is a way to speed up the execution of the fetch-decode-execute cycle by exploiting inherent parallelism and running multiple instructions at the same time as they will not all need to use the same sub-component of the processor at the same time

Question 35

What are the three types of pipeline hazards?

Answer 35

1. Control hazards when a control-transfer instructions changes the flow of execution from being sequential 2. Data hazards when instruciton n depends on the result from n-1 so it needs to be executed first 3. Structural hazards when two parts of the pipeline needs to use the same hardware

Question 36

How do we fix a pipeline stall?

Answer 36

When a pipeline hazard occurs the pipeline stalls so we must flush it which costs us a lot of time

Question 37

What are the two main problems with connecting I/O devices?

Answer 37

1. The speed gap challenge where the two processors are running at different orders of magnitude faster or slower 2. The diversity challenge because peripheral devices are extermely diverse and they all have different ways of performing different instructions

Question 38

How do we solve the speed-gap problem?

Answer 38

The device driver acts like a middle man. The device driver is a software plug in

Question 39

How do we soolve the diversity problem?

Answer 39

Devcie drivers abstract the diversity of the peripherals by grouping devices that are essentially the same

Question 40

FUnctions of a device driver

Answer 40

Registering the device with the OS and initialising it Initiating data transfers to and from the device Monitoring status events from the device Managing device/suystem shutdown

Question 41

What is the traditional two fold classification of device type

Answer 41

Character devices which send and receive one byte at a time Block devices which send and receive multi-byte blocks at a time

Question 42

How does a hard disk store information?

Answer 42

The information is stored in tracks which are then split into sectors, these sectors are stacked to form cylinders and cylinders are then grouped into heads

Question 43

What does an isolated I/O do?

Answer 43

Provide physical pins for the connection of the I/O device and dedicated instructions for doing I/O operations, better for simple devices as you can achieve diversity

Question 44

What does a memory-mapped I/O do?

Answer 44

It has a device that sits within the CPU/s linear memory address space