15 - Hardware and Virtual Machines

Describe Reduced Instruction Set Computers (RISC). [6]

• Uses simple instructions

• Uses fixed length instructions

• Instructions only require one clock cycle

• Uses many registers

• Makes use of pipelining

• Hardwired CU

Describe Complex Instruction Set Computers (CISC). [9]

• Uses many instruction formats

• Uses variable length instructions

• Makes use of different addressing modes

• Uses few registers

• Has a large instruction set

• Requires complex circuits

• Frequently uses cache

• Instructions (converted to sub-instructions that) may require many clock cycles

• Programmable CU

Describe interrupt handling on RISC and CISC processors. [4]

• Once the processor detects an interrupt at the start/end of the fetch-execute cycle

• ... the current program is temporarily stopped and the status of each register stored on the stack.

• After the interrupt has been serviced/the Interrupt Service Register (ISR) has been executed ...

• ... the registers can be restored to its original status before the interrupt was detected // ... the data can be restored from the stack.

Describe the importance and use of pipelining in RISC processors. [6]

• Pipelining allows several instructions to be processed simultaneously / concurrently

• ...therefore, increasing the CPU instruction throughout / the number of instructions completed per unit of time.

• Instructions are divided into 5 stages

• Each instruction stage / subtask is completed during one clock cycle

• No two instructions can execute their same stage of instruction / subtask at the same clock cycle.

• ...e.g. while one instruction is being decoded, the next instruction can be fetched, etc.

Explain how pipelining affects interrupt handling. [4]

• Pipelining adds an additional complexity // there could be a number of instructions still in the pipeline when the interrupt is received

• All the instructions currently in operation are usually discarded except for the last one/the one at write back

• ... the interrupt handler routine is applied to the remaining instruction.

• Once the interrupt has been serviced the processor can restart with the next instruction in the sequence.

Describe the Single Instruction, Single Data (SISD) computer architecture. [3]

• Contains one processor, a control unit and a memory unit

• Executing a single instruction from a single source

• Executes instructions sequentially

Describe the Single Instruction, Multiple Data (SIMD) computer architecture. [3]

• Performs the same operation on multiple different data streams simultaneously.

• The instructions can be performed sequentially, taking advantage of pipelining.

• Parallel computers with multiple processors.

Describe the Multiple Instruction, Single Data (MISD) computer architecture. [3]

• Performs different operations on the same data stream.

• Each processor works on the same data stream independently.

• Parallel computers with multiple processors.

Describe the Multiple Instruction, Multiple Data (MIMD) computer architecture. [3]

• Contains many processors

• Processors operate asynchronously / independently

• Can execute different instructions on different data sets

Describe massively parallel computers. [4]

• A large number of computer processors / separate computers connected together

• ...simultaneously performing a set of coordinated computations // collaborative processing

• network infrastructure

• communicate using a message interface / by sending messages.

State the purpose of a flip-flop circuit. [1]

The purpose of a flip-flop is to store a binary digit / (single) bit.

Draw the circuit diagram for an SR flip-flop. [1]

Draw the circuit diagram of a JK flip-flop. [1]

Draw the circuit diagram of a half adder and write an expression for each of the outputs. [3]

S = P XOR Q

C = P AND Q

Draw the circuit diagram of a full adder and write an expression for each of the outputs. [3]

X = (P XOR Q) XOR R

Y = ((P XOR Q) AND R) OR (P AND Q)

Describe virtual machines. [3]

• The emulation of a computer system / hardware and/or software

• ...using a host computer system.

• Using guest operating system(s) for emulation.

Describe the benefits of virtual machines. [6]

• Multiple guest operating systems / VMs can be used on the same computer.

• Different instruction set architectures can be emulated on a single computer.

• A virtual machine can crash without affecting the host machine.

• There are security benefits // Trying a piece of suspicious software and if it is / has a virus, it will only infect the virtual machine.

• Cost savings due to not needing to purchase extra hardware.

• Can run legacy applications that are currently incompatible.

Describe the drawbacks of virtual machines. [6]

• A virtual machine is less efficient / has poorer performance than real machines because of extra load on the host computer

• Performance of the guest system cannot be adequately measured.

• A virtual machine may be affected by any weaknesses of the host machine.

• Costly and/or complex to maintain / implement / manage.

• Cannot emulate new hardware

• ...because the hardware may have been developed since the virtual machine was developed.