9618 SYSTEM SOFTWARE & VIRTUAL MACHINES

Front

The management and scheduling of processes are tasks carried out by an operating system.

(a) Identify three process states. [3]

Back

One mark per point

• Running state

• Ready state

• Blocked state

Front

(b) Describe the function of the shortest job first scheduling routine and give a benefit of this routine. [4]

Back

One mark per mark point (Max 3)

MP1 The processes are queued as they arrive

MP2 The process with the shortest time to complete/burst time is selected first and executed

MP3 The process will continue until complete or put in a waiting state once execution has begun // it is non-pre-emptive

MP4 The scheduler will continue to choose shorter processes over longer processes if they continue to be added to the queue can cause starvation for longer jobs

One mark for a benefit (Max 1)

MP5 Shorter jobs don’t have to wait for longer jobs to complete before processing // Significantly reduces the average overall waiting time for processes // Ensures starvation doesn’t occur for process with shorter burst times

MP6 Higher throughput of processes

Front

Explain what is meant by lexical analysis during program compilation. [4]

Back

One mark per mark point (Max 4)

MP1 It is the first stage of compilation

MP2 White space and comments are removed

MP3 It takes modified source code and breaks it into a series of tokens

MP4 Each token is categorised and assigned types

MP5 Identifiers are stored in a symbol table

MP6 If the lexical analyser finds invalid tokens, it generates an error

MP7 Acceptable data from the lexical analyser passes to the syntax

analyser

Front

Describe the process of executing a program using an interpreter. [4]

Back

One mark for each mark point (Max 4)

MP1 The interpreter translates the source code one line at a time

MP2 If the line is syntax error free it is executed

MP3 It is not stored in executable format

MP4 If an error is found, the program halts with an error message

MP5 Each line must be translated every time it is run, including lines running multiple times for example in loops

Front

(a) Describe what is meant by multi-tasking and how it benefits process management. [2]

Back

One mark for each mark point

MP1 Running multiple processes concurrently

MP2 … which benefits process management by allowing more tasks to complete than would be the case if they had to run one task after another.

Front

(b) Explain the function of the shortest remaining time scheduling routine and give a benefit of this routine. [4]

Back

One mark for each mark point (Max 3)

MP1 The processes are queued as they arrive

MP2 Processes with the shortest burst time are executed first

MP3 It is a pre-emptive scheduling function // When a process with a shorter burst time arrives the existing process is replaced by the shorter process.

MP4 The scheduler will continue to choose shorter processes over longer processes if they continue to be added to the queue can cause starvation for longer jobs

One mark for benefit (Max 1) e.g.

MP5 Processes with a short burst time are processed very quickly

MP6 Waiting time is minimised

Front

(a) The kernel is the central component of an Operating System (OS).

Outline how the kernel of an OS acts as an interrupt handler. [2]

Back

One mark per mark point (Max 2)

MP1 the kernel receives a signal when an interrupt is generated

MP2 the kernel checks the priority and reviews the status/priority of the current interrupts

MP3 system enters kernel mode if the type of interrupt is of higher priority than the current process

MP4 the kernel consults the interrupt dispatch table / IDT

MP5 … and saves the state of the interrupted process / contents of the registers on the kernel stack

MP6 the kernel restores the process state e.g. contents of registers once the interrupt is serviced

Front

(b) (i) State what is meant by the term multi‑tasking in an Operating System. [1]

Back

One mark per mark point (Max 1)

MP1 multi-tasking allows computers to carry out / seem to carry out more than one process at a time

Front

(ii) Describe how multi‑tasking is implemented in an Operating System. [2]

Back

One mark per mark point (Max 2)

MP1 processor time/common hardware and resources is/are shared between tasks

MP2 scheduling is used to decide on the processes to be carried out to ensure multi-tasking operates correctly / efficiently/without clashes

MP3 one task of a higher priority can interrupt another task that is currently running

Front

(a) Outline two benefits and two limitations of a virtual machine. [2]

Benefit 1 .....................................................................................................

Benefit 2 .....................................................................................................

Limitation 1 .....................................................................................................

Limitation 2

.....................................................................................................

Back

One mark per mark point for up to two benefits (Max 2)

MP1 COMPATIBILTY e.g. Applications that aren’t compatible with the host computer can be run on the virtual machine // It is possible to emulate old software on a new system by running a compatible guest operating system as a virtual machine // Software can be tried on different OS on the same hardware.

MP2 PROTECTION e.g. The guest operating system has no effect on anything outside the virtual machine other virtual machines or the host computer//Virtual machines are useful for testing as they will not crash the host computer if something goes wrong // Easier to recover if software causes a system crash as virtual machine software protects

the host system.

MP3 COST e.g. No need to buy extra computers / hardware as multiple virtual machines can be implemented on the same hardware.

One mark per mark point for up to two limitations (Max 2)

MP4 PERFORMANCE e.g. The performance of the guest operating system will not be as good on a virtual machine as it would be on its own compatible machine because of the extra code / using more RAM/memory space // The performance of the VM is dependent on the capabilities of the host computer // Response times cannot be accurately measured using a virtual machine.

MP5 COMPLEXITY e.g. Building an in-house virtual machine can be expensive, time consuming and complex to maintain / set-up.

MP6 HARDWARE/SOFTWARE ISSUES e.g. Some hardware/software can’t be emulated with a virtual machine // Some of the host machine’s hardware can’t be directly accessed by the virtual machine