Week 9

Question 1

What is an operating system?

Answer 1

It’s a software that runs and makes a machine useable when there’s no application process running.
It’s a collection of a large number of services, some of which are provided all the time and others available to user programs via machine code requests (usually via an instruction like TRAP).

Question 2

What happens when a computer starts up?

Answer 2

It executes its reset handler which is in read only memory (ROM). Programs that are permanently in ROM on a computer are known as firmware.

Question 3

Where is the operating system held?

Answer 3

Some in ROM (basic input-output system B|OS) and some in firmware. The reset handler will normally go into the BIOS.

Question 4

What is multitasking or concurrency?

Answer 4

When an OS can run several programs at once. When two or more processes are switched between when we are waiting for I/O to complete, several processes can progress at the same time on one CPU. This is called multi-tasking.

Question 5

What are examples of operating systems?

Answer 5

Windows, unix, iOS, Android

Question 6

Can multiple users be supported on multitasking operating systems?

Answer 6

Yes, eg Unix

Question 7

What happened before operating systems existed in computers when the user wanted to run a program?

Answer 7

Operators using toggle switches loaded the program.

Question 8

What does physical input/output involve?

Answer 8

Input and output (collectively I/O) involves the movement of binary codewords from/to external input or output peripherals.

Question 9

What are examples of I/O devices?

Answer 9

Keyboard (input-only)
Screen (output-only)
Disk (both)

Question 10

Why does running a single program using I/O take so long?

Answer 10

After instructions have been executed, there is a wait time for the I/O devices to complete.

Question 11

What is a process?

Answer 11

It’s related to a program but differs from it as follows:

• A program is a static entity, either a source program written as text, or a binary image containing machine code
• A process is a running program and so is dynamic, continually changing as time passes and its instructions are executed
• When a program is loaded into primary memory and is ready to run, it becomes a process. When it is ready to run, it’s seen to be in a “waiting state”.

Question 12

Why would a process stop running?

Answer 12

• The programmer has designed it that way by adding in a TRAP instruction.
• It can be aborted by the OS or a user with no intention of resuming it.
• It can be suspended by the OS with the intention of resuming it later.
• It is less complicated to resume a process if it is suspended between instruction cycles.
• To resume a process successfully, the contents of all the CPU registers and memory locations it is using must be preserved.

Question 13

What is the “state” in the context of processes?

Answer 13

The collection of information needed to resume a process later.

Question 14

What is a context switch?

Answer 14

When a process is waiting on I/O, it can be suspended with it’s state carefully saved, and another process can then start up. When we switch from one process to another, this is called a context switch.

Question 15

What is the “ready” state in multi-tasking?

Answer 15

When the processes are not actually executing but are ready to go when the CPU becomes available.

Question 16

What is a “timer” in multitasking?

Answer 16

It sets time periods to interrupt the processor and stop the CPU doing what it’s doing and jump to a handler routine.

Question 17

What is a handler routine?

Answer 17

Some kind of OS and would decide of the several processes that are active which one will go next.

Question 18

What is an “interrupt” in multitasking?

Answer 18

It is a hardware signal from a device to the CPU which will stop it executing the current process. This is called a process break. The processes are executing in memory.

Question 19

What issues occur with multitasking?

Answer 19

• A programmer cannot know what other processes are running if one of the processes encounters a process break or an I/O block.
• How should I/O devices be shared?
• If several processes have their images in memory at the same time, those images are going to be at different addresses. There has to be something in place to prevent over-writing each other’s memory.
• This is why we have an operating system which supports multitasking.

Question 20

How is the operating system designed to support multitasking?

Answer 20

• It maintains a table of all running processes
• It mediates every process break, deciding what to do next. This is called process scheduling.
• The OS has to control with process gets access to which I/O device at any given time.
• The OS has to control access to the physical memory.

Question 21

What is Process Control?

Answer 21

It is implemented by the central core of machine code in the operating system (i.e. the Kernel). It is a fundamental service by any multitasking OS.

Question 22

What are some points on multitasking on one CPU?

Answer 22

The CPU is executing just one instruction. That instruction must belong to a process or to the OS.

Question 23

How often does the CPU switch between processes?

Answer 23

Depends on when the timer is set for (but around 100 times per second)

Question 24

What states will the process in a multitasking system always be in?

Answer 24

Either:

• Running
• Blocked (when the process is waiting for I/O)
• Ready

Question 25

What is the difference between Concurrency and Parallelism?

Answer 25

- Concurrency means several processes at the same time on a user's time scale. Requires multiple CUs and EUs. - Processes run in parallel when their instructions execute truly simultaneously (exactly at the same time) on separate CUs and execution units (EUs). They are usually run on multiple CUs. Parallelism is a type of concurrency.

Question 26

What is so confusing about the core and the CPU?

Answer 26

Often a core is referred to as the CPU. But with a multicore chip, this is also referred to as the CPU (even though it is a collection of CPUs!).

Question 27

What do we do to get around the fact that there's usually a greater amount of processes running concurrently in a typical OS than number of available cores?

Answer 27

Processes are run in a combination of true parallelism and context switching on each core.

Question 28

What approach doesn't rely on a timer for context switching?

Answer 28

Co-operative scheduling. Applications must be written to process break voluntarily, giving up control to the OS at regular intervals. This is usually programmed using a TRAP instruction with appropriate parameter. Programmers have to be careful because if even one app fails to process break, it has all the machine resources and other processes are starved.

Question 29

What do we called timer-based scheduling of multitasking?

Answer 29

pre-emptive scheduling. The hardware and OS enforce a policy of regular process breaks. These are provided via an external hardware timer with an interrupt line to the CPU. On interrupt the CPU stops the process it is executing and jumps to a special interrupt handler routine which is part of the OS and which now decides what to do. The handler for the timer interrupt is called the process scheduler (part of the kernel). The process scheduler uses a scheduling algorithm to choose which process goes next if more than one is ready to run.

Question 30

Where are internal disks located usually?

Answer 30

Outside the CPU/primary memory/bus structure, therefore we will consider them to be (external) I/O peripherals.

Question 31

What does it mean to say that I/O is synchronous?

Answer 31

The process doing I/O cannot do anything in the meantime until I/O completes

Question 32

What does it mean to say that the I/O is asynchronous?

Answer 32

The process can do useful work while doing I/O. In this case, we have an interrupt handler to signal when the request completes. This causes the CPU to jump to an interrupt handler.

Question 33

Do you need an OS to do multitasking?

Answer 33

Yes

Question 34

What are some examples of I/O devices that cannot be shared?

Answer 34

Printer, keyboard

Question 35

What is software interrupt?

Answer 35

Basically just a TRAP instruction.

Question 36

Which are more complex to write? Synchronous or asynchronous?

Answer 36

Async

Question 37

What is an interrupt?

Answer 37

A hardware interrupt allows an external hardware device to indicate to the processor to stop what it's doing and jump to an interrupt handler. A timer can have an interrupt line straight to the processor and a I/O port can have an interrupt line straight to the processor