FIT1047 Study Guide - Final Guide: Context Switch, Subroutine, Interrupt Handler
31 May 2018
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Virtualising the CPU
Saturday, 26 May 2018
8:35 PM
• Goal of Virtualising the CPU is to run multiple processes simultaneously, but in a way that
individual processes don't know that they are running in parallel with other processes.
• The Operating System creates this illusion of many processes running in parallel by switching
between the processes several times per second
o A process shouldn't notice that it's not running continuously.
o Each process would get a fair share of the overall CPU time
Virtualisation mechanisms
• Mechanisms for virtualising a CPU:
o Ready
o Running
o Blocked
• When the process is created by loading program code into memory, it is put into the ready
state
o Ready means ready for execution, but not currently being executed by the CPU
• When the OS decides that the process should now be executed, it puts it into running state.
(OS schedules the process)
o In the running state, the code for the process is actually executed on the CPU
• 2 things can happen after:
o Either the OS decides that the time for this process is up
• Process is de-scheduled, and put back into ready state, from which it will be
scheduled again in the future
o Or the process requests some I/O to happen
• Opening a file from an external storage
• I/O can take a long time, so the process will have to wait until the file has been
opened. The OS takes advantage of this opportunity and puts the process into the
blocked state
• As soon as the I/O is finished, the process will be put back into ready, from which
it will be scheduled again
find more resources at oneclass.com
find more resources at oneclass.com
Limited Directed Execution
CPU Virtualisation via process switching has a number of challenges:
• Performance
o Virtualisation should not create a huge overhead, and most of the CPU time should be
spent on actually running processes, not managing them
• Control
o The OS should enable fair scheduling of processes and offer a certain level of protection
against malicious or buggy code
The solution to these challenges is to get some support from the hardware in a mechanism called
limited directed execution (LDE).
In order to achieve good performance, each process is allowed to run directly on the CPU
This is what direct means in LDE.
CPUs have different modes of operations that enables the OS to restrict what an application
program can do
User Mode
• Only a subset of all instructions are allowed to be executed
• Instructions that perform I/O is not allowed in user mode
• Normal applications are executed in user mode
Kernel Mode
• Code is run without any restrictions
• Has full access to I/O devices and memory
• OS runs in Kernel mode
• Whenever an interrupt happens, the CPU switches into kernel mode to execute the interrupt
handler
• Sometimes called supervisor mode
Notes:
• User modes places quite severe restrictions on application programs
o Without access to I/O instructions, a program cannot read any files, send or receive
messages over networks, or draw characters on the screen
o Applications need to do all these things
• In order to give user mode processes access to these "privileged" operations, the OS provides
a number of "hooks" that a user mode process can use
o Much like subroutines, to perform functions like reading from a disk or sending a
message over the network
o These hooks are called system calls
System Calls
A system call is a special CPU instruction that switches the CPU from user mode into kernel mode,
then jumps to a special subroutine of the OS
• Many CPU architectures simple provide an instruction that causes in interrupt, which causes
the switch to kernel mode
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
I/o can take a long time, so the process will have to wait until the file has been opened. The os takes advantage of this opportunity and puts the process into the blocked state: as soon as the i/o is finished, the process will be put back into ready, from which it will be scheduled again. The solution to these challenges is to get some support from the hardware in a mechanism called limited directed execution (lde). In order to achieve good performance, each process is allowed to run directly on the cpu. Cpus have different modes of operations that enables the os to restrict what an application program can do. User mode: only a subset of all instructions are allowed to be executed, normal applications are executed in user mode. Instructions that perform i/o is not allowed in user mode.
