Page 85 - Computer_Science_F5

Computer Science Privileged and non-privileged operating system or specific programs
with authorisation. Examples of such
instruction
instructions in x86 architecture are;
Imagine a well-organised restaurant
(a) MOV to/from Control Registers (such
kitchen called CPU, where tasks are
as CR0 and CR3): These instructions
divided for efficiency and security. The
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head chef represents the operating system,
like memory protection and paging.
having complete control over ingredients configure essential CPU behavior,
(resources) and appliances (hardware). Tampering with them could lead to
Other cooks represent user programs and system instability. Example: MOV
are responsible for specific dishes (tasks), CR0, eax - This instruction moves
but they need permission to access certain the contents of the EAX register
ingredients and appliances. This analogy to the Control Register 0 (CR0),
explains privileged and non-privileged potentially enabling features like
instructions in ISA. paging or write-protection.
(b) I/O instructions (IN, OUT): Direct
Again, consider a real-world example: a interaction with hardware devices
word processing program (user program) like disks or network cards requires
wants to save a document to the hard drive privileged access to prevent conflicts
(a privileged operation). However, it can’t and data corruption.
directly access the disk. Instead, it uses (c) HLT (Halt): This instruction puts
a system call (like asking the head chef the processor into a low-power state,
for permission) to request the operating effectively stopping all program
system to perform the save operation on execution. Only the OS should have
its behalf. The operating system then uses the authority to do this.
privileged instructions to interact with
the disk controller and write the data.
Privileged instructions are like secret Non-privileged instructions
recipes or master control panels in the These instructions are like the everyday
kitchen. They ensure smooth operation cooking tasks assigned to regular cooks.
and protect valuable resources. By limiting They can be used freely by user programs
access to these important functions, the (Ring 3) without special permission.
operating system maintains a stable and The operating system monitors non-
secure environment for user programs to privileged instructions to prevent
carry out their tasks unauthorised access to critical resources
through privilege checks. An attempt to
Privileged instructions execute a privileged instruction without
Like the head chef’s control panel for permission results in a protection fault
adjusting ovens, privileged instructions exception (like setting off an alarm in the
require a higher level of access (Ring kitchen). Examples of such instructions
0) and can only be executed by the in x86 architecture are;

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Computer Science Form 5.indd 76 23/07/2024 12:32