Dual-mode Protection

202310061634
Status: #idea
Tags: OS

Dual-mode Protection

flowchart TD

0[Dual-mode protection] --- 1[User mode]

0 --- 2[System/monitor mode]

Hardware has 1 bit for the 2 modes
- $1$ - User mode
- $0$ - System mode (monitor mode)
Switch to monitor mode if:
- Interrupt is raised
- Fault occurs
- Privileged instructions need to be run
- Kernel needs to perform some work

Info

A system call changes to kernel mode, and after executing, returns to user mode

Note

Now, CPUs support multi-mode operations (like Virtual Memory Manager for VMs)

CPU Protection

OS sets a timer to prevent infinite loops
Updated with the help of a physical clock
Used to calculate running time
Setting the timer is a privileged operation

I/O Protection

All I/O instructions are privileged
Must ensure that the user program does not control the computer in monitor mode

Memory Protection

flowchart LR

0[Logical address] -->|"Memory management\nunit (MMU)"| 2[Physical address]

Needed for IVT and ISR (interrupt service routine)
Determine the range of legal addresses a program may access
- Base register: Holds smallest legal memory address
- Limit register: Contains size of the range
Memory outside said range is protected

References