Hardware protection¶

Why?¶

Prevents user programs to break computer by executing bad instructions

Dual-Mode Operation¶

• User mode : may disable some instructions (cannot execute privileged instructions)
• Kernel mode (Monitor mode) : can execute all instructions
• Uses a bit to indicate current mode

IO protection¶

• All IO instructions are privileged instructions

Because IO device is shared between users

But still can modify memory to do something bad

Memory protection¶

• Protect interrupt vector and the interrupt service routines

HW support¶

• Base register : smallest leagal physical memory address
• Limit register : contains the size of the range

Outside of the range is protected

flowchart LR
    A@{ shape: circle, label: "CPU" } --> le{addr ≥ base}
    le --yes--> g{addr < base + limit}
    g --yes--> memory
    le --no--> trap@{ shape: trap-t, label: "trap to OS (addressing error)" }
    g --no--> trap
    trap --> q{end}
    memory --> q

CPU protection¶

• Prevent user program from not returning control

HW support¶

• Timer: interrupts computer after specified period
• Timer reaches \(0\) an interrupt occurs

Implement time sharing

• Load-timer is a privileged instruction, it writes the timer value

FAQ¶

• Is ... instruction a privileged instruction?