Dual-Mode Operation

• sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
• provide hardware support to differetiate between at least two modes of operations

1. User mode - execution done on behalf od a user.
2. Monitor Mode - (also supervisor mode or system mode) -execution done on behalf of operating system.

I/O Protection

• All I/O isntructions are privileged instructions.
• Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part od its execution, stores a new address in the interrupt vector).

Memory Protection

• Must provide memory protection at least for the interrupt vector and the interrupt service routines
• In order to have memory protection, add two registers that determine the range of legal addresses a program may access:

- base register - holds the smallest legal physical memory address.

- limit register - contains the size of the range.

• Memory outside the defined range is protected.

CPU Protection

• The VRM design approachremoves cable inductance fromthe distribution and reducesboard inductance. A load-changetransient occurs when coming outof or entering a low power mode.For some CPUs this load-changetransient can be on the order of13A. These are not only quickchanges in current demand, butalso long-lasting average currentrequirements. Even during nor-mal operation the currentdemand can still change by asmuch as 7A as activity levelschange within the processorcomponent. Maintaining voltagetolerance during these changesin current requires high-densitybulk capacitors with low EffectiveSeries Resistance (ESR). Thesehigh-current immediate demandson the circuits can cause compo-nents to fail. Circuit protectionprevents the VRM from damag-ing the CPU in the event of aVRM fault. If the VRM fails, theprocessor tries to pull too muchpower. A PolySwitch device canbe placed on the input pins to theVRMs that supply power to theprocessors, therefore protectingthe processors. If there is a fail-ure, only the VRM needs to bereplaced, rather than the moreexpensive CPU.