Hardware Reference
In-Depth Information
restores the previously saved context or state of the processor and programs, and the pro-
cessor resumes running exactly where it left off.
Whileinitiallyusedmainlyforpowermanagement,SMMwasdesignedtobeusedbyany
low-levelsystemfunctionsthatneedtofunctionindependentoftheOSandothersoftware
on the system. In modern systems, this includes the following:
• ACPI and APM power management functions
• Universal serial bus (USB) legacy (keyboard and mouse) support
• USB boot (drive emulation)
• Password and security functions
• Thermal monitoring
• Fan speed monitoring
• Reading/writing Complementary Metal Oxide Semiconductor (CMOS) RAM
• BIOS updating
• Logging memory error-correcting code (ECC) errors
• Logging hardware errors besides memory
• Wake and Alert functions such as Wake on LAN (WOL)
One example of SMM in operation occurs when the system tries to access a peripheral
device that had been previously powered down to save energy. For example, say that a
program requests to read a file on a hard drive, but the drive had previously spun down to
save energy. Upon access, the host adapter generates an SMI to invoke SMM. The SMM
software then issues commands to spin up the drive and make it ready. Consequently,
SMM returns control to the OS, and the file load continues as if the drive had been spin-
ning all along.
Superscalar Execution
The fifth-generation Pentium and newer processors feature multiple internal instruction
execution pipelines, which enable them to execute multiple instructions at the same time.
The 486 and all preceding chips can perform only a single instruction at a time. Intel calls
the capability to execute more than one instruction at a time
superscalar
technology.
Superscalar architecture was initially associated with high-output reduced instruction set
computer (RISC) chips. A RISC chip has a less complicated instruction set with fewer
and simpler instructions. Although each instruction accomplishes less, the overall clock
speed can be higher, which usually increases performance. The Pentium is one of the first
complexinstructionsetcomputer(CISC)chipstobeconsideredsuperscalar.ACISCchip
