Hardware Reference
In-Depth Information
to the interrupted program as well as any other software on the system. Once the SMM task is
complete, a resume instruction restores the previously saved context or state of the processor and
programs, and the processor resumes running exactly where it left off.
Although initially used mainly for power management, SMM was designed to be used by any low-
level system functions that need to function independent of the OS and other software 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 spinning 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 complex instruction set computer (CISC) chips
to be considered superscalar. A CISC chip uses a richer, fuller-featured instruction set, which has
more complicated instructions. As an example, say you wanted to instruct a robot to screw in a light
bulb. Using CISC instructions, you would say the following:
1.
Pick up the bulb.
2.
Insert it into the socket.
3.
Rotate clockwise until tight.
Using RISC instructions, you would say something more along the lines of the following:
1.
Lower hand.
