PC Hardware

System Memory Management

System Memory Management (SMM) is a hardware-based function that allows the microprocessor to selectively shut down the monitor, hard drives, and any other peripherals not in use. SMM works at the chip level; the microprocessor can be operating in real, protected, or virtual 8086 mode. SMM is transparent to all software running on the system, which decreases the likelihood of lockups.

Clock-Doubling Debuts

The need for speed spurred the introduction of new models of the 486 family through the spring of 1994, the last variations being the DX2 and DX4. These chips were models with faster clock speeds of up to 100 MHz. The processors were either 25- or 33-MHz versions that had been altered to run internally at double or triple their external speed. For example, the DX4 version of the 486 33-MHz processor ran at 33 MHz externally, but at 100 MHz internally (3 x 33.3 MHz). This meant that internal operations, such as numeric calculations or moving data from one register to another, occurred at 100 MHz, while external operations, like loading data from memory, took place at 33 MHz.

Slower external clock speeds allowed existing motherboard and memory designs to be used. Upgrades were less expensive, and new machines based on the DX technology could quote faster benchmarks at lower costs. The DX4 offered 16 KB of on-board cache, further boosting performance. The DX2 50-MHz-based machines should not be confused with machines designed around the 50-MHz 486DX processor-the latter performed much better.

Vendors such as AMD rode the wave with their own editions of the 486 for users feeling a need for greater speed. The following table lists the most popular 486 chips and third-party work-alikes.

Chip CPU Speed (MHz) Register Width External Data Bus Address Bus Internal Cache
Intel 80486DX 25, 33, 50 32-bit 32-bit 32-bit 8 KB
Intel 80486DX/2 50, 66 32-bit 32-bit 32-bit 8 KB
Intel 80486DX/4 75, 100 32-bit 32-bit 32-bit 16 KB
Intel 80486SX 16, 20, 25 32-bit 32-bit 32-bit 8 KB
Intel 80486SL 16, 20, 25 32-bit 32-bit 32-bit 8 KB
AMD AM486DX 33, 40 32-bit 32-bit 32-bit 8 KB
AMD AM486DXLV 33 32-bit 32-bit 32-bit 8 KB
AMD AM486DX2 50, 80 32-bit 32-bit 32-bit 8 KB
AMD AM486DX4 100, 120 32-bit 32-bit 32-bit 8 KB
AMD AM486DX
"Enhanced"
120, 133 32-bit 32-bit 32-bit 16 KB W/B
AMD AM486DXL2 50, 80 32-bit 32-bit 32-bit 8 KB
AMD AM486SX 33, 40 32-bit 32-bit 32-bit 8 KB
AMD AM486SXLV 33 32-bit 32-bit 32-bit 8 KB
AMD AM486SX2 33 32-bit 32-bit 32-bit 8 KB
CYRX CX486DX 33 32-bit 32-bit 32-bit 8 KB W/B
CYRX CX486DX2 50-80 32-bit 32-bit 32-bit 8 KB W/B
CYRX CX486DLC 33-40 32-bit 32-bit 32-bit 1 KB W/B
CYRX CX486SLC 20-33 32-bit 32-bit 32-bit 1 KB W/B
CYRX CX486SLC2 50 32-bit 32-bit 32-bit 1 KB W/B

Heat Sinks and Fans

The 486 is notable for one other item, the addition of a standard heat sink and, usually, a fan mounted on the CPU and powered by the PC. To maintain stable operation, the PC must provide proper cooling for the 5486 and newer CPUs. Failure of the cooling apparatus can lead to erratic behavior and-uncorrected-can damage the chip. If a customer complains of strange noises inside the PC, the CPU fan is a good place to look. As their bearings age, they start to whine.

by BrainBellupdated
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