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.