486 CLASS CPUS

8/14/98

486 Class CPUs: Intel introduced the 486CPU in August of 1988. The 486 featured a full 32 bit external interface in a 168 pin grid array package. Some variants used a 208 pin SQFP package, or various 80386 packages. The 486 included an 8KB cache memory on chip and an internal math coprocessor. The 486 was the first CPU to support burst mode memory access that allowed memory accesses to consecutive addresses to pipeline data in as quickly as 1 clock per transfer. The instruction set was substantially speeded up relative to the 386 yielding an average of about 2 clocks per instruction. Intel claims that the clocks per instruction is 1.2, but that seems quite optimistic. Typical MIPS ratings ranged from 70 for the DX4-100 down to 20 for the 486SX25. For the most part the 486 memory management and instructions were identical to the 386DX. Cache management was added and power management was introduced late in the product cycle.

The 486 was originally released as a 5 volt CPU running at 20, 25 or 33 MHz. Later, a 50 MHz version was released. 50MHz bus speed often exceeded both the actual and theoretical limits of peripheral cards, and 50MHz systems were frequently unstable.

A cheaper variant -- the 486SX (Mar91) was identical to the 486DX but lacked a functioning math coprocessor. A later variant was the introduction of clock multiplied CPUs. These ran the CPU at multiples of the bus speed. These included x2 (Mar92 - the DX2-50 and DX2-66) and x3 (Mar94 -- the DX4-75 DX4-100). Intel also sold high priced Overdrive processors intended to upgrade 486 motherboards. Some of these were actually identical to the normal CPUs while others had a slightly different pinout to accommodate an Overdrive socket present on many motherboards. Not all Overdrive sockets worked. In mid product cycle, the CPU voltage was changed from 5 volts to 3.3 volts. The 486DX2-66 was produced in both 3 and 5 volt versions. DX4s were initially 3 volt only, but a 5Volt Overdrive processor was sold also. the DX4 expanded the internal cache to 16KByte. A low power variant, the 486SL (Nov92), was available in 20-25-33MHz versions for use in portable computers

Other manufacturers also made 486s. AMD had access to the 486 as a result of prior licensing arrangements, and produced a line of fully compatible CPUs although the later designs showed some variations. The final AMD 486s were x4 multiplied versions known as the 5x86 which were claimed to operate as fast as the low end Pentiums such as the P-75. IBM also had access to the design, and opted to produce a variety of somewhat odd variants with 16 bit external interfaces, clock multiplication, and often with larger caches than the 486DX. These were usually identified as SLCs or "Blue Lightnings". Cyrix produced 486 variants specifically intended to plug into 386 sockets. These CPUs with 1K caches were dubbed the SLC (not to be confused with the IBM offering of the same name) for the 386SX replacement and the DLC for the 386DX replacement. Cyrix later produced independently designed 486DX2-66, 80 and 100 CPUs. The Cyrix 486DX2s were also produced by IBM, SGS-Thompson and Texas Instruments and came in both the standard pinout (so marked) and a non-standard pinout with features like cache mode controlled externally. Some motherboards had Floppy I/O problems with Cyrix CPUs set for Write Back caching.

Power consumption varied with CPU speed and voltage. Experience has shown that most 486s can be run without heat sinks although the 5 volt 486DX2-66 generally is used with at least a passive heatsink. CPU fans were often used with 486DX2-66 when the CPU was first introduced and was very expensive and therefore was worth considerable expense to protect. However, the early fans often were not very reliable and will often be found to be long since frozen up when the machines are serviced. Many of the highest speed 486s were sold with passive heat sinks permanently affixed.

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