The UMC Green CPU was an
x86-compatible
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
produced by
UMC, a Taiwanese semiconductor company, in the early- to mid-1990s. It was offered as an alternative to the
Intel 80486
The Intel 486, officially named i486 and also known as 80486, is a microprocessor. It is a higher-performance follow-up to the Intel 386. The i486 was introduced in 1989. It represents the fourth generation of binary compatible CPUs following t ...
with which it was
pin compatible
In electronics, pin-compatible devices are electronic components, generally integrated circuits or expansion cards, sharing a common footprint and with the same functions assigned or usable on the same pins. Pin compatibility is a property des ...
, enabling it to be installed in most 80486 motherboards. All models had power management features intended to reduce electricity consumption.
Models produced
The UMC Green CPU was available with different features, physical characteristics and clock speeds. Some of which were only sold in limited quantities.
Available models
All models feature an 8 KB
level 1 cache
A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, which ...
and operate at clock speeds of 25 MHz, 33 MHz, or 40 MHz.
Functionally all models except U5D are identical and only differed in their intended application, voltage rating or physical packaging. The U5SD does not contain a floating point unit and is indistinguishable from other U5S chips in operation, though it is unusual because it features a 486DX pinout as opposed to the more common 486SX pinout versions. This would allow installation into certain older motherboards which may have had upgrade sockets hardwired to fit only 486DX chips.
Production and fabrication
As one of the largest chip foundry owners in Taiwan, UMC owns several fabrication plants which allowed them to fabricate their own designs, whereas some other manufacturers, notably
Cyrix
Cyrix Corporation was a microprocessor developer that was founded in 1988 in Richardson, Texas, as a specialist supplier of floating point units for 286 and 386 microprocessors. The company was founded by Tom Brightman and Jerry Rogers.
In 1992 ...
, had to contract this process out to third parties such as
IBM and
Texas Instruments
Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
. All available models of Green CPU were produced on a 0.6
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
CMOS process. The chips were available in both plastic and ceramic packages for different applications. The majority of PQFP variants were sold to motherboard manufacturers as a low-cost embedded solution.
Performance

The UMC U5 Series design was focused on microcode optimizations. An equivalently clocked Intel or AMD processor required 40 cycles to perform an
integer division
Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new numbers. The other operations are addition, subtraction, and multiplication.
At an elementary level the division of two natural numbers ...
whereas the UMC processors required only 7, allowing the instruction to complete significantly faster. Performance is generally observed to be higher than competing processors on a clock-for-clock basis; The 40 MHz U5S model being comparable to an AMD Am486SX2 at 66 MHz with correctly configured motherboards.
Due to an error in the microcode,
Microsoft Windows 98 Second Edition identifies some processors as an
Intel Pentium MMX which could cause the operating system and software running within it to crash or exhibit undefined behavior.
Clock doubling version
UMC produced a small quantity of clock doubling processors labeled as the U486DX2. These processors were designed to compete with other clock doubling solutions such as the
Intel 80486DX2
The Intel i486DX2, rumored as 80486DX2 (later renamed IntelDX2) is a CPU produced by Intel that was first introduced in 1992. The i486DX2 was nearly identical to the i486DX, but it had additional clock multiplier circuitry. It was the first chi ...
,
AMD Am486DX2 and
Cyrix Cx486DX2, but due to ongoing legal troubles UMC withdrew the U486DX2 from production. The processor was only ever produced as an engineering sample and never made it to market, it was manufactured with a 0.35 μm CMOS process and is contained within a ceramic package.
Legal dispute
In 1994, Intel alleged that UMC had infringed upon its patent for the 80486 microprocessor and filed complaints against UMC and its distributors.
[http://www.cpushack.com/2012/09/06/intel-vs-the-world-the-338-patent/ An article which talks about the Intel VS. UMC case.] UMC countered the claims with an anti-trust suit and the case was eventually settled out of court with UMC withdrawing their product and ceasing production of 80486-compatible microprocessors.
As a result of the dispute, all processors were prohibited from sale within the United States and were visibly labelled as such. Placement of this label varies, the ceramic U5S and U5D models typically display "Not for U.S. sale or import" as part of the silkscreen on the top of the chip where the U5SX and U5SD models usually had "NOT FOR U.S. SALE" printed onto the golden die cover on the underside of the package. This labelling is often absent from plastic packages.
Gallery
References
{{reflist
X86 microprocessors