The NEC V20 is a

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 circ ...

that was designed and produced by

NEC is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network soluti ...

. It is both

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 desir ...

and object code compatible with the

Intel 8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and ...

, with an

instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...

(ISA) similar to that of the Intel 80188 with some extensions. The V20 was introduced in March 1984.

Features

The V20's

die Die, as a verb, refers to death, the cessation of life. Die may also refer to: Games * Die, singular of dice, small throwable objects used for producing random numbers Manufacturing * Die (integrated circuit), a rectangular piece of a semicondu ...

comprised 63,000

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s; more than double the 29,000 of the 8088 CPU. The chip was designed for a clock

duty cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a form ...

of 50%, compared to the 33% duty cycle used by the 8088. The V20 has two, 16-bit wide internal databuses, allowing two data transfers to occur concurrently. Differences like that meant that a V20 could typically complete more instructions in a given time than an Intel 8088 running at the same frequency. The V20 was fabricated in 2-micron CMOS technology. Early versions ran at speeds of 5, 8, and 10 MHz. In 1990, an upgrade to the fabrication process technology resulted in the V20H and V20HL, with improved performance and reduced power consumption. Later versions added speeds of 12 and 16 MHz. The V20HLs were also completely static, allowing their clock to be stopped. The V20 was described as 16 bits wide internally. It used an 8-bit external data bus that was multiplexed onto the same pins as the low byte of the address bus. Its 20-bit wide address bus was able to address 1 MB. The V20 was reported to have been compatible with the Intel 8087

floating-point unit In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be ...

(FPU) coprocessor. NEC also designed their own FPU, the μPD72091, which was cancelled before reaching production. They followed this with a revised design, the μPD72191, but it is unclear how many, if any, of this second part were produced. The V30, a nearly identical CPU with a

16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two ...

wide external data bus, debuted on March 1, 1984. It was pin and object-code compatible with the Intel 8086.

ISA extensions

The V20's ISA includes several instructions not executed by the 8088, with instructions for bit manipulation, packed BCD operations, multiplication, and division. They also include new real-mode instructions from the Intel 80286. The ADD4S, SUB4S, and CMP4S instructions were able to add, subtract, and compare huge packed

binary-coded decimal In computing and electronic systems, binary-coded decimal (BCD) is a class of binary encodings of decimal numbers where each digit is represented by a fixed number of bits, usually four or eight. Sometimes, special bit patterns are used ...

numbers stored in memory. Instructions ROL4 and ROR4 rotate four-bit

nibble In computing, a nibble (occasionally nybble, nyble, or nybl to match the spelling of byte) is a four- bit aggregation, or half an octet. It is also known as half-byte or tetrade. In a networking or telecommunication context, the nibble is oft ...

s. Another family consisted of the TEST1, SET1, CLR1, and NOT1 instructions, which test, set, clear, and invert single bits of their operands, but are far less efficient than the later

i80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsBT, BTS, BTR, and BTC; neither are their encodings compatible. There were two instructions to extract and insert bit fields of arbitrary lengths (EXT, INS). And finally, there were two additional repeat prefixes, REPC and REPNC, which amended the original REPE and REPNE instructions for scanning a string of bytes or words (with instructions SCAS and CMPS) while a less or not less condition remained true. The V20 offered a mode that emulated an

Intel 8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...

CPU. A BRKEM instruction is issued to start 8080 emulation. The operand of the instruction specifies an interrupt number whose vector contains the segment:offset where emulation is to begin. To end, a RETEM instruction is issued in 8080 code. One feature not often employed is the CALLN (call native) which issues an 8086-type interrupt call that enables x86 code (which returns using an IRET) to be mixed in with 8080 code. Another mode put the processor into a power-saving state via a HALT instruction.

Lawsuit

In 1982 Intel sued NEC over the latter's μPD8086 and μPD8088. This suit was settled out of court, with NEC agreeing to license the designs from Intel. In late 1984 Intel again filed suit against NEC, claiming that the microcode in the V20 and V30 infringed its patents for the 8088 and 8086 processors. NEC software engineer Hiroaki Kaneko had studied both the hardware design of the Intel CPUs and the original Intel microcode. In its ruling, the court determined that the microcode in the control store constitutes a computer program, and so is protected by copyright. They further found Intel had forfeited their copyright by neglecting to ensure that all second-source chips were suitably marked. The court also determined that NEC did not simply copy Intel's microcode, and that the microcode in the V20 and V30 was sufficiently different from Intel's to not infringe Intel's patents. The judge in the case accepted NEC's ''

cleanroom A cleanroom or clean room is an engineered space, which maintains a very low concentration of airborne particulates. It is well isolated, well-controlled from contamination, and actively cleansed. Such rooms are commonly needed for scientif ...

'' evidence. He also approved of NEC's use of

reverse engineering Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompli ...

with respect to the creation of NEC's Rev.2 microcode, without commenting on it with respect to the Rev.0 code.

Variants and successors

Image:NEC_V30_die.JPG, NEC V30 Image:NEC_V50_die.JPG, NEC V50 Image:NEC_V53_die.JPG, NEC V53

References

External links

* * * {{NEC Corporation NEC V20 V20 16-bit microprocessors