NEC V30

The NEC V20 (μPD70108) was a processor made by NEC that was a reverse-engineered, pin-compatible version of the Intel 8088 with an instruction set compatible with the Intel 80186. The V20 was introduced in 1982, and the V30 debuted in 1983.

The chip featured much more than the 29,000 transistors of the simpler 8088 CPU, ran at 5 to 10 MHz and was around 30% faster (application dependent) than the 8088 at the same clock speed, primarily due to faster effective address calculation, along with faster loop counters, shift registers and multiplier.

NEC V20 was used in "turbo" versions of some PC clones such as Commodore PC compatible systems, Copam, and Tandy 1000 laptop series. It was also used in the Casio PV-S450 PDA and Hewlett-Packard's HP 95LX. It was also used in the Bandai WonderSwan, a handheld gaming system released in Japan in 1999. Sony also produced this microprocessor under license from NEC as the V20H (Sony CXQ70108).

An unusual feature of the NEC V20 was that it added an Intel 8080 emulation mode, in which it could execute programs written for the Intel 8080 processors. The instructions BRKEM executed in 8086 mode (NEC used a different notation for the instructions than Intel and BRK in NEC notation = INT in Intel notation) and RETEM and CALLN executed in 8080 mode was used to switch or return to or from the emulation mode. There were some programs which allowed to run 8080-based CP/M-80 programs on a MS-DOS machine: V2080 CPMulator (later ZRUN) by Michael Day and 22nice from SYDEX.

Another unusual feature was the existence of several families of unique instructions. Instructions ADD4S, SUB4S, CMP4S were able to add, subtract and compare huge packed binary-coded decimal numbers stored in memory. Instructions ROL4 and ROR4 rotate nibbles. Another family consisted of instructions TEST1, SET1, CLR1, NOT1. These instructions test, set, clear and invert single bits but are far less efficient than the later I80386 equivalents BT, 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, namely REPC and REPNC, which amended the original REPE and REPNE and allowed a string of bytes of words to be scanned (with instructions SCAS and CMPS) while less or not less.

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