Electronic Control Units (ECUs) for automobiles are usually composed of a main single-chip microcomputer and peripheral circuits with some standard and/or custom ICs. The peripheral circuits vary with the kinds of control or models of automobiles.When the peripheral circuits are replaced with a single-chip microcomputer, the ECU becomes compact and low in cost. This is because the ECU is constructed with only two LSIs and can be used for various kinds of control and various models of automobiles only by changing the program of the microcomputer. The microcomputer, however, requires many I/O functions and high reliability.We have developed a new 4-bit microcomputer suitable for these requirements. The new microcomputer has two remarkable features. One is powerful I/O functions such as high speed I/O, serial I/O, parallel I/O, analog I/O, and default output that is generated in place of the calculated output by the main CPU when it fails. The other is highly reliable architecture avoiding program runaways to the utmost with the following characteristics: (1) The instruction set is simplified. (2) All instructions are 1-byte long including a parity bit and an operand field. (3) No operation is executed when a parity error occurs. (4) Branch instructions on which the address goes back are limited to only one. (5) A power-on and supply voltage-drop reset circuit is included. The new microcomputer has been fabricated on an LSI chip using a standard CMOS process technology with a 2-μm design rule. The chip contains about 47,000 transistors and measures 5.4× 4.6mm2.