The electronic systems in modern cars and trucks -- under new scrutiny as regulators continue to raise concerns about Toyota vehicles -- are packed with up to 100 million lines of computer code, more than in some jet fighters.

"It would be easy to say the modern car is a computer on wheels, but it's more like 30 or more computers on wheels," said Bruce Emaus, the chairman of SAE International's embedded software standards committee.

Even basic vehicles have at least 30 of these microprocessor-controlled devices, known as electronic control units, and some luxury cars have as many as 100.

These electronic brains control dozens of functions, including brake and cruise control and entertainment systems. Software in each unit is also made to work with others. So, for example, when a driver pushes a button on a key remote to unlock the doors, a module in the trunk might rouse separate computers to unlock all four doors.

The evolution of automotive control electronics has been rapid. IEEE Spectrum, an American technical publication, reported that electronics, as a percentage of vehicle costs, climbed to 15 percent in 2005 from 5 percent in the late 1970s -- and would be higher today.

According to Bob Hrtanek, a spokesman for the auto supplier Delphi Powertrain Systems, the first Delphi units were introduced around 1980 to improve emissions systems.

Throttle-by-wire technology, also known as electronic throttle control, replaced cables or mechanical connections. In modern systems, when the driver pushes on the accelerator, a sensor in the pedal sends a signal to a control unit, which analyzes several factors (including engine and vehicle speed) and relays a command to the throttle body. Among other things, throttle by wire makes it easier for carmakers to add advanced cruise and traction control features.

These systems are engineered to protect against the kind of false signals or electronic interference that could cause sudden acceleration.

Emaus says that cars are engineered with "defensive programming" to counter erroneous signals.

"There is a tremendous engineering effort, and testing and validation, to guard against problems," he said. "But given the complexity of the car, can they test against every eventuality? Probably not."

Perhaps one in 100 new microprocessor designs had "an issue," Emaus said, and might need reprogramming or replacing, usually before it reached customers.

And he identified the metal-to-metal connections between electronic control units and wiring harnesses as a potential weak point.