Types of Serial EEPROM Chips

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There are several types of Serial EEPROMs, but most of them fall into either a 2-wire or 3-wire interface category. Usually, the 3-wire devices require an addition wire (beyond the 3 for data transfer) for each chip to be used. The 2-wire interface, called I2C or IIC or "I squared C" uses only two wires, regardless of how many chips are attached. I2C is a trademard of Philips. The three wire interfaces include SPI and Microwire, which is a trademark of National Semiconductor.
As the pressure on engineers to make products smaller has grown, semiconductor manufacturers have introduced several new interfaces, usually aimed at lower a product's size and cost... and undoubtedly many more will appear in the future.
As a practical matter, the code offered below only works with the Microwire 3 Wire interface, and is specifically intended to work with the 93C46, which is a standard part available from a variety of distributors.

What is it, Why use a serial EEPROM?

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Serial EEPROMs are small Electronically Erasable Programmable Read Only Memory chips. These devices are usually used to store user configurable parameters and device serial numbers. They use a serial bus interface, which allows them to be packaged in inexpensive 8 pin packages.
To you, the 8051 system designer, these little chips offer a nifty way to store a small amount of data in non-volatile memory, using only a few of the port pins, and without raising the system's cost much. They are usually specified to retain the data for 10 years and to endure 100,000 write operations before failure. They only require a 5 volt power supply (some 3V only versions exist too).
Because these chips use a serial interface, they can not be read quickly enough to serve as conventional memory. In addition, a considerable length of time (milliseconds) is required to perform a write operation. They typically hold less than 1024 bytes of memory. However, they usually cost less than $1.00 (US) for single piece quantities, making them pretty desirable memory devices for storing configuration parameters or other bits of information that should be retained when the power is lost.