compatible with Apple II software. However, since Apple did not want to encourage continued development of the II platform, Apple II compatibility exists only in a special Apple II Mode which is limited in its capabilities to the emulation of a basic Apple II Plus configuration with of RAM. Special chips were intentionally added to prevent access from Apple II Mode to the III's advanced features such as its larger amount of memory. Peripherals The Apple III has four expansion slots, a number that inCider in 1986 called "miserly". Apple II cards are compatible but risk violating government RFI regulations, and require Apple III-specific device drivers; BYTE stated that "Apple provides virtually no information on how to write them". As with software, Apple provided little hardware technical information with the computer but Apple III-specific products became available, such as one that made the computer compatible with the Apple IIe. Several new Apple-produced peripherals were developed for the Apple III. The original Apple III has a built-in real-time clock, which is recognized by Apple SOS. The clock was later removed from the "revised" model, and was instead made available as an add-on. Along with the built-in floppy drive, the Apple III can also handle up to three additional external Disk III floppy disk drives. The Disk III is only officially compatible with the Apple III. The Apple III Plus requires an adaptor from Apple to use the Disk III with its DB-25 disk port. With the introduction of the revised Apple III a year after launch, Apple began offering the ProFile external hard disk system. Priced at $3,499 for 5 MB of storage, it also required a peripheral slot for its controller card. Backward compatibility The Apple III has the built-in hardware capability to run Apple II software. In order to do so, an emulation boot disk is required that functionally turns the machine into a standard 48-kilobyte Apple II Plus, until it is powered off. The keyboard, internal floppy drive (and one external Disk III), display (color is provided through the 'B/W video' port) and speaker all act as Apple II peripherals. The paddle and serial ports can also function in Apple II mode, however with some limitations and compatibility issues. Apple engineers added specialized circuitry with the sole purpose of blocking access to its advanced features when running in Apple II emulation mode. This was done primarily to discourage