Inside the Apple III

The Hardware

The Apple III was based on the 6502 microprocessor, the same one used by the Apple II and Apple II Plus computers (the Apple IIe and the Apple IIc used the 65C02 processor). This microprocessor was also used by some other microcomputers at that time, such as the Commodore 64, the Commodore VIC-20, the Atari 400, and the Atari 800, just to name the most famous ones. The 6502 was a stripped-down version of the Motorola 6800 processor. The 6502 was originally designed by MOS Technology, but it was also licensed and manufactured by other companies. In the case of the Apple III, the 6502 vendor was Synertek.

The Apple II used the “original” 6502 running at 1 MHz; however, on the Apple III the processor worked at 2 MHz, so from a pure processing standpoint, the Apple III was twice as fast as the Apple II. The original Apple III used the 6502A processor, which was rated at 2 MHz. The revised Apple III and the Apple III Plus used the 6502B processor, which could handle up to 3 M
Hz, but its clock rate was fixed at 2 MHz, so using a “faster” CPU in this case was nonsense. With microprocessors from the 1970s and early 1980s, a letter after the “name” of the processor indicated the maximum supported clock rate (that doesn’t mean that the CPU would work at that rate).

The 6502 could only access up to 64 kB of memory (RAM and ROM), but through a technique called bank switching, the Apple III could access up to 512 kB of RAM, although it came with 128 kB (original version) or 256 kB (revised version and the Apple III Plus). The memory chips were located on a daughterboard installed on the motherboard. On the 256 kB version, 32 chips of 64 Kbits were used (64 Kbits x 32 = 256 kB).

Figure 6: The Apple III motherboard (revised version)

Figure 7: The memory board

Figure 8: The Apple III motherboard with the memory board removed