Inside the Macintosh 128K

The Motherboard

As mentioned, the Macintosh 128K was based on the Motorola 68000, which is a 32-bit microprocessor using a 16-bit data bus, and a 24-bit address bus, allowing it to access up to 16 MB of memory. The 128 kB of RAM available was comprised of 16 chips with 64 kbits each. Other notable chips available were the 6522 “Versatile Interface Adapter,” in charge of mouse and keyboard communications; the Z8530 serial communications controller, in charge of the two serial ports; and the custom-made IWM (Integrated Woz Machine), in charge of controlling the floppy disk drive.

Figure 12: The motherboard of the Macintosh 128K

The Macintosh used six PAL (Programmable Array Logic) chips named LAG (Linear Address Generator), TSM (Timing State Machine), BMU0 and BMU1 (Bus Management Unit), TSG (Timing Signal Generator), and ASG (Analog Sound Generator).

The motherboard had a reset and an interrupt button (seen at the top right corner in Figure 12) targeted to programmers. These buttons were normally not accessible from outside the computer. However, as these buttons were located in front of the side ventilation slits of the computer, programmers could buy a special “programmer’s switch” that could be attached to this vent (located on the left-side of the computer) and therefore access these buttons.

More about the motherboard of the Macintosh 128K can be found here.