The original Macintosh was a relatively simple machine, now of interest for its simplicity and for the fact that it was the first computer produced by Apple under the name Macintosh. The Macintosh used standard off-the-shelf components to the greatest extent possible, achieving a moderate price point by mixing complex LSI chips, readily customizable programmable array logic, and off-the-shelf components.
Overall architecture
The Macintosh used the Motorola 68000. The 68000's bus was wired directly to the other programmable components of the computer: the IWM floppy controller, the Zilog 8530 SCC, and the MOS Technology 6522.
The bus also connected the 68000 to the 128 or 512 KiB of main memory (DRAM), which was shared between the processor and the multimedia circuits in a direct memory access (DMA) arrangement. Either the processor or the video/sound engine could access the memory, but not both, resulting in up to a 10% loss in performance; the DMA circuit also performed necessary maintenance on the RAM which would otherwise add overhead, a trick previously used in the Apple II.
Precise timing information was relayed to the 68000 by interrupts. The 68000 provides three interrupt inputs, which in the Macintosh 128K/512K were connected to the 6522, the 8530, and a human input designed for programmers, in order of increasing priority. Thus typing on the keyboard (attached to the 6522) did not reduce serial data (8530) performance, yet the program controlling the serial bus could be debugged by the programmer.
To further reduce the cost of manufacture, as compared with its predecessor the Lisa, Apple did not include an MMU. As a result, the Macintosh did not support protected memory, and this feature remained absent from the OS until 2001 with the Mac OS X operating system.
According to Andy Hertzfeld the Macintosh used for the introduction demo on January 24, 1984, was a prototype with 512k RAM, even though the first model offered for sale implemented just 128k of non-expandable memory. This prototype was used to provide adequate RAM to run the memory-intensive demo, which showcased speech synthesis software intended to impress the crowd.
Components
This is a comprehensive list of the integrated circuits in the original Macintosh: a Motorola MC68000 microprocessor at clock speed 7.8336 MHz 64 or 128 KB of ROM in two chips containing parts of the operating system 128 KB of RAM in 16 chips eight TTL chips implementing a video and sound DMA controller, plus two TTL chips providing a 16-bit video buffer (74166 type) one PAL chip generating video timing signals (LAG) two TTL chips providing an 8-bit Pulse-width modulation sound driver (74LS161 type) two analog chips providing sound amplification (MC14016 switch, LF353 op-amp) a Zilog 8530 chip controlling two RS-422 buses through two driver chips an Integrated Woz Machine 400 KB floppy-disk controller plus support PAL (ASG) a 6522 VIA bridge chip connecting to the keyboard and clock an Apple real-time clock chip plus a 32.768 kHz quartz oscillator an Intel 8021 microcontroller in the keyboard bus control and extra logic including two PAL chips to activate the other chips (BMU0/1) two PAL chips to convert the 16 MHz clock to other timing signals (TSM, TSG) two TTL chips buffering the RAM to the 68000 (74LS244 type) some inverters (74LS04 type)
This personal computer was implemented in four special-purpose LSI chips, six MSI PALs, 19 chips of standard SSI/MSI logic and analog circuits, plus memory. Most of the simpler chips would be consolidated into a few custom chips in the next generation, much reducing cost.
Features
The above components implemented the Macintosh GUI and networking as described below.
Mouse
The centerpiece of the new interface was mouse-driven control. The mouse contained only electromechanical components: a button, and two optical encoders. The button was connected to the 6522. The encoders' four outputs were connected two to the 8530 and two to the 6522.
The optical encoders detected movement by quadrature. Each encoder had a wheel with radial stripes which would interrupt the light passing between an LED and a light-detecting photodiode, producing electrical pulses with mouse movement. Both the X and the Y encoders were turned by frictional contact with the mouse ball. Two pairs of emitters and detectors were used on each encoder. A first set of pulses is enough to detect the rate of rotation without indicating the direction of rotation, and a second set of synchronized but 90° out of phase pulses provides the direction of rotation. Therefore, two emitter-detector pairs were used for X and Y each.