Macintosh 128K/512K technical details

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. However, Apple did not compromise on the features which now define personal computers. The resulting design implemented an aggressive feature set at a moderate price point by mixing complex LSI chips, readily customizable programmable array logic, and off-the-shelf components.

Overall architecture

The Motorola 68000, like most microprocessors of the 1980s, was designed for ease of use as well as speed. The 68000 chip was wired directly to the other programmable components of the computer: the IWM floppy controller, the Zilog 8530 SCC, and the MOS Technology 6522 which provided miscellaneous services. The connection used by the processor to transfer information is called the bus.

The bus also connected the 68000 to the 128 or 512 KiB of main memory (DRAM), but not directly. The connection to the RAM 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. This remains a popular way to trade performance for reduced cost. There was less trade-off in the Macintosh, however, because the DMA circuit simultaneously performed necessary maintenance on the RAM which would otherwise add overhead. Newer designs use a DRAM controller which inevitably reduces performance. The Macintosh graphics and sound engine doubles as a DRAM controller, 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 choose to give the Macintosh protected memory. Protected memory came to the Macintosh platform in 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 included a showcasing of speech synthesis software intended to impress the crowd.[1]

From a programmer's perspective, the Macintosh used memory-mapped I/O. The chips on the motherboard could be treated as memory locations in the computer's virtual space. Graphics and sound likewise appeared as regions inside the main memory. Given the simplicity of the hardware, most functions were also simple and efficient to program.

Components

This is a comprehensive list of the integrated circuits in the original Macintosh:

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 connected to the 8530.

The button connected to a general-purpose I/O input of the 6522. Pressing or releasing it would immediately stop the processor, and the operating system would note the new user input.

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 (one detector blocked when the other is clear) is required to detect the direction of rotation. Therefore, two emitter-detector pairs were used for X and Y each.

The motion detection signals connected to the 8530 chip using two non-essential pins used for obsolete modems. Originally these signaled modem connection or disconnection. When the mouse moved by a certain amount, the 8530 would interrupt the processor. The operating system would detect "modem activity" and act to move the cursor.

The direction signals connected to two more general-purpose I/O inputs of the 6522. When the operating system detected the mouse movement, these inputs differentiated left from right, and up from down.

Cursor and video

The mouse cursor was drawn on the screen by software, and moreover the Macintosh supported real-time animation. The screen timing PAL circuit sent a pulse to the 6522 once per refresh. This was the basis for an operating system service called the VBL (vertical blanking) Manager. When the screen was to be redrawn, the cursor would be moved and games had an opportunity to update the display.

It could sometimes be difficult to avoid a race condition between the game and the raster display. Flicker could result from the processor writing to the image while it was being sent to the CRT. Therefore, the Macintosh provided a choice of two images in memory, so one could be read while the other was written. The "page" was selected by a general-purpose I/O output connected from the 6522 to the video DMA. As the images together took 42.75 KiB of precious RAM, however, this feature was ultimately unpopular.

As described above, the DMA graphics controller operated independently. One-bit pixels were fetched over a 16-bit data bus and output at 16 MHz, necessitating one million fetches per second. Each fetch took two cycles out of eight per microsecond, implying a memory bandwidth tax of 25%. This was mitigated to 17% by disabling DMA while the CRT scanned right to left.

Keyboard

The 6522 provided general-purpose serial bus. The keyboard contained an Intel 8021 microprocessor which transmitted user input to the 6522 over standard phone patch cable. A new keystroke resulted in a processor interrupt.

Sound

The sampled sound engine piggybacked on the video circuit. As the raster scan returned from the right side of the screen to the left, one byte of data was placed into a PWM generator instead of the screen. This provided 8-bit sampled monaural sound sampled at the 22.25 kHz horizontal blanking rate. A general-purpose 6522 output could mute the sampled sound.

A square wave generator was included on the 6522. One of its two timer circuits could be set to toggle the mute output periodically. This could produce frequencies higher than 11 kHz.

The sound volume was controlled by an analog circuit providing 8 levels of attenuation, controlled by three general-purpose outputs. The user controlled this through software.

This system was not compatible with the Lisa / Mac XL hardware, which in other respects could run Mac software with commonly available software/firmware modifications. Running programs on Lisas which made use of the Mac sound features would cause severe video problems and system crashes.

Communication

The Zilog 8530 SCC was clocked at 3.7 MHz. At this speed each serial channel was nearly half as fast as the main memory. The RS-422 protocol was implemented except for the connection-established line, which was used to support the mouse. Apple later changed to an 8-pin connector which dropped it entirely.

Storage

The Macintosh's persistent storage medium was Sony's floppy diskette drive. This drive replaced the Apple ]['s Shugart drive, and later, the less reliable but more storage dense Apple 871K FileWare/"Twiggy" floppy drive used in the original Lisa, as the storage medium chosen to be implemented in the original Macintosh. The single-sided 3.5 inch floppy stored 400 KB by spinning the disk slower when the outer edge was used. A separate microcontroller, the IWM (Integrated Woz Machine), was dedicated to disk control. The floppy operated by polled I/O. Disk access was not seamless as in today's computers, and loading and saving files were operations that stopped the entire machine.

Twenty bytes of memory were included in the real-time clock counter chip. This data was retained using a 4.5 Volt alkaline battery.

Timekeeping

The Macintosh featured a real-time clock counting seconds, and a countdown timer with near-microsecond resolution. The former was connected to the 6522 by a serial bus on three general-purpose I/O lines. It functioned much as a quartz watch when the machine was powered off. The latter was built into the 6522 itself. Either could generate interrupts.

The countdown timer provided the hardware necessary for preemptive multitasking. The Macintosh operating system did not feature standard multitasking until 1987 with the release of the MultiFinder, unlike the Lisa, which used cooperative multitasking. Preemptive multitasking was only later adopted on the Apple Macintosh by Mac OS 9.x as an additional API, i.e. the application could be programmed to use the preemptive or cooperative model, and all legacy applications were multitasked cooperatively within a single process. Mac OS X, being a Unix-like system, uses preemptive multitasking for all native applications.

Note that the RAM map is organised so that the system globals, system and application heaps grow upwards from low memory, everything else grows downwards from MemTop, from high memory towards low memory. On the 512K Macintosh, the "extra" RAM thus appears as a wider gap between the application heap and the stack, where it is available for application use.

References

  1. ↑ Andy Hertzfeld "It Sure Is Great To Get Out Of That Bag!"

External links

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