Macintosh 128K/512K Technical Details - Overall Architecture

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 2002 with the Mac OS X operating system. According to Andy Hertzfeld, also, the Mac used for the introduction demo was a prototype with 512k RAM, even though the only model offered for sale initially had just 128k and could not be expanded. This was done in order to give the Macintosh the ability to run speech synthesis software to impress the crowd.

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.