robertm@dartvax.UUCP (Robert P. Munafo) (02/03/84)
If you've read the article in the February BYTE regarding the Macintosh,
you've seen some good reasons why Apple did not provide system bus expansion
slots on the Mac's digital board ('mother board'). Absence of expansion
slots allows Apple to use a small power supply (since the power requirements
of the peripherals need not be handled), a fixed I/O memory map, and a
smaller main system unit. All peripherals generally used with computers,
such as the Winchester disk and music synthesizer mentioned by Mike K. @
ihnss can be connected via the high-speed 'Apple-bus' connector on the back
of the Macintosh, with the exception of DMA devices - processor cards and
direct-to-hires frame grabber cards. The Macintosh is designed in such a way
that only the 68000 will ever be able to access the RAM. This makes imple-
menting the operating system routines in ROM much easier. (The BYTE
article refers to 'virtual slots' on the Apple Bus which are maintained by
the 68000. Any peripheral which operates by sending and receiving data to/
from the processor and/or RAM can be implemented for the Macintosh using
an Apple Bus compatible peripheral and a peripheral driver routine on disk.
The Apple Bus runs at 250,000 bits/sec. with imbedded clock bits and
1 million bits/sec. with an external clock. )
Robert P. Munafo ...|{decvax,linus}|dartvax|robertm