This article is adapted from a series of articles and
sidebars in the February 1984 issue of Byte magazine. Although some of the
details included in this article are specific to the original Mac, many
also apply to other compact Macs, such as the Plus and SE/30,
The Macintosh was
introduced on January 24, 1984, so the staff of Byte was working with late prototypes or
early production units when they wrote these articles, which include a
long interview with the Macintosh development team.
The goal of the Macintosh project was to bring the concepts and
technology of the $10,000 Lisa to a much broader audience,
mostly by figuring out how to adapt it to a far less costly computer.
The original Macintosh was released at $2,500, a fraction of the Lisa's
original cost. Steve Jobs said this fulfilled the design goal to "build
a really cheap implementation of Lisa's technology...."
Bill Atkinson summed up that part of the design: "We want to most
computer that you can get for the least dollars so that the most people
can have it...." Maybe that's the origin of "The computer for the rest of us," although the
Mac has never really sold for "the least dollars."
Memory
The biggest compromise was memory - Lisa came with 1 MB and
could be expanded to 2, while the Macintosh had 128 KB of memory.
(Apple hoped to expand that to 512 KB by the end of 1984; they had to
wait for 256 kilobit memory chips. Once they became available, Apple
released the Macintosh
512K in September 1984.) The operating system and programs had to
be completely recreated for the low-RAM footprint of the Mac, since the
Lisa OS alone required more RAM than the Mac had.
Making life even more difficult, just over 21 KB of that precious
128 KB of memory was used for video, leaving under 107 KB available for
the operating system and applications. (That memory used for video is a
big part of the reason Apple didn't produce a 64 KB Macintosh - video
would have used one-third of its memory.)
Perhaps the first thing to go was multitasking; with so little RAM,
the Mac couldn't be expected to run multiple programs at the same time,
although Apple did provide for Desk Accessories, applets that could run
at the same time as full-fledged applications.
Another clever memory-saving, speed-boosting move was switching from
compiled Pascal to 68000 machine language for the operating system.
A third way of getting around the limitation of 128 KB of RAM (due
to high memory prices, from what I've heard, since even the original
Mac OS really cried out for 256 KB of RAM) was building 480
handcrafted, optimized toolbox routines into ROM, where the OS and all
Mac applications could access them. Not only did this reduce the need
for RAM, since these routines were run directly from ROM, but it also
helped enforce a standard user interface, since all programmers could
use these routines.
Speed & Video
Although we call this an 8 MHz computer, nit-pickers are quick
to point out that the 8 MHz processor "only" ran at 7.83 MHz. (Why they
get so bent out of shape over this 2% difference is beyond me.)
Compared with the 5 MHz Lisa, the CPU in the Macintosh was a speed
demon. And compared with the 4.77 MHz 8088 processor common in DOS
machines, the Mac's 68000 was both faster and more efficient, since it
accessed memory 16 bits at a time vs. the 8-bit architecture of the IBM
PC.
Working to squeeze the most out of every component, ROM and RAM ran
at full speed on the system bus, so there was no speed penalty using
the toolbox routines in ROM. However, because RAM was triple-ported
(that is, it was used by the CPU, for screen display, and for sound),
the 68000 processor had access to RAM only part of the time.
Video and sound are time-critical; they had to run at fixed
intervals. In simple terms, that means that when the Mac needed to draw
on the screen or make a sound, the CPU was temporarily shut out from
access to RAM.
According to "Macintosh System Architecture" by Burrell C. Smith,
this was handled in a very clever way where the CPU and video
alternated access to memory:
Each 512-pixel horizontal line consists of 32 words
[16 bits or 2 bytes] of data . . . followed by 12 words of
horizontal blanking. The last memory bus cycle of each horizontal line
is reserved for sound DMA . . . The update rate of the sound
channel is equal to the horizontal video rate, or 22,254.55 Hz.
Access to RAM is divided into synchronous time slots,
with the 68000 and video circuits sharing alternate word accesses
during the love portion of the horizontal video display line
. . . Although access to RAM is divided three ways, the
68000's share is maximized by giving it access to unused cycles during
horizontal and vertical blanking. This way, 68000 access to RAM
averages to a speed of about 6 MHz.
The Mac only displayed black and white pixels - no grays - and had a
relatively small screen of 512 x 342 pixels, which also helped the
display run quickly. (The video was also internally clocked to 60.15
Hz, making it possible to run the Mac on 50 Hz current without messing
up the display.)
Even the fact that the Mac uses square pixels, something we take for
granted today, greatly simplified the display routines and helped make
for faster screen displays.
Beyond 8 MHz
Because of the video architecture, moving to a 16 MHz CPU, as with
the SE/30 or Brainstorm upgrade for the Mac Plus, meant
that the CPU didn't share time 50:50 with the display, but instead used
three cycles for every one needed by video. Because of this, 16 MHz
compact Macs could run over twice as fast as 8 MHz ones.
This is also a reason that Apple never released a grayscale compact
Mac - 8-bit video would require eight times as much data going to the
display, which would mean 8 video accesses for every CPU access to
memory on an 8 MHz Mac (4:1 on a 16 MHz Mac), which would have slowed
the computer to a crawl. Color and grayscale video had to wait for
dedicated video cards (on the Mac
II and SE) and faster CPUs
with 32-bit data paths (the IIci).
Reliability and Simplicity
Mac prototype with Twiggy drive.
The Mac had a fraction as many chips as the Lisa and used only two
circuit boards, one for analog circuitry (power) and on for digital
(the motherboard). Decreasing the number of parts not only helped
reduce costs, but also meant greater reliability. Reliability was also
one of the reasons Apple moved from 5.25" floppies to Sony's new 3.5"
floppy. (As the photo to the right shows, Apple built some Mac
prototypes using the Twiggy drive from the Lisa - look at how wide the
floppy slot is.)
One reason Apple was able to squeeze 400 KB onto a 3.5" floppy was
Steve Wozniak's clever disk interface work done for the Apple II. By
creating a high-speed version of the Woz design, Apple had "twice the
margin of MFM" (the standard way of encoding data) and was able to
store 400 KB on a single-sided floppy that other vendors used at 270 KB
capacity.
Another clever hack was running the floppy at different speeds
depending on how close the track was to the outer edge, since there was
much more room to store data there. This also contributed to the higher
capacity - and was a key reason other computers would have a difficult
time reading Mac floppies.
The 3.5" Sony drive was customized for Apple to include 80 tracks
(earlier drives had only 70 tracks) and support rotation speeds of
390-600 rpm. This allowed the Mac to store data at a constant linear
density under control of the IWM (integrated Woz machine) chip. The
drive had 400 possible speeds, and the IWM chose the proper speed by
monitoring data read speed and attempting to maintain 489.6 Kbps.
If all that sounds complex, remember that the whole thing is
controlled by the IWM, while IBM's floppy controller needed 45-50 chips
to run a drive with a single rotation rate. Apple's elegant solution
was simpler and more reliable - and definitely a sign of a company that
thinks different.
To this day, Macs have a reputation for greater reliability than
other personal computers.
Hardware
Bucking conventional wisdom, Apple adopted a single-button mouse
because their usability tests found that new users were often unsure
which button to use on multi-button mice. Apple has never produced a
multi-button mouse.
The two serial ports were not the typical RS232C ports found on
other computers, but RS422A ports, which not only supported RS232
functionality, but allowed Apple to develop LocalTalk, an inexpensive,
easy, relatively fast (230.4 kbps) networking protocol that was
standard on all Macs until the iMac. With an external
clock, the serial port could support 1 Mbps, a feature which some
companies would later use to accelerate LocalTalk.
In addition to the mouse and serial ports, the only other data port
on the Mac was for a second floppy, although this would later be
adapted for serial hard drives as well.
These serial ports remained in use until Apple adopted USB with the
iMac in 1998.
The keyboard connected to the front of the Mac using a coiled cable
that looked like it came from a telephone handset, although it was not
interchangeable with telephone cables. The keyboard itself had no arrow
keys (you were expected to use the mouse), no numeric keypad, and no
function keys. It did have shift, option, and command keys, but no
escape or delete keys.
Apple added arrow keys to the keyboard with the Mac Plus, and
included function keys on the Extended Keyboard for the Mac II and
SE.
The Physical Design
An early design goal was to make the system compact and portable,
but early designs had a horizontal layout with the floppy drive next to
the screen - just like the Osborne, Compaq, and most other portables of
that era.
One day Steve Jobs "said that we didn't want portability to be the
primary aspect of this, but we did want it to take minimum desk space."
Putting the floppy below the display solved that and also put the
monitor at a better working height.
Not only did Apple begin the Macintosh with this kind of design, but
this also forms the foundation of the iMac's design.
The Operating System
One of Apple's clever hacks in adapting the Lisa OS to a one program
at a time environment was retaining the Lisa's clipboard, which let the
Mac user copy a piece of text or a graphic from one program, quit that
application, launch another, and paste the item into the second
application. We may expect that today, but with the Lisa and Mac, it
was an innovative feature.
Another key issue for both the Lisa and Mac operating systems was
full integration of graphics and mouse with the operating system. This
is in contrast to operating systems such as the consumer version of
Windows (a graphical shell running on top of DOS) and Mac OS X (Aqua
running on top of Unix). While it is possible to break out of the GUI
(Graphical User Interface) into a programmers mode, even that uses
toolbox routines to display text; you cannot use the Mac OS without
accessing graphics.
Desk accessories were Apple's solution to the occasional need to run
more than one application at a time. These applets were specifically
designed to run at the same time as other applications to display the
time, manage a scrapbook, etc.
The Mac OS responds to peripherals on an asynchronous basis, which
permits the computer to work with more than one peripheral at a time.
For example, this means the user can move the mouse while accessing a
floppy disk.
Peripherals
The Mac's two peripherals were the ImageWriter printer and an
external floppy drive. The $595 ImageWriter could easily (albeit
slowly) print in graphics mode to emulate what was on the screen. The
$495 external floppy was a practical necessity, since it meant you
could switch programs or save files on a work disk without ejecting the
system/application disk.
It was two years before Apple produced the HD20, a hard drive for
the Mac that connected via the slow floppy port. Shortly thereafter
they released the Mac Plus,
the first Mac with a SCSI port, which made it easy to add faster hard
drives. Apple still offers SCSI as a build-to-order option for the
Power Mac G4.
Trivia
McIntosh is the apple, but Macintosh is the Apple computer - and
also an overcoat in the UK. The original code name, McIntosh, was
misspelled, and Apple decided to stick with it.
MacWrite was developed under the name MacAuthor, and MacDraw was
going to be called Mackelangelo. Thank goodness they changed them.
The Macintosh was designed for an international market; the only
text on the rear panel was Apple. Everything else was marked with
icons.
All the early Macs (all models prior to 1987) were fanless, just
like current iMacs and the
Cube.
