Remember When
The
first Apple II, or Apple ][ computers went on sale on June 5, 1977 with
a MOS Technology 6502 microprocessor running at 1 MHz, 4 kB of RAM, an
audio cassette interface for loading programs and storing data, and the
Integer BASIC programming language built into the ROMs. The video
controller displayed 24 lines by 40 columns of monochrome,
upper-case-only (the original character set matches ASCII characters
0x20 to 0x5F) text on the screen, with NTSC composite video output
suitable for display on a TV monitor, or on a regular TV set by way of a
separate RF modulator. The original retail price of the computer was
US$1298 (with 4 kB of RAM) and US$2638 (with the maximum 48 kB of RAM).
To reflect the computer's color graphics capability, the Apple logo on
the casing was represented using rainbow stripes, which remained a part
of Apple's corporate logo until early 1998. The earliest Apple II's were
assembled in Silicon Valley, and later in Texas; printed circuit boards
were manufactured in Ireland and Singapore.
An external 5¼-inch floppy disk drive, the Disk II, attached via a
controller card that plugged into one of the computer's expansion slots
(usually slot 6), was used for data storage and retrieval to replace
cassettes. The Disk II interface, created by Steve Wozniak, was regarded
as an engineering masterpiece at the time for its economy of electronic
components. While other controllers had dozens of chips for
synchronizing data I/O with disk rotation, seeking the head to the
appropriate track, and encoding the data into magnetic pulses, Wozniak's
controller card had few chips; instead, the Apple DOS used software to
perform these functions. The Group Code Recording used by the controller
was simpler and easier to implement in software than the more common MFM.
In the end, the low chip count of the controller contributed to making
Apple's Disk II the first affordable floppy drive system for personal
computers. As a side effect, Wozniak's scheme made it easy for
proprietary software developers to copy-protect the media on which their
software shipped by changing the low-level sector format or stepping the
drive's head between the tracks; inevitably, other companies eventually
sold software to foil this protection. Another Wozniak optimization
allowed him to omit Shugart's Track-0 sensor. When the Operating System
wants to go to track 0, the controller simply moves forty times toward
the next-lower-numbered track, relying on the mechanical stop to prevent
it going any further down than track 0. This process, called
"recalibration", made a loud buzzing (rapid mechanical chattering) sound
that often frightened Apple novices.
The approach taken in the Disk II controller was typical of Wozniak's
design sensibility. The Apple II used several engineering shortcuts to
save hardware and reduce costs. For example, taking advantage of the way
that 6502 instructions only access memory every other clock cycle, the
video generation circuitry's memory access on the otherwise unused
cycles avoided memory contention issues and also eliminated the need for
a separate refresh circuit for the DRAM chips. Rather than using a
complex analog-to-digital circuit to read the outputs of the game
controller, Wozniak used a simple timer circuit whose period was
proportional to the resistance of the game controller, and used a
software loop to measure the timer.
The text and graphics screens had a somewhat outdated arrangement (the
scan lines were not stored in sequential areas of memory) which was
reputedly due to Wozniak's realization that doing it that way would save
a chip; it was less expensive to have software calculate or look up the
address of the required scan line than to include the extra hardware.
Similarly, in the high-resolution graphics mode, color was determined by
pixel position and could thus be implemented in software, saving Wozniak
the chips needed to convert bit patterns to colors. This also allowed
for sub-pixel font rendering since orange and blue pixels appeared half
a pixel-width further to the right on the screen than green and purple
pixels.
Color on the Apple II series took advantage of a quirk of the NTSC
television signal standard, which made color display relatively easy and
inexpensive to implement. The original NTSC television signal
specification was black-and-white. Color was tacked on later by adding a
3.58 MHz subcarrier signal that was partially ignored by B&W TV sets.
Color is encoded based on the phase of this signal in relation to a
reference color burst signal. The result is that the position, size, and
intensity of a series of pulses define color information. These pulses
can translate into pixels on the computer screen.
The Apple II display provided two pixels per subcarrier cycle. When the
color burst reference signal was turned on and the computer attached to
a color display, it could display green by showing one alternating
pattern of pixels, magenta with an opposite pattern of alternating
pixels, and white by placing two pixels next to each other. Later, blue
and orange became available by tweaking the offset of the pixels by half
a pixel-width in relation to the color burst signal. The high-resolution
enhanced display offered more colors simply by compressing more,
narrower pixels into each subcarrier cycle. The coarse, low-resolution
graphics display mode worked differently, as it could output a short
burst of high-frequency signal per pixel to offer more color options.
The epitome of the Apple II design philosophy was the Apple II sound
circuitry. Rather than having a dedicated sound-synthesis chip, the
Apple II had a toggle circuit that could only emit a click through a
built-in speaker or a line out jack; all other sounds (including two,
three and, eventually, four-voice music and playback of audio samples
and speech synthesis) were generated entirely by software that clicked
the speaker at just the right times. Not for nearly a decade would an
Apple II be released with a dedicated sound chip. Similar techniques
were used for cassette storage: the cassette output worked the same as
the speaker, and the input was a simple zero-crossing detector that
served as a relatively crude (1-bit) audio digitizer. Routines in the
ROM were used to encode and decode data in frequency-shift keying for
the cassette.
Wozniak's open design and the Apple II's multiple expansion slots
permitted a wide variety of third-party devices to expand the
capabilities of the machine. Apple II peripheral cards such as Serial
controllers, improved display controllers, memory boards, hard disks,
and networking components were available for this system in its day.
There were plug-in expansion cards such as the Z80-card that permitted
the Apple to use the Z80 processor and run a multitude of programs
developed under the CP/M operating system, including the dBase II
database and the WordStar word processor. There was also a third-party
6809 card that would allow OS-9 Level One to be run. The Mockingboard
sound card greatly improved the audio capabilities of the Apple, with
simple music synthesis and text-to-speech functions. Eventually, Apple
II accelerator cards were created to double or quadruple the computer's
speed.
Please visit
www.oldcomputers.net and take a step back in time to see how
computers used to be.