The first Apple II computers went on sale on June 10, 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 40 columns by 24 lines 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 character cell 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 IIs 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 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 helped make Apple's Disk II the first affordable floppy drive 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 40 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 use 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 complex arrangement (the scanlines 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 scanline 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 colorburst signal. The high-resolution 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 (though with six expansion slots, users could add sound functionality via a soundcard like the Mockingboard). 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. Since the Apple II mainboard had no interrupts, it was impossible to use the speaker without taking CPU time and so most games had little sound.
Wozniak's open design and the Apple II's multiple expansion slots permitted a wide variety of third-party devices, including Apple II peripheral cards such as Serial controllers, display controllers, memory boards, hard disks, and networking components. 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 audio capabilities, allowing simple music synthesis and text-to-speech functions. Eventually, Apple II accelerator cards were created to double or quadruple the computer's speed.
The original Apple II was discontinued at the start of 1981, having been superseded by the II+. An estimated 40,000 machines were sold for its 4-year production run.
... See also Apple II graphics The Apple II series features a 16-color composite video YPbPr palette, that comprises 15 different colors plus a duplicate gray ... The original Apple II has two graphic modes, along with a 40 columns text mode and some mixed graphic+text modes ... When an RF modulator is employed to plug the Apple II into an NTSC TV set, many colors vary, and, dramatically, the brown and light blue, which when converted from YCbCr to YIQ ...
... See also Apple II graphics modes The Apple II featured not only the graphics modes of its precursors, but several new modes similar to ones found on the Atari ST and Commodore Amiga ...
... Rod Holt is credited (for example in the Walter Isaacson biography of Jobs) with the solution ... Holt employed a Switched-mode power supply design ...
... The Apple II graphics were composed of idiosyncratic modes and settings that could be exploited ... This graphics system debuted on the original Apple II, continued with the Apple II Plus and was carried forward and expanded with the Apple IIe, Enhanced IIe, IIc, IIc Plus ...
... Platform License Link Agat Emulator 1.21 02011-05-05May 5, 2011 Apple II, Agat 7, Agat 9 Windows GPL AppleIIGo 1.0.8 02011-06-14June 14, 2011 Apple IIe Cross-platform GPL AppleWin 1.2 ...
Famous quotes containing the word apple:
“An apple cleft in two is not more twin
Than these two creatures.”
—William Shakespeare (15641616)