IBM PCjr was a home computer produced and marketed by IBM from March 1984 to May 1985, intended as a lower-cost variant of the IBM PC with hardware capabilities better suited for video games, in order to compete more directly with other home computers such as the Apple II and Commodore 64.
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The PCjr supported expansion via "sidecar" modules, which could be attached to the side of the unit.
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PCjr chassis is made entirely of plastic, unlike the all-steel chassis of the IBM PC.
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Front of the PCjr exposes a pair of cartridge slots in which the user can insert software on ROM cartridges, as was common with other home computers.
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IBM claimed that an average of two wait states are added, but the designers of the Tandy 1000, a clone of the PCjr, claimed that six was a more accurate figure.
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PCjr graphics were similar to CGA, with several new video modes:.
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The CGA did not provide a VBLANK interrupt, making it hard to detect when the screen was beginning to be drawn, but the PCjr provides this on IRQ 5, an important feature for smooth page-flipping.
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Unlike CGA, PCjr has palette registers which allow the colors in all modes to be chosen from the full 16-color RGBI palette.
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Monitor included with the PCjr is a TTL RGBI display like those supported by the CGA, but including an internal amplified speaker.
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Also like CGA, the PCjr supported composite video out for use with a TV or composite monitor.
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PCjr's sound is provided by a Texas Instruments SN76496 which can produce three square waves of varying amplitude and frequency along with a noise channel powered by a shift register.
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The PCjr design allows for an analog sound source in an expansion-bus "sidecar" module, and a software-controlled internal analog switch can select the source for the sound output from among the PC speaker, the SN76489, the cassette port, or the expansion-bus sound source.
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The PCjr keyboard has 62 keys rather than the 83 of the PC keyboard, and the remaining keys must be entered by holding a shift key.
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Internally the PCjr did have expansion slots to support specific upgrades: a RAM upgrade, a modem, and a floppy drive.
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The PCjr has a "gate" register to which software writes the number of the video register to be accessed, followed by the value to be written into it.
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PCjr was released in March 1984, missing the 1983 Christmas sales season due to production delays.
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PCjr's cost was its biggest disadvantage, even more so than the keyboard.
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IBM was surprised to learn that many of the initial customers for the PCjr were not home users as they assumed, but instead businesses who wanted a cheaper PC that took less space on a desk.
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The PCjr's price was close to that of the Coleco Adam, but the Adam included a tape drive, a printer, and software.
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Ultimately, the PCjr was perceived as not having a killer app to make up for these limitations.
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The PCjr lacks a DMA controller, so the 8088 CPU has to service floppy disk transfers directly, causing the system to momentarily freeze while accessing a disk.
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PCjr reportedly became the best-selling computer, outselling the Apple IIe and IIc by four to one in some stores and even the C64.
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The PCjr's enhanced graphics and sound standards became known as "Tandy-compatible", and many PC games advertised their Tandy support.
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The PCjr was able to run other software designed for the PC, such as word processor, database and spreadsheet programs ran well on the PCjr with 128K of memory.
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