IBM PCjr
IBM PCJr with original "chiclet" keyboard, PCjr color display, and 64 KB memory expansion card
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| Manufacturer | Teledyne, Lewisburg, Tennessee |
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| Type | Personal computer |
| Release date | March 1984 |
| Introductory price | US$1,269 with 128 KB memory and without monitor |
| Discontinued | May 1985 |
| Units shipped | 500,000 |
| Operating system | IBM PC DOS 2.10 |
| CPU | Intel 8088 @ 4.77 MHz |
| Memory | 64 KB |
| Graphics | Video Gate Array |
| Sound | Texas Instruments SN76489 |
| Predecessor | IBM Personal Computer |
| Successor | IBM PS/1 |
The IBM PCjr (read "PC junior") was IBM's first attempt to enter the home computer market. The PCjr, IBM model number 4860, retained the IBM PC's 8088 CPU and BIOS interface for compatibility, but various design and implementation decisions led the PCjr to be a commercial failure.
Announced November 1, 1983, and first shipped in late January 1984, the PCjr—nicknamed "Peanut" before its debut—came in two models: the 4860-004, with 64 KB of memory, priced at US$669 (equivalent to $1,609 in 2016); and the 4860-067, with 128 KB of memory and a 360 KB 5.25-inch floppy disk drive, priced at US$1,269 (equivalent to $3,051 in 2016). It was manufactured for IBM in Lewisburg, Tennessee by Teledyne. The PCjr promised a high degree of compatibility with the IBM PC, which was already a popular business computer, in addition to offering built-in color graphics and 3 voice sound that was better than the standard PC-speaker sound and color graphics of the standard IBM PC and compatibles of the day. The PCjr is also the first PC compatible machine that supports page flipping for graphics operation. Since the PCjr uses system RAM to store video content and the location of this storage area can be changed, it could perform flicker-free animation and other effects that were either difficult or impossible to produce on contemporary PC clones.
The video is produced by on-board display hardware, which is capable of all seven BIOS-supported modes of the Color Graphics Adapter (CGA) plus additional 160×200 16-color, 320×200 16-color, and 640×200 4-color modes. The latter two modes and the 80×25 text mode, collectively called the "high-bandwidth" modes by IBM, require the optional 64 KB internal memory upgrade card, which doubles the amount of system RAM from the base 64 KB to 128 KB (specifically by providing the second 64 KB starting at address 0x10000). (Upgrading the memory by other means, such as adding a RAM "sidecar" adapter, is not adequate to support the high-bandwidth graphics modes.) Like the CGA, the PCjr has a composite pseudo-NTSC video output that supports using a color or black-and-white television set, and the PCjr's video adapter hardware is also capable of the common undocumented composite artifact colors mode of CGA, which is entered by enabling composite color (referred to as "color burst") in the 640×200 graphics mode. The PCjr is also capable of the semi-documented 160×100 16-color CGA graphics mode, although except for programming considerations, there is no advantage to using this mode instead of the 160×200 16-color PCjr graphics mode. The fully programmable 16-color palette logic in the PCjr allows any set of colors from among the 16-color RGBI color set—the same set of colors available in CGA text modes—in each mode. That is, in each graphics mode, each pixel "color" value can be independently mapped to any one of the 16 real RGBI colors. (The IBM Technical Reference is unclear as to whether the programmable palette is active in the text modes, where all 16 colors are available even without palette mapping.) When the BIOS is used to set a video mode, it always sets up the PCjr palette table (i.e. the 16 palette registers) to emulate the CGA color palette for that mode. Programs specifically written to use PCjr graphics can subsequently reprogram the palette table to use any colors desired. Palette changes must be made during horizontal or vertical blanking periods of a video frame in order to avoid corrupting the display. However, the provision of a vertical retrace interrupt (on IRQ5) simplifies this and also makes seamless page-flipping much easier. The video hardware of the PCjr is the first to offer a vertical retrace interrupt, or any raster interrupt. The PCjr video subsystem also has a little-known graphics blink feature, which toggles the palette between the first and second groups of eight palette registers at the same rate used for the text blink feature, and a palette bit-masking feature that could be used to switch between palette subsets without reprogramming palette registers, by forcing one or more bits of each pixel value to zero before the value is used to look up the color in the palette table.
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