IBM PC jr Color Display

The monitor included with the PCjr is a 12" TTL RGBI display like those supported by the CGA, but including an internal amplified speaker. Also like CGA, the PCjr supported composite video out for use with a TV or composite monitor.

A Motorola 6845 CRTC like the one used in the MDA and CGA adapters, and a custom IBM chip called the Video Gate Array (VGA) constitute the bulk of the PCjr video hardware. Several discrete standard logic devices complete the video subsystem. The 6845 is responsible for the basic raster timing and video data address sequencing, and the Video Gate Array contains all the additional timing logic, video data demultiplexing logic, color processing logic, and programmable palette table logic, as well as the logic for multiplexing RAM access between the 8088 CPU and the video generation circuitry. For programming, the CRTC is generally compatible with the CGA at the hardware register level. Some other CGA programming details, in particular the Mode Control Register and the Color Select Register (at I/O addresses 3D8h and 3D9h respectively), are not compatible, as the PCjr provides the equivalent functionality through different registers inside the Video Gate Array which are accessed in a completely different manner through a single I/O address (3DAh).

The 6845 CRTC and the VGA together are responsible for refreshing the internal DRAM of the PCjr, which complicates the process of switching video modes on the PCjr. Resetting the VGA, which must be done during certain video mode switches, must be done by code not running from the system RAM controlled by the VGA, and if the CRTC or the VGA is disabled for too long, the contents of the internal RAM can be lost. Additional external DRAM (in sidecar expansion modules) is refreshed independently and never affected by a video mode switch.