Editing IBM PCjr (section)

=== Compatibility ===
The register mapping of the PCjr's video hardware is different from the IBM CGA card, so software that tries to modify or read registers directly will not work. 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. Alteration of other CRTC registers cannot be assumed to produce the same results from the PCjr video system as from the CGA.

Programs for the CGA that manipulate the CRTC start address, and that rely on address wrap-around above address 0xBC000, may not work correctly on the PCjr because it always has a 32&nbsp;KB contiguous block of RAM in the video area from address 0xB8000 through 0xBFFFF. The PCjr's video memory cannot be moved above 128k if expansion memory is added, so some PC software that ran off of self-booting disks would not work on a PCjr if the software required more than 128k of RAM.<ref name="norton19840626">{{cite news|author=Norton, Peter|date=June 26, 1984|title=Adding to Junior's Memory|pages=98|work=PC Magazine|url=https://books.google.com/books?id=IGkuDrOYjOcC&pg=PA98|access-date=October 24, 2013}}</ref>{{r|norton19840320}}

The floppy controller on the PCjr also had its I/O registers mapped into different ports than on the PC, and since the PCjr did not have [[Direct memory access|DMA]], the BIOS routines for handling floppy access were different and more complex than those on the PC. Software that tried to perform direct, low-level disk access (mainly utilities, but also the occasional game such as ''Dunzhin: Warrior of Ras'') would not work unless it was rewritten for the PCjr.