How much memory can I add to a PCjr?
A PCjr has a minimum of 64KB of memory, which is soldered onto the motherboard. (Eight chips, each 64 kilobits – no parity protection in this setup.) To get to 128KB, you need the internal expansion card. To go beyond that, you need to start adding sidecars or other boards. Some of the sidecars were sold as 128KB units, which would bring memory up to 256KB. The IBM and Microsoft Booster sidecars could be modified to go up to 512KB, giving the PCjr up to 640KB of memory. The way the PCjr memory map is laid out, it is possible to get up to 736KB on a PCjr system. Of course you need that much memory installed and some software patches to make DOS recognize the extra memory. Remember that on a PCjr, the graphics subsystem “steals” memory from the system to use for the video display. Therefore, you are usually short 16KB or 32MB on a PCjr. Device drivers to use extra memory usually reserve the unused memory in the first 128KB for performance reasons; that means on a 640 KB system you have a
