by tezza » Fri Jun 11, 2010 10:27 am
Ok, I just answered my own question. You must indeed have the old ROM. From Wikipedia...
"The system operated PC-DOS 2.11 and Advanced BASIC. Like the IBM PC, if the system was left to boot without inserting a diskette into one of the drives the BASIC interpreter would be loaded, which was compatible with IBM PCjr BASIC, including Cartridge BASIC. PC-DOS 2.11 could only use half of the tracks of a 3.5" drive, however, since it didn't really understand what a 3.5" drive even was. The PCjx's BIOS pretended that it was a 5.25" drive.
The PCjx later had a BIOS upgrade chip, sold together with DOS 3.21, which permitted the full 720kB capacity of the diskette drives to be used. Some popular options for the PCjx were a 10MB external hard disk (as a stackable unit the same size as the JX itself) and a joystick. IBM never released any kind of 3270 emulation adapter for the PCjx in order to steer enterprise customers to more expensive PCs and PC XTs."