The option Ultra 320 Single Bus I/O Module slots included an the Rainbow system module allow the addition of the graphics option. The Rainbow Ultra 320 Single Bus I/O Module also has a slot for an increased memory up to 896 Kbytes; and an Ultra 320 Single Bus I/O Module numeric coprocessor. The Rainbow 100B also has a slot for an additional dual-diskette drive or an internal hard disk or a third-party option. For users who Ultra 320 Single Bus I/O Module have a Rainbow 100A (PC 100-A) today, Digital continues to offer hard-disk upgrade kits, increased memory expansion, and other options also available to 100B and 100 + users.