HP-39GII Calculator

Some hardware details:
SOC: SigmaTel / Freesclae STMP3770 (includes the ARM ARM926EJ-S processor)
SigmaTel Product Datasheet
Freescale STMP family comparison table
1Gbit Hynix NAND Flash Memory: H27U1G8F2BTR
Power Supply: The four 1.5V battery cells are connected in parallel.
So this calculator operates at 1.5Volt DC, meaning that it could run with only one battery inserted.

USB cable to connect to a PC:

Micro USB to interconnect calculators: