The TX-0 (Transistorized eXperimental computer 0) was built to aid in the testing of it’s big brother, the TX-2, and many of the features used in the TX-2 were first tested on the TX-0. This was one of the reasons why the TX-0 was originally equipped with 64Kword memory (each word being 18 bits). When work on the TX-0 began, it was not clear if such large magnetic core memories could be built.
The memory was stripped down to 4Kword memory when it was delivered to RLE but was later upgraded to 8Kword. The 18 bit words used 16 bits to address the original 64Kword memory leaving two bits for instructions. When it became apparent that it would never be equipped with its original 64Kword memory, the three unused address bits were used to add a set of new instructions.
