126 BEGINNING OF THE MINICOMPUTER

Figure 3. Lincoln Laboratory TX-0 computer (courtesy of M.I.T. Lincoln Laboratory).

first 16 memory locations, permitting direct en try of variables. However, despite the multiple I/O devices, the TX-0 had no program interrupt mechanism.

The two program accessible registers were called the Accumulator and the Live Register. The Accumulator was used for logic functions and the Live Register was used for controlling and buffering transfers to various I/O equipment. The initial version of the TX-O had only four instructions encoded in two bits, leaving sixteen bits to access the large, 64-Kword memory. Three of the instructions accessed memory: "store in location," ''add from location,'' and "transfer if Accumulator is negative to location." The fourth instruction, "operate," was for program controlled I/O transfers and included commands that could be combined to produce a large number of instructions. The combining process was called "micro programming" because bits in the instruction specified particular register transfer operations and could be programmed. Among the instructions that could be created were "clear the right half of the Accumulator," "cycle the Accumulator right one position," and "start the paper tape reader." The operations encoded in the instruction could occur at any one of six possible times during the instruction; thus, a multi- function instruction could be formed, such as one to display a point on the screen and to generate a new pseudo-random point.

In 1958 the TX-0 was transferred (by Earl Pugh and John MacKenzie) from Lincoln Lab oratory to the M.I.T. campus for laboratory experiment control and for teaching. The memory size was reduced from 64 Kwords to 4 Kwords but used one of the first all-transistor driven core memories. A second memory stack was later added to provide 8 Kwords. In 1960 Professor Jack Dennis assumed the management of TX-0 and extended the architecture in an up-