128 BEGINNING OF THE MINICOMPUTER

cluded 4 X 9-bit, 2 X 18-bit, 9/27-bit, and 36- bit arithmetic.*

By the time the PDP-1 was designed in 1959, most of the important ideas of logical organization, such as addressing, address modification, sequencing control, arithmetic, and I/O control, had been invented. However, the major advances in the hardware realizations of these concepts were yet to come. Machines were just entering the second (transistor) generation. A review of the state of the art in logical organization is given in [Beckman et al., 1961]. A review of the state of the hardware art in core memories is given in Rajchman [1961], and examples of the transistor circuitry used at the time are given in Chapter 4.

There is no record of the goals, constraints, and objectives of the PDP-l design. It is clear that the PDP-l instruction set processor was a reaction to the TX-0, but it is unclear whether an effort to make the PDP-l compatible to the TX-0 was ever considered. It seems unlikely because there was little software when TX-0 arrived at M.I.T. As it turned out, it is fortunate that no such effort was pursued because the TX-0 was continuously extended, making compatibility a difficult goal to achieve. Instead of being program compatible with the TX-0, the PDP-l was oriented toward being producible by a commercial enterprise and usable by a variety of programmers. To this end, it had more instructions than the TX-0 and a simpler I/O structure for ease in interfacing. In contrast to the existing large-scale scientific and business computers, the PDP-1 had a much shorter word length (18 bits) and a simpler instruction set (28 instructions). The I/O structure included a sequence break option (the name given to the sixteen channel interrupt mechanism) and a high speed channel (now called Direct Memory Access). The hardware implementation of the ma chine used DEC's 5 MHz 1000-series system modules and a 4-Kword memory which was later expanded to 64 Kwords. The processor and memory occupied four cabinets.

The registers and functional units of the PDP-l are shown in Figure 5, a diagram taken from the original PDP-1 programming manual. The PDP-l registers were named after those of

Figure 5. PDP-1 processor register transfer diagram.

_______________________
*TX..2 operated until 1977, when it was dismantled. In the last decade of its use, it was modified and operated as a multi- programmed timesharing system [Forgie, 1965]. The machine was used for a variety of applications. Two notable works included Sutherland's Sketchpad [1963], an interactive graphic design program, and the first computer network experiment between Lincoln Laboratory and the System Development Corporation computer [Marill and Roberts, 1966].