THE PDP-1 AND OTHER 18-BIT COMPUTERS 161

parallel with the basic data path. In this way, significant overlap occurred, permitting the 800-nanosecond cycle time. The contrast between this design and the PDP-4 design is noteworthy. The PDP-4 had only four registers in the basic machine, but the use of integrated circuits in the PDP-15 permitted more registers to be used without so much concern for cost.

The first major extension of the PDP-15 was the addition of the Floating-Point Processor (Figure 32) to enable it to perform well in the scientific/computation marketplace using FORTRAN and other algorithmic languages. With the addition of the Floating-Point Processor, the time for a programmed floating-point operation was reduced from 100-200 microseconds to 10-15 microseconds, giving nearly a factor of 10 increase in FORTRAN performance - depending on the mix of floating-point

Figure 32. PDP-1 5 Floating-Point Processor register transfer diagram.

operations. For most machines, the difference between built-in and programmed data-types is higher; but, because the machine was originally designed to operate effectively without hard-wiring, the difference is quite low. Table 4 gives a summary of the performance improvements offered by the floating-point option.

The addition of the floating-point unit required that a number of instructions be added to the machine. The irony of this extension is that the PDP-l 1 and nearly all minicomputer instruction set extensions exactly follow this evolution.

A low cost multi-user protection system was added in the form of a relocation register and a boundary register. Because this was marketed as an add-on option, it degraded the machine performance more than necessary. However, the minimum machine cost maintained the performance/cost target.

The first PDP-15 was shipped in February 1970, 18 months after the project had started. A number of difficulties had been encountered, including personnel turnover, that caused a two- month slip. However, the project at first customer ship was within the budget and, by 1977,