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Turning Cousins into Sisters:

An Example of Software Smoothing of Hardware Differences



In 1970, the PDP-11 was Digital Equipment Corporation's newly announced minicomputer and its first offering in the 16-bit world. Among the many software components needed to complement the hardware, a FORTRAN system was high on the list. A FORTRAN project was begun in 1970 and the first release of the resulting product took place in mid-1971. In the succeeding years, the number of PDP- 11 CPUs and related options increased dramatically to provide a wide range of price/performance alternatives. What makes the original FORTRAN interesting, even today, is the extent to which the basic implementation approach was able to be extended gracefully to span the entire family with modest incremental effort.

This paper describes the design concepts, threaded code and a FORTRAN virtual machine, used to implement the original PDP-11 FORTRAN product. As the PDP-11 family of processors expanded with new models and options, these original design concepts proved both stable enough and flexible enough to be employed successfully across the entire family.

When this FORTRAN was finally superseded in early 1975, it had two successors. One, called FORTRAN IV, continued the threaded code and virtual machine concepts of the earlier product with similar execution performance across the PDP-11 family, but offered much faster compilation rates in smaller memory. The other successor, called FORTRAN IV-PLUS, produced direct PDP-11 code and obtained significantly improved execution performance for the PDP-l1/45, PDP-1l/70, and PDP-l1/60 with FP11 floating-point hardware relative to both of the other FORTRANs.

In the Beginning

The PDP-l 1/20 was a significant advance over other minicomputers of its time, but was a bare machine architecture by today's standards. There was no floating-point hardware of any kind (even as an option) and integer multiply and divide operations were available only by means of an I/O bus option, the Extended Arithmetic Element (EAE). (The EAE also provided multiple-bit arithmetic shift operations;


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