FOCAL (programming language)

FOCAL (acronym for Formulating On-line Calculations in Algebraic Language, or FOrmula CALculator) is an interactive interpreted programming language based on JOSS and mostly used on Digital Equipment Corporation (DEC) PDP series machines.

JOSS was designed to be a simple language to allow programs to be easily written by non-programmers. FOCAL is very similar to JOSS in the commands it supports and the general syntax of the language. It differs in that many of JOSS' advanced features like ranges and user-defined functions were removed to simplify the parser and allow the system to run in less memory. Some of the reserved words (keywords) were renamed so that they all start with a unique first letter. This allows users to type in programs using one-character statements, further reducing memory needs. This was an important consideration on the PDP-8, which was often limited to a few kilobytes (KB).

Like JOSS, and later BASICs, FOCAL on the PDP-8 was a complete environment that included a line editor, an interpreter, and input/output routines. The package as a whole was named FOCAL-8, which also ran on the PDP-5 and PDP-12. When ported to the PDP-11, the resulting FOCAL-11 relied on the underlying operating system, RT-11, to provide file support and editing. The language definition was updated twice, to FOCAL-69 and a very slightly modified FOCAL-71. A port to the Intel 8080 was also available.

FOCAL is notable as the language in which the original versions of the early video games Hamurabi and Lunar Lander were written. Both were later ported to BASIC, where they became much better known. FOCAL was not popular outside the PDP platform and largely disappeared during the move to the VAX-11. It had a strong revival in the Soviet Union where PDP-11 clones were used as educational and home computers (BK series).

JOSS was released in May 1963 on the one-off JOHNNIAC computer at RAND Corporation. In RAND, use grew rapidly, and the machine, originally built in 1953, quickly ran out of capability. JOHNNIAC was decommissioned in 1966 and JOSS was reimplemented on a newly purchased PDP-6, Digital Equipment Corporation's (DEC) first "big" machine. Use continued to grow and by 1970, the system was being used by 500 to 600 users across the country and had spawned several innovations such as mobile computer terminals that could be wheeled from room to room and plugged in for quick access.

JOSS was highly influential. It emerged just as time-sharing was being introduced. There was significant interest in man-machine interaction and computers were seeing wider use. Whereas most time-sharing operating systems of the era concentrated on user account and file management, leaving the users to do their own programming, JOSS provided file editing and a programming language in one package. RAND showed the system to a parade of people in the industry.

The PDP-6 was DEC's first mainframe, and JOSS took full advantage of its power and memory capacity. The DEC programmers were familiar with the system and interested in JOSS, but most of the computers DEC sold had nowhere near the power needed to run it. Written by Richard Merrill, FOCAL removed features from JOSS as required in order to be able to run in the much more limited memory space of the PDP-8 and other 12-bit machines in the DEC lineup. To achieve this, a major change was made to reduce the amount of temporary data, or state, needed to parse the statements. One noticeable effect of this decision was that conditionals could only be used for branches, in contrast to JOSS, where conditionals can be applied to any statement.

The other noticeable change was to rearrange the keywords so each started with a unique letter. This simplified the parser, which needed to read only the first letter of the command on a line of code, and could then skip forward to the next whitespace character. It would then compare that against an internal list of possible keywords, which was one character per entry. In contrast, JOSS had to continue reading until it had read the entire command word and then compared that against a list containing complete words. Not only did this change save memory in the parser, users could also save memory by typing in only that letter, reducing the size of the source code.