FOCAL (programming language)


FOCAL is an interactive interpreted programming language based on JOSS and mostly used on Digital Equipment Corporation 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 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.
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.

History

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 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.

FOCAL

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.
The first version of FOCAL was released in 1968 for the PDP-8. An updated version followed the next year, which was later known as FOCAL-69. The system as a whole, regardless of version, was known as FOCAL-8 when it was ported to other machines in the 12-bit series, including the PDP-5 and PDP-12. It was popular as it was highly efficient in memory use, which was often severely limited on these machines to a few kilobytes. The similar FOCAL-11 ran on the PDP-11 under RT-11.
Processor Technology also offered a version of 8k FOCAL, complete with Lunar Lander, for the Intel 8080-based Altair 8800 system. This was provided for the copying charge of the tape, while the source code was provided for free with other purchases.

BASIC

While FOCAL was becoming popular on DEC machines, BASIC was becoming a more popular alternative on other platforms. By the late 1960s, several companies were making inroads in DEC's minicomputer stronghold, selling similar machines running time-sharing versions of BASIC. Notable among these was the HP 2100 series, running HP Time-Shared BASIC.
David H. Ahl had recently joined DEC's PDP-8 Group, just as the company became interested in selling the machine into educational settings. Due to the popularity of BASIC in the education market, especially with the growing library of BASIC programs from the Minnesota Educational Computing Consortium, Ahl found selling the machine with FOCAL was difficult despite its advantages. As he later noted:
Ahl took it upon himself to produce a BASIC system for the platform, hiring a company he later learned was a single programmer in Brooklyn to produce a version for the 4 kWord PDP-8. DEC began selling packaged versions of the PDP-8 with terminals and the BASIC as the "EduSystem" lineup, with larger systems in the series having expanded versions of BASIC, and in some cases, also FOCAL and FORTRAN. Some PDP-11 based EduSystems were also created.
Ahl then began porting programs from FOCAL to BASIC, notably The Sumer Game, a version of Lunar Lander, and many smaller FOCAL demos. Combining his ports with submissions from outside programmers, he managed to collect enough material to have DEC publish 101 BASIC Computer Games in 1973. The book was an immediate success and ultimately ran through three printings to 1975.
By the mid-1970s BASIC was a standard feature of all DEC machines and FOCAL use evaporated.

Rebirth in the Soviet Union

The PDP-11 had been cloned in the Soviet Union in the 1970s for military purposes. In the 1980s, single-chip variations similar to the LSI-11 were produced that gave rise to a series of PDP-11 compatible home computers. Most notable among several models were the Electronika BK series, released in 1985. These were initially supplied with FOCAL on a ROM cartridge, while a BASIC cartridge was an optional add-on. Later models supplied BASIC by default.

Language

The following description is based on FOCAL-69 as seen in the FOCAL-8 language reference manual.

Direct and indirect modes

FOCAL followed the JOSS model for interaction via a command line interface. This allowed users to type in commands in "direct mode", which were performed immediately, or to prefix them with a line number, in which case they were added to the program if they were unique, or overrode existing code if the same number had previously been used.
The method of operation is similar to BASIC's "immediate mode" vs. "program mode". It contrasts with JOSS, in which all commands, both program and direct, were saved and loaded as part of the user's workspace. This allowed JOSS to have commands without line numbers, which they used for definitions and other tasks. FOCAL lacked this capability, so those instructions that needed to be loaded and saved became options on other program-mode commands like.

Program statements

Every line in a FOCAL program must start with a line number. As with JOSS, line numbers are fixed-point numbers consisting of two two-digit integers separated by a period. In FOCAL-8, valid line numbers range from 1.01 through 31.99. When printed out, using, the FOCAL equivalent to BASIC's, leading zeros will be added; 1.10 will be printed as. This makes all lines numbers five characters long when output so that all line numbers align. Statements referring to those lines do not need the leading zeros, for example,.
The number on the left of the period is known as the "group number". Groups provide some level of code organization that is lacking in languages like Fortran or BASIC. The primary use of these was to use a group as a subroutine which can be called with, for example, will jump to the subroutine written in group 5. The editor also used these during edit-time, for example, one could to produce a listing of the code in group 2, or to delete all of the lines in group 4.
Every line must start with a command keyword following the line number. There is no concept of a "default command" as is the case in BASIC with its optional statement. Multiple statements can be placed on a single line, separated by semicolons. Usually, the behavior is no different than if the statements had been on separate lines, except in the case of FOR loops.

Commands

ASK

The command will take a list of strings and variables, echo the strings and store the user input in variables. Equivalent to BASIC's.
01.01 ASK "NAME", NAME
01.02 ASK "COORDINATES", X, Y
01.03 ASK "A1",A1,"OMEGA",W,"T0",T0,"DAMPING FACTOR",DAMPINGFACTOR
If the user doesn't enter a number but enters text, the system will convert the initial character to a number with "A"=1, "B"=2, etc.

COMMENT

The command creates a remark. Equivalent to BASIC's. The original manual lists as a synonym of COMMENT, used to mark empty lines, but this has no internal difference.
01.01 COMMENT: THE SUMER GAME, BY RICHARD MERRILL