Forth (programming Language) - Facilities

Facilities

Forth parsing is simple, despite having no explicit grammar. The interpreter reads a line of input from the user input device, which is then parsed for a word using spaces as a delimiter; some systems recognise additional whitespace characters. When the interpreter finds a word, it tries to look the word up in the dictionary. If the word is found, the interpreter executes the code associated with the word, and then returns to parse the rest of the input stream. If the word isn't found, the word is assumed to be a number, and an attempt is made to convert it into a number and push it on the stack; if successful, the interpreter continues parsing the input stream. Otherwise, if both the lookup and number conversion fails, the interpreter prints the word followed by an error message indicating the word is not recognised, flushes the input stream, and waits for new user input.

The definition of a new word is started with the word : (colon) and ends with the word ; (semi-colon). For example

will compile the word X, and makes the name findable in the dictionary. When executed by typing 10 X at the console this will print 11 10.

Most Forth systems include a specialized assembler that produces executable words. The assembler is a special dialect of the compiler. Forth assemblers often use a reverse-polish syntax in which the parameters of an instruction precede the instruction. The usual design of a Forth assembler is to construct the instruction on the stack, then copy it into memory as the last step. Registers may be referenced by the name used by the manufacturer, numbered (0..n, as used in the actual operation code) or named for their purpose in the Forth system: e.g. "S" for the register used as a stack pointer.