📝Forth cheatsheet

tags

Forth

  • help
    • see <word> to show word definition
    • .S display stack without destroying it
  • I/O
    • . print number
    • emit show number as char
    • ." Hello" print string “Hello” (space required after .")
  • Functions
    • : <funcname> <body...> ; define function
  • Variables
    • variable <varname> create variable
    • <varname> pushes variable address onto stack
    • <varname> @ fetch from variable
    • <value> <varname> ! store to variable
    • 2variable <varname> create double-length variable
  • Constants
    • <value> CONSTANT <constname> create constant
    • <constname> pushes value of the constant to the stack
  • Values (CORE EXTENSION)
    • <value> VALUE <valname> creates value and initializes it
    • <valname> push value to the stack
    • <value> TO <valname> re-assign value
  • Locals (LOCALS EXTENSION)
    • local variables
    • locals act as values (not variables)
    • LOCALS| a b c ... |

    • example

      : v+    ( a b c d -- a+c b+d)
    LOCALS| d c b a |
    a c +  b d +  ;
  • Return stack (rstack)
    • >R store to rstack
    • R> retrieve from rstack
    • R@ copy from rstack
  • Memory
    • @=/!= fetch/store numbers
    • D@=/=D! fetch/store double numbers
    • C@=/=C! fetch/store chars (chars are always positive)
    • CREATE <name> makes a new dictionary entry
    • HERE return address of the next available space
    • 13 CELLS ALLOT allocate 13 cells (kind of array) (used as in create x 10 cells allot)
  • Control flow
    • : test 0 = invert IF cr ." Not zero!" THEN ; if-then
    • : truth cr 0 = IF ." false" ELSE ." true" THEN ; if-then-else
    • recurse call itself
    • BEGIN xxx ( -- flag) UNTIL loop until flag is true
    • BEGIN xxx ( -- flag) WHILE yyy REPEAT loop until flag is false (execute yyy only if flag is true for given iteration)
    • <to> <from> DO ... LOOP (i can be used to get iteration number)
    • <to> <from> DO ... <step> +LOOP
    • ?DO skip looping if <to> and <from> are equal
  • create
    • CREATE is a component of the compiler, whose function is to make a new dictionary entry with a given name (the next name in the input stream)
    • : VARIABLE CREATE 1 CELLS ALLOT ; definition of variable
    • or : VARIABLE CREATE 0 , ;
    • , put top of stack to the dictionary pointer and increment dictionary pointer
    • : CONSTANT CREATE , DOES> @ ; definition of constant (it is usually implemented in machine code for speed, though)
    • DOES>
    • : : CREATE ] DOES> doLIST ; definition of :
    • : ; next [ ; IMMEDIATE definition of ;
    • [ switch to interpret mode, ] switch to compile mode

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