📝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

Backlinks

• 📝 Forth