Basic Operations

Term	Example	Explanation
Number	300	( -- n ) push value onto stack (separate floating point stack in gforth)
Arithmetic operation	+ - * / mod	( n1 n2 -- n ) pop appropriate number of operands; push result
SWAP	1 2 .s swap .s	( n1 n2 -- n2 n1 ) swap top two stack values
DROP		( n -- ) discard top value
2DROP		( n1 n2 -- ) discard top two values
DUP		( n -- n n ) duplicate top value
2DUP		( n1 n2 -- n1 n2 n1 n2 ) duplicate top 2 values
ROT		( n1 n2 n3 -- n2 n3 n1 ) rotate top 3 values
OVER		( n1 n2 -- n1 n2 n1 ) copy 2nd value to top
PICK		( wn .. w1 w0 n -- wn .. w1 w0 wn ) copy wn to top of stack
ROLL		( wn .. w1 w0 n -- wn-1 .. w1 w0 wn ) rotate wn to top of stack
More on arithmetic operations
MOD		( n1 n2 -- remainder ) remainder upon dividing n1 by n2
/		( n1 n2 -- quotient ) quotient upon dividing n1 by n2

Defining Words

Term	Example	Explanation
WORDS		display all the words in the dictionary
: word definition ;	: add-top-4 + + + ;	Forth programmers, like C programmers, would be averse to using such a descriptively long word
CONSTANT	2 constant coin-sides	define a constant using the top of stack value; it is an r-value; its word represents a value
VARIABLE	variable x	declare a variable; it is an l-value; its word represents an address
ALLOT	10 allot	( n -- ) allocate n address units
CELLS	10 cells allot	( n1 -- n2 ) number of address units in a cell
CHARS	create data 10 chars allot	( n1 -- n2 ) number of address units in n characters
, (comma)	create values 1 , 2 , 3 ,	( x -- ) reserve one cell and store x there
CREATE	create data 10 chars allot	( -- ) define a word, which is bound to an address
@	variable x x @	( a-addr -- n ) way to get value stored in variable
!	variable x 1 x ! (set x to 1)	( n a-addr -- ) way to store a value into a variable
HERE		( -- addr ) contents of data space pointer (an address)

Address Calculation

Term	Example	Explanation
char+		( c-addr1 -- c-addr2 ) address of next character
c@		( c-addr -- char ) get character at address
c!		( char c-addr -- ) put character at address
cell+		( a-addr1 -- a-addr2 ) address of next cell
c@		( c-addr -- char ) get character at address
@		( a-addr -- x ) get value at address
!		( x a-addr -- ) put value at address
sp@		( -- a-addr ) get value of stack pointer
sp!		( a-addr -- ) set value of stack pointer
ALIGNED	sp@ 1 aligned + add one to stack pointer	( addr -- a-addr ) align the address
rp@		( -- a-addr ) get value of return stack pointer
rp!		( a-addr -- ) set value of return stack pointer
>r		( w -- ) R:( -- w) transfer from data to return stack
r>		( w -- ) R:( -- w) transfer from return to data stack

Selection

Term	Example	Explanation
if	: order2 dup > if swap then ;	Note - if is a compile only word, so can only be used in a subroutine definition
relational operators, e.g., < , > , < , u<	0 1 = leaves false, i.e., 0 on top of stack	compare top two values and replace with 0 (false) or true; special cases, e.g., 0= and 0<
logical operators, i.e., 0= , and , or	0 0= leaves non-zero on top of stack	( x -- flag) operate on top or top two values and replace with 0 (false) or true

Iteration

Term	Example	Explanation
do loop	: drop-n-items ( w1 .. wn n -- ) 0 ?do drop loop ;	( limit start -- ) Uses two stack parameters - in this example the loop stop value is already on the stack
LEAVE	: drop-at-most-n-items ( w1 .. wn n -- ) 1 ?do dup 0= if leave then drop loop drop ;	( R: start limit -- ) remove loop control parameters from return stack and leave the innermost loop
while repeat	: drop-until-zero ( 0 w1 .. wn -- 0) begin dup 0 <> while drop repeat ;	( -- ) if code between begin -- while tests non-zero, execute code between while -- repeat
Subroutine actions
EXIT	: test-for-prime ( n -- flag ) dup 1 = if drop 0 exit then dup 2 = if drop 1 exit then dup 2 ?do \ ( n n ) check-done if unloop exit then other-stuff loop ;	( R: return-addr -- ) return from subroutine (be sure to unloop if inside a loop)
UNLOOP		See EXIT

Input/Output

Term	Example	Explanation
key	key	( -- n ) push ascii code of key press
emit	67 66 65 emit emit emit	( n -- ) display character with ascii code n
accept		( c-addr +n1 -- +n2 ) get at most n1 characters of input; n2 is actual number read; store at location c-addr;
type		( c-addr u -- ) display u characters beginning at address c-addr
. (dot)	.s 3 . .s	pop and display top of stack
.r (dot-r)	.s 11 5 .r	pop field width then pop and display top of stack ( n1 n2 -- )
.S	1 2 3 .s	display stack (without altering it)
CR	cr ." hello world"	display a carriage return
."	cr ." hello world"	display string (compilation only)
.( string)	cr .( hello world)	display string (immediate effect)
SPACE	cr space .( hello) space .( world)	display a space
SPACES	cr 10 spaces .( indented text)	pop value and display value spaces