You can create a global variable
variable v ( -- addr )
v pushes the address of a cell in memory on the stack. This cell
was reserved by
variable. You can use
! (store) to store
values into this cell and
@ (fetch) to load the value from the
stack into memory:
v . 5 v ! .s v @ .
You can see a raw dump of memory with
v 1 cells .s dump
Cells ( n1 -- n2 ) gives you the number of bytes (or, more
generally, address units (aus)) that
n1 cells occupy. You can
also reserve more memory:
create v2 20 cells allot v2 20 cells dump
creates a variable-like word
v2 and reserves 20 uninitialized
cells; the address pushed by
v2 points to the start of these 20
cells (see CREATE). You can use address arithmetic to access
3 v2 5 cells + ! v2 20 cells dump
You can reserve and initialize memory with
create v3 5 , 4 , 3 , 2 , 1 , v3 @ . v3 cell+ @ . v3 2 cells + @ . v3 5 cells dump
Assignment: Write a definition
vsum ( addr u -- n )that computes the sum of
ucells, with the first of these cells at
addr, the next one at
The difference between
create is that
variable allots a cell, and that you cannot allot additional
memory to a variable in standard Forth.
You can also reserve memory without creating a new word:
here 10 cells allot . here .
here pushes the start address of the memory area, the
here the address after the dictionary area. You should
store the start address somewhere, or you will have a hard time
finding the memory area again.
Allot manages dictionary memory. The dictionary memory contains
the system’s data structures for words etc. on Gforth and most other
Forth systems. It is managed like a stack: You can free the memory that
you have just
-10 cells allot here .
Note that you cannot do this if you have created a new word in the
meantime (because then your
alloted memory is no longer on the
top of the dictionary “stack”).
Alternatively, you can use
free which allow
freeing memory in any order:
10 cells allocate throw .s 20 cells allocate throw .s swap free throw free throw
throws deal with errors (e.g., out of memory).
And there is also a
garbage collector, which eliminates the need to