Defining and using Arrays

The traditional approach is the following:

create my-array 42 cells allot

This creates the dictionary entry named my-array and allocates 42 cells in RAM. BUT: the my-array dictionary entry is not connected to the allocated RAM. The correct solution is:

variable my-array  42 cells allot

This makes the dictionary entry named my-array, sets up the link to the RAM address and allocates an additional amount of 42 cells in RAM.

Forth 200x introduced a new word named Buffer:. With it the above code turns into

43 buffer: my-array

please note the different sizes! The buffer:-implementation allocates the exact number of bytes whereas the variable version adds the given size parameter to the 1 cell it allocates anyways.

The use of the array is quite simple:

: my-array-@ cells my-array + @ ;
: my-array-! cells my-array + ! ;

Arrays of structures

This example uses structures. Structures can be used after including of the structures.frt file. First a hash data structure consisting of two elements is defined. This structure is used to create an array of a few elements afterwards.

begin-structure hash
 field: hash.key
 field: hash.value

\ inspired by CELLS
\ ( n -- size )
\ calculates the size of n items of the
\ type hash
: hash-cells hash * ;

\ define a hash-array
: hash:
    hash-cells buffer:
    swap hash-cells  +

The helper word hash-cells calculates the size of the data structure in terms of byes, just like the standard word cells does it.

Now we’re using the words (using the amforth-shell). First define an array of 4 hash pairs. After that store a key/value pair at a particular position and retrieve it again later.

(ATmega16)> 4 hash: my-hash
(ATmega16)> 42 3 my-hash hash.key !
(ATmega16)> 4711 3 my-hash hash.value !
(ATmega16)> 3 my-hash hash.key @ .
  42  ok
(ATmega16)> 3 my-hash hash.value @ .
  4711  ok

If you place the data structure in a different memory (e.g. the EEPROM) adapt the code accordingly. buffer: needs to be replaced with a similiar allocation word and @/! with the proper memory access words. Remember, memory is not always 2 bytes per cell.

See also