Deferred Words

Deferred words a technique that allows to change the behaviour of a word at runtime. This is done by storing an execution token under a certain name that is executed whenever that name is called. The stack effect is entirely that of the stored execution token code. The basic specification is at which is a must-read now.

Amforth supports different locations to store the execution token. The AVR8 provides 3 different variants: Edefer stores in EEPROM, Rdefer stores in RAM and Udefer stores in the USER area. The MSP430 has only RAM (Rdefer) since flash is not changeable, except the info flash area.

Depending on the storage location, different initalization actions may be required at startup. Only the AVR8 EEPROM based defers work without further actions and every changes are kept likewise.

Assigning a new execution token uses the command IS for all defers, regardless of the actual location used.

AmForth uses the deferred words technique internally:

  • turnkey is an EEPROM (AVR8) or info flash (MSP430) based deferred word that is executed from QUIT during startup and reset.
  • the words key, key?, emit, and emit? are USER deferred words for low level terminal IO. (AVR8)
  • refill and source are USER deferred words used by the forth interpreter to get the next command line.
  • pause is a RAM based deferred word that is called whenever a task switch can be done. It is set to noop per default.
  • !i does the actual flash write of a single cell. It is intended for Unbreakable AmForth (AVR8)

Since there is no standard defer word, the developer has to choose where to store the execution tokens. An EEPROM location is keept over resets/restarts and is valid without further initialization. A USER based deferred word can be targeted to different words in a multitasking environment and finally a RAM based deferred word can be changed frequently.

How Defers work

Defers store an execution token. When the name of the defered word is called, they fetch this token and execute it. When the name is compiled into another definition, this fetch-execute happens when calling this other word. That way even a compiled deferred word can be changed later on since it’s only the defer definition that got compiled, not its content.

> Xdefer foo
> : bar foo ;
> ' words is foo
> bar
 <long list of words>
> ' noop is foo
> bar

Xdefer is one of the various defer defining word. Regardless of the actual type, all defers behave the same way.

The defer defining words are created with the same design:

: Rdefer ( "name" -- )
    here , 2 allot
    [: @i @ ;] , \ used to read
    [: @i ! ;] , \ used by IS

The first command (defer) creates the dictionary entry “name” and sets up the runtime behaviour (execution token). The next line allocates a memory region (RAM in the example) and compiles its address. The two quotations are called to access the data item. They are called with the address of the compiled address (thus the @i). That way two memory accesses are performed: first is to get the address from the dictionary entry the second to fetch/store from/to the address in the right memory pool.

Sealing Defers

It is sometimes necessairy to prevent a deferred word from changing. This can be achieved with the following word

: defer:seal ( XT -- )
 dup defer@ ( -- XT' XT )
 swap ( -- XT XT')
 dup ['] quit @i ( get DO_COLON) swap !i
 1+   dup rot swap !i
 1+ ['] exit swap !i

With it, the dictionary entry is patched directly to change it from beeing a defer to a colon word named as the deferred word calling only the current XT stored in it

(ATmega32)> Edefer mytest
(ATmega32)> ' ver is mytest
(ATmega32)> mytest
 amforth 5.3 ATmega32 ok
(ATmega32)> ' mytest 5 - 10 idump
 10E0 - FF06 796D 6574 7473 10CB 0836 005C 07D6   ..mytest..6.\...
 10E8 - 07E0 FFFF ...
(ATmega32)> ' mytest defer:seal
(ATmega32)> ' mytest 5 - 10 idump
 10E0 - FF06 796D 6574 7473 10CB 3800 078C 381A   ..mytest...8...8
 10E8 - 07E0 FFFF ...
(ATmega32)> mytest
 amforth 5.3 ATmega32 ok

Technically the word mytest is changed to the same dictionary content as if it was defined as

: mytest ver ;

This is possible since a deferred word occupies 3 flash cells in the body and the faked colon definition needs only 2: the XT of the deferred word and the exit call.