Re: defered word options

On Fri, 23 Nov 2007 11:48:14 -0800 (PST), foxchip
<fox@xxxxxxxxxxxxxxxxxxx> wrote:

Nested decsions trees may introduce the greatest overhead.
Traditional Forth code might look like outside the loop:

VARIABLE 'FUNCTION
' MYFUCTION 'FUNCTION !

and inside the loop

'FUNCTION @ PUSH ;

Portable ANSI Forth cannot rely on the existence of an
actual return stack and has added a layer of abstraction:

'FUNCTION @ EXECUTE

(some systems provide an @EXECUTE function similar to
the operation of the machineforth sequence

IF PUSH ; THEN DROP ;

DEFERed words are not part of the standard but are
common practice. Similar to the traditional style

'FUNCTION @EXECUTE

in their implementation they are often used with syntax

' MYFUCTION IS THEFUNCTION

I have argued for many years that the most efficient
implementation of defered words is simply a jump opcode.
It is of course an example of using self-modifying code
to implement DEFER.

DEFER FOO

: t ... FOO ... ;

Are you are talking about the implementation of FOO or the
code laid by referencing FOO ? If the latter, it's really
architecture dependent. On the i32, 68k and MSP430
architectures and other CISC machines it often just reduces
to an indirect call.

defer foo ok
: t ... foo ... ; ok
dis t
T
....
( FF1510AA4A00 ) CALL [004AAA10] FOO
....
( C3 ) NEXT,
ok

variable poo ok
: t2 ... poo @ execute ... ; ok
dis t2
T2
....
( 004AAA80 FF1560AA4A00 ) CALL [004AAA60] POO
....
( 004AAA86 C3 ) NEXT,
ok

The code density vs speed tradeoff is also dependent on CPU and
memory architecture, as well as on application demands.

Stephen


--
Stephen Pelc, stephenXXX@xxxxxxxxxxxx
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
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