Subject:
Re: Optimiziing Forth compiler for mid-range PICs
From:
Francisco Rodrigo Escobedo Robles ####@####.####
Date:
7 Mar 2000 02:46:02 -0000
Message-Id: <200003070233.CAA02085@xinef.pepix.net>
I noticed the small typo in the subject of my announce, but in order not to
break the thread, i will not correct it :)
> > I mailed Scott Dattalo at the address listed in the gnupic.org pages, as I
>
> That page is hopelessly outdated. My current e-mail address is the one
> where I'm sending this response.
Thanks, it seems our messages get crossed, as I recently joined the list.
> > Any suggestion (and code patches :) are welcome. Right now, PIC models
> > supported are 16C84 and 16C63, but adding more mid-range models is easy.
> > Low-range PIC support is not planned. High-range may be supported in the
> > future.
>
> >From what I can tell, the compiler is written Forth as well. I don't know
> Forth at all. Could you perhaps provide some sample pic programs written
> in Forth?
It's my intention to make a sort of tutorial, but I lack the time right now.
There is a very small example in the docs, and I have just sent a message to
the list in response to Wojciech Zabolotny in which there is a snippet of code
showing an interrupt service routine.
Some words about Forth (funny, because 'word' is the term for a callable
routine, a variable, etc. Everything is a word in Forth):
- Forth uses a stack for intermediate results, local variables and parameter
passing. This mechanism is not transparent to the user. Every word takes its
parameters and leaves its results in the stack.
- @ is used to fetch the value of a memory address. So, if PORTA is defined
as the address of port A (in fact, it is in the currently supported processors),
PORTA @
effectively leaves in the stack the current value at the port A pins.
- ! stores the value in the second on stack into the address in the top of
stack, so if PORTB is defined as the address of port B,
PORTA @ PORTB !
effectively copies the current value at the port A pins into port B.
- there are high level flow control structures, as IF and loops as WHILE,
UNTIL and infinite loops.
- a word definition is like a subroutine, but in mary this is extended so a
routine can have several entry points.
For example, suppose you want to make a toy in which a led bar "moves" in a
ping-pong fashion (or a "Knight Rider" fashion, if you like). A possible (not
complete) code could be:
----8<----
( AN EXAMPLE OF A PING-PONG LED BAR )
PICFORTH HEX
INIT-PIC
100 CONSTANT MYDELAY ( ALTER VALUE OF MYDELAY FOR CHANGING SPEED )
VARIABLE COUNTER
0 ORG ( START OF MEMORY )
FLABEL MAIN GOTO ( MAIN CODE )
4 ORG ( INTERRUPT VECTOR )
FLABEL ISR GOTO ( INCONDITIONAL JUMP )
( SUPPORT WORDS )
: DELAY ( N -- ) ( A TIMED DELAY )
COUNTER !
BEGIN
COUNTER @ ( LOOPS UNTIL 0 )
UNTIL
;
: PINGPONG ( -- ) ( A FULL CYCLE OF LED BAR PING PONG )
1 CONST, ( INITIAL VALUE, FIRST HALF CYCLE )
BEGIN
DUP
WHILE
DUP PORTB !
MYDELAY DELAY
2* ( EQUIVALENT TO A LEFT SHIFT )
REPEAT
40 CONST, ( INITIAL VALUE, SECOND HALF CYCLE )
BEGIN
DUP
WHILE
DUP PORTB !
MYDELAY DELAY
2/ ( EQUIVALENT TO A RIGHT SHIFT )
REPEAT
;
( HERE COMES MAIN PROGRAM CODE )
MAIN
BEGIN
PINGPONG
AGAIN
( INTERRUPT SERVICE ROUTINE. MUST BE CALLED PERIODICALLY )
ISR
COUNTER @ IF ( IS COUNTER NON-ZERO? )
-1 CONST, COUNTER +! ( YES, DECREMENT IT )
THEN
RETFIE
----8<----
Again, too lazy of me for testing it right now (it's too late), the interrupts
must be properly initialized, but they aren't in this code.
There are a pair of tricks in the example, one being the testing for the end
of the WHILE loops, the other being the reliance on a periodic interrupt.
There is a kludge in mary in the for of the (rather ugly) CONST, word, whose
purpose is to push into the stack a previous unnamed constant. This will
disappear in the future, but for the time being, we must live with this.
Remember mary is in alpha state.
Oops, too long a message. Sorry.
Regards.
---
Francisco Rodrigo Escobedo Robles - ####@####.####
El surrealismo vive en Pepix Labs - http://www.pepix.net/
Este mensaje expresa unicamente mi opinion en este momento
