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Setting W register as a function variable

 
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Guest
Setting W register as a function variable
PostPosted: Thu Jan 13, 2005 2:51 pm     Reply with quote
Hi,

I'm trying to optimize my code and I would like to know if there's any clean way to do this.

Let's assume that I'm using a function

void TRANSMIT(int8 data)

If I call it like this:

TRANSMIT(0x88);

I'll always get:

;----------------- TRANSMIT(0x88)
MOVLW 88
MOVWF 71 <---This is not necessary
CALL 108

Is there a way to force the variable to be W so there would not be a MOVWF everytime the function is called. (The MOVWF would be inside the function)??

Thanks
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Jan 14, 2005 2:35 pm     Reply with quote
I tryed using #inline and it didn't help with the problem.
You could use a macro. If you use a macro, then it
doesn't make a function call. It just replicates the code
in each instance. I don't know if that's going to help your
efficiency problem, though.

The program below creates this ASM code:
Code:
0000                00296 ....................   
0000                00297 .................... TRANSMIT(0x55); 
0012 3055       00298 MOVLW  55
0013 1E0C       00299 BTFSS  0C.4
0014 2813       00300 GOTO   013
0015 0099       00301 MOVWF  19
0000                00302 .................... TRANSMIT(0xAA); 
0016 30AA       00303 MOVLW  AA
0017 1E0C       00304 BTFSS  0C.4
0018 2817       00305 GOTO   017
0019 0099       00306 MOVWF  19

Code:
#include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

/*
void TRANSMIT(int8 data)
{
putc(data);
}
*/

#define TRANSMIT(data)  putc(data)

//============================
void main()
{

TRANSMIT(0x55);
TRANSMIT(0xAA);

while(1);
}
dperron



Joined: 13 Jan 2005
Posts: 1

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PostPosted: Fri Jan 14, 2005 3:56 pm     Reply with quote
The problem with #define is that it's limited to only 1 line, you cannot do a multiline macro with it.

I really wished there were some way to set W a the variable...

So it's not so trivial after all...
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Jan 14, 2005 4:20 pm     Reply with quote
Quote:
The problem with #define is that it's limited to only 1 line, you cannot do a multiline macro with it.

Actually you can, if you have a modern version of the compiler.
You can use the backlash symbol to indicate line continuation. Example:
Code:

// This macro transmits the data byte 4 times.

#define TRANSMIT(data)  putc(data);  \
putc(data);         \
putc(data);         \
putc(data)
Ttelmah
Guest
PostPosted: Sat Jan 15, 2005 11:25 am     Reply with quote
dperron wrote:
The problem with #define is that it's limited to only 1 line, you cannot do a multiline macro with it.

I really wished there were some way to set W a the variable...

So it's not so trivial after all...

As PCM programmer has pointed out, you can have multi-line macros. You can also go further, and use a very rarely used feature of C, and have a multi-line macro, returning a value. If you generate a macro like:
Code:

#define from_locn(x) (temp=x,\
    x=(x+1)%maxval,\
    buffer[temp])

The code will save 'x', increment it, and generate the remainder after division my the 'maxval' value (ideal for incrementing a buffer pointer), then return the contents of 'buffer[temp]', as the macro return value!.
The 'comma' seperator, as opposed to the normal semi-colon seprator, in C, allows multiple statements to be executed, and only returns the value of the last statement.
The macro language in C, can do a lot more than is often realised...

Best Wishes
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