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Optimization High/Low byte

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RLScott

Joined: 10 Jul 2007
Posts: 465

Optimization High/Low byte

Posted: Fri Feb 25, 2011 11:59 am

In the listing below, I am trying to get the most optimum 16F1824 code for the following function:

Code:

.................... unsigned int16 GetTimer1(void)
.................... {
.................... unsigned int k;
.................... unsigned int16 retGetTimer1;
.................... #byte gt_low=retGetTimer1
.................... #byte gt_high=retGetTimer1+1
....................
.................... k = TMR1H;
*
00BB: MOVF 17,W
00BC: MOVWF 37
.................... gt_low = TMR1L;
00BD: MOVF 16,W
00BE: MOVWF 38
.................... if(k != (gt_high=TMR1H)) //..high byte changed..
00BF: MOVF 17,W
00C0: MOVWF 39
00C1: SUBWF 37,W
00C2: BTFSC 03.2
00C3: GOTO 0C6
.................... gt_low = TMR1L; //..then re-read low byte
00C4: MOVF 16,W
00C5: MOVWF 38
.................... return retGetTimer1;
00C6: MOVF 38,W
00C7: MOVWF 78
00C8: MOVF 39,W
00C9: MOVWF 79

As you can see this is very optimum. But why was it necessary for me to resort to the #byte directives to get it? I was hoping to get the same sort of code from the following:

Code:

.................... unsigned int16 GetTimer1(void)
.................... {
.................... unsigned int8 k,m,n;
.................... k = TMR1H;
*
00BB: MOVF 17,W
00BC: MOVWF 37
.................... m = TMR1L;
00BD: MOVF 16,W
00BE: MOVWF 38
.................... //n = TMR1H;
.................... if(k != (n=TMR1H)) //..high byte changed..
00BF: MOVF 17,W
00C0: MOVWF 39
00C1: SUBWF 37,W
00C2: BTFSC 03.2
00C3: GOTO 0C6
.................... m = TMR1L; //..then re-read low byte
00C4: MOVF 16,W
00C5: MOVWF 38
.................... return ((unsigned int16)n<<8) + m;
00C6: CLRF 3B
00C7: MOVF 39,W
00C8: MOVWF 3A
00C9: MOVWF 3B
00CA: CLRF 3A
00CB: MOVF 38,W
00CC: ADDWF 3A,W
00CD: MOVWF 78
00CE: MOVLW 00
00CF: ADDWFC 3B,W
00D0: MOVWF 7A
00D1: MOVF 7A,W
00D2: MOVWF 79

As you can see, it is nowhere near as good as the first implementation. Is there some way I can coax the optimization of the first listing from an implementation that did not resort to such compiler-dependent constructs as #byte and and assumed endian order? (My build options are set for maximum optimization.)
_________________
Robert Scott
Real-Time Specialties
Embedded Systems Consulting

PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Fri Feb 25, 2011 1:07 pm

You can use make16() to do it efficiently:

Code:

.................... return(make16(n,m));
000E: MOVF n,W
000F: MOVWF @7A
0010: MOVF m,W
0011: MOVWF @78
0012: MOVF n,W
0013: MOVWF @79

Code:

#include <16F1824.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT //, NOLVP
#use delay(clock=4000000)

#byte TMR1H = 0x17
#byte TMR1L = 0x16

unsigned int16 GetTimer1(void)
{
unsigned int8 k,m,n;
k = TMR1H;
m = TMR1L;
//n = TMR1H;
if(k != (n=TMR1H)) //..high byte changed..
m = TMR1L; //..then re-read low byte

return(make16(n,m));
}

//==========================================
void main()
{
int16 result;

result = GetTimer1();

while(1);
}

RLScott

Joined: 10 Jul 2007
Posts: 465

Posted: Fri Feb 25, 2011 1:45 pm

PCM programmer wrote:

You can use make16() to do it efficiently:

Thanks. That is neater than my solution.
_________________
Robert Scott
Real-Time Specialties
Embedded Systems Consulting

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