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spi_write()... or just handling int8

 
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spi_write()... or just handling int8
PostPosted: Mon Aug 31, 2009 10:22 am     Reply with quote
Hi

Take a look at this.

The first ex. is clean and nice:-)

The second ex. is not so clean, why?
I have no success with casting (int8) in the second ex.
any good explanation?
Code:

....................  tmp=x>>8&0xff;
0090:  MOVFF  x+1,tmp
....................  spi_write( tmp );
0094:  MOVF   SSPBUF,W
0096:  MOVFF  tmp,SSPBUF
009A:  RRCF   SSPSTAT,W
009C:  BNC   009A
....................   
....................  spi_write( x>>8&0xff );
009E:  MOVFF  x+1,@@60
00A2:  CLRF   @@x61
00A4:  CLRF   x61
00A6:  MOVF   SSPBUF,W
00A8:  MOVFF  x+1,SSPBUF
00AC:  RRCF   SSPSTAT,W
00AE:  BNC   00AC
....................
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Mon Aug 31, 2009 3:01 pm     Reply with quote
Quote:
The second ex. is not so clean, why?

It appears that no matter how it's done, the compiler puts in 2 or 3
ROM words of unnecessary instructions. The "make8" method has
the least number (only 2 words) of all the methods shown below. But
how important is this ? Why worry about optimizing out 2 instructions ?
Code:

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

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

x = 0x1234;

// Try different ways of getting the MSB of 'x' for the
// spi_write() function:
spi_write(x >> 8);
spi_write(make8(x, 1));
spi_write(x/256);

while(1);
}
 
John P



Joined: 17 Sep 2003
Posts: 331

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PostPosted: Tue Sep 01, 2009 7:24 am     Reply with quote
My favorite way to get an 8-bit quantity out of an int16 would do it like this:

Code:

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

//======================================
void main()
{
int16 x;
#byte x_low = x
#byte x_hi = x+1

x = 0x1234;

spi_write(x_low);           // Should send 0x34
spi_write(x_hi);             // Should send 0x12

while(1);
}
 
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