CHAINED IF COMPILATION IN AVR STUDIO - TIPS ON AVR PROGRAMMING

CHAINED IF COMPILATION IN AVR STUDIO 

     Often chained if loop will be necessary for checking a port input status, but it may take up some unnecessary space & delay if not handled properly.

     For an example code shown below:

C code:

prv=PIND & MSK;

      if(prv != 0xF8) {

            Wait(TIME);

            if(prv != (PIND & MSK))

             { ……… }

      }

the compiled assembly code will be as follows:

Assembly code:

42:        prv=PIND & MSK;

IN        R20,0x16       In from I/O location

ANDI      R20,0xF8       Logical AND with immediate

MOV       R17,R20        Copy register

43:         if(prv != 0xF8) { //chk for any key press

CPI       R20,0xF8       Compare with immediate

BREQ      PC+0x26        Branch if equal

LDI       R24,0x40       Load immediate

LDI       R25,0x9C       Load immediate

SBIW      R24,0x01       Subtract immediate from word

BRNE      PC-0x01        Branch if not equal

46:               if(prv != (PIND & MSK))

IN        R24,0x16       In from I/O location

MOV       R18,R20        Copy register

LDI       R19,0x00       Load immediate

LDI       R25,0x00       Load immediate

ANDI      R24,0xF8       Logical AND with immediate

ANDI      R25,0x00       Logical AND with immediate

CP        R18,R24        Compare

CPC       R19,R25        Compare with carry

BREQ      PC+0x19        Branch if equal

We can see there is unnecessary usage of R19, R25. To overcome this, the chained if must be broken. The comparison must include only 2 variables as follows.

C code:

  unsigned char p;

 prv=PIND & MSK;

      if(prv != 0xF8) {

            Wait(TIME);

            p=PIND & MSK;

            if(prv != p)

             { ………  }

}

 Assembly code:

42:        prv=PIND & MSK;

IN        R18,0x16       In from I/O location

ANDI      R18,0xF8       Logical AND with immediate

MOV       R17,R18        Copy register

43:         if(prv != 0xF8) { //chk for any key press

CPI       R18,0xF8       Compare with immediate

BREQ      PC+0x21        Branch if equal

LDI       R24,0x40       Load immediate

LDI       R25,0x9C       Load immediate

SBIW      R24,0x01       Subtract immediate from word

BRNE      PC-0x01        Branch if not equal

45:               p=PIND & MSK;

IN        R24,0x16       In from I/O location

46:               if(prv != p) //(PIND & MSK))

ANDI      R24,0xF8       Logical AND with immediate

CP        R18,R24        Compare

BREQ      PC+0x19        Branch if equal

We might think that adding a variable could increase memory space & memory access, but the compiler optimizes it with using a register. So memory space & memory access cycles are not required.

This code uses lesser space & is faster the previous code.

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