How to XOR?

[Mike]

I have this line in my program:

T1CON_TMR1ON = T1CON_TMR1ON ^ 0b0000_0001

which toggles Timer1.  I thought it should result in only two assembly instructions:

MOVLW    1

XORWF    v_t1con,f

But it actually results in five:

;  220       T1CON_TMR1ON = T1CON_TMR1ON ^ 0b0000_0001
             btfss    v_t1con, 0 ; t1con_tmr1on
             goto     l__l718
             bcf      v_t1con, 0 ; t1con_tmr1on
             goto     l__l717
l__l718
             bsf      v_t1con, 0 ; t1con_tmr1on
l__l717

But I  noticed that another part of the code:

ch = ch ^ 0xFF

correctly generates assembly using the XORWF instruction.

Do I have unreasonable expectations?  Why does it work in some situations, but not others?

-Mike

[Pinhead]

Hi,

If you just want to flip the T1CON_TMR1ON bit, you can use

T1CON_TMR1ON = !T1CON_TMR1ON

The ^ operator is useful when you want to flip an entire byte (or word or dword)

Le 27-04-26 à 20:04, 'Mike' via jallist a écrit :

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[Mike]

I tried that.  It still doesn't use the XORWF in assembly.  I'm not saying this is definitely wrong, or awful, I just want to know why the compiler doesn't choose the simpler and shorter code.

[Mike]

I think it must be a rule in the compiler.  It generates an XORWF when using a regular variable, but doesn't when using a Special Function Register such as T1CON.  It may have been done to avoid the problem with the PORT registers and just implemented as a general rule for all SFR's.