Locating 2 parameters at the same address

Having mocked you of having less curiosity than me, I could hardly ignore your request. (I had to change your header files as I don't have those - I'm using an 8051F930 - so perhaps it's not an apples-apples comparison.)

With optimization 4+ var1/var2 are at the same address. However the actual assembler code doesn't reference these and uses registers. Not a fair test.

With optimization at 2, var1/var2 do not overlay each other. I would not expect this program to write anything other than 36 to P2.

I have to change the code as follows to get the data to overlay:

static void (*mHandler)(unsigned int uData);

void sub1(unsigned char var1) {
        var1=0;                                                 //reset var1/var2
        if (var1) var1=0;                               //make sure var1 is always 0
        //P2=var1;                                              //make sure tmr0 is working
}

void sub2(unsigned char var2) {
        var2=var2+1;                                    //increment var1/var2
        if (var2==0) var2=1;                    //make sure var2 is never zero
        NOP256(); NOP256();                             //delay 512 ticks - create opportunity for tmr0 to fire at least once
        P2=var2;                                                //output var2 on P0
        if ( var2 == 0 )
        {
                P2 = 1;
        }
}

void tmr0_isr(void) interrupt 1 {
//              sub1(0);
        (*mHandler)(0);                                                //call sub1 periodically to reset var1/var2
}

int main(void) {
                mHandler = sub1;
        TR0=0;                                                  //turn off tmr0
        TMOD = (TMOD & 0xf0) | 0x02;        //tmr0 in mode 2 (auto reload tl0 with th0
        TH0=-100;                                               //tmr strikes every 100 ticks
        TL0=TH0;
        ET0=1;                                                  //enable tmr0 interrupt
        TR0=1;                                                  //turn on tmr0
        EA=1;                                                   //enable global interrupt

        while (1) {
                sub2(35);
        }
}

This code seems to suffer from the same problem as my program. In fact, from within Sub2 I got var2 == 0.

Having mocked you of having less curiosity than me, I could hardly ignore your request. (I had to change your header files as I don't have those - I'm using an 8051F930 - so perhaps it's not an apples-apples comparison.)

With optimization 4+ var1/var2 are at the same address. However the actual assembler code doesn't reference these and uses registers. Not a fair test.

With optimization at 2, var1/var2 do not overlay each other. I would not expect this program to write anything other than 36 to P2.

I have to change the code as follows to get the data to overlay:

static void (*mHandler)(unsigned int uData);

void sub1(unsigned char var1) {
        var1=0;                                                 //reset var1/var2
        if (var1) var1=0;                               //make sure var1 is always 0
        //P2=var1;                                              //make sure tmr0 is working
}

void sub2(unsigned char var2) {
        var2=var2+1;                                    //increment var1/var2
        if (var2==0) var2=1;                    //make sure var2 is never zero
        NOP256(); NOP256();                             //delay 512 ticks - create opportunity for tmr0 to fire at least once
        P2=var2;                                                //output var2 on P0
        if ( var2 == 0 )
        {
                P2 = 1;
        }
}

void tmr0_isr(void) interrupt 1 {
//              sub1(0);
        (*mHandler)(0);                                                //call sub1 periodically to reset var1/var2
}

int main(void) {
                mHandler = sub1;
        TR0=0;                                                  //turn off tmr0
        TMOD = (TMOD & 0xf0) | 0x02;        //tmr0 in mode 2 (auto reload tl0 with th0
        TH0=-100;                                               //tmr strikes every 100 ticks
        TL0=TH0;
        ET0=1;                                                  //enable tmr0 interrupt
        TR0=1;                                                  //turn on tmr0
        EA=1;                                                   //enable global interrupt

        while (1) {
                sub2(35);
        }
}

This code seems to suffer from the same problem as my program. In fact, from within Sub2 I got var2 == 0.

"This code seems to suffer from the same problem as my program. In fact, from within Sub2 I got var2 == 0."

if you really had curiosity, you probably would have run it on different platforms and observed the differences in behavior and asked yourself why.

I actually claimed to have a lack of curiosity - and proud of it.

Thanks to everyone here to for their contributions to a case solved. Especially to Ron who, strangely enough, found the solution but didn't recommend it.