Signed 32 bit numbers

[Nazoa]

Hello,

I want to get the PIC to substract two 32 bit numbers. Now, I know that the result (including a sign bit) will always fit in 3 bytes, so I want to send these three bytes out to the RS232 port. I want the MSB of the 3 byte number to be the sign digit and I do not want 2s complement. I am proposing to use something as follows.

amp2=amp2-amp3; //Get difference
if bit_test(amp2,31) bit_set(amp2,23); //Negative sign for 3 byte

// Send out data
putc(make8(amp2,2)); // CH2-3, msb first
putc(make8(amp2,1));
putc(make8(amp2,0));

I need some guidance as I am not sure how 32 bit numbers are handled. Any comments or suggestions?

Thanks
Nils

[PCM programmer]

32-bit signed integers are stored in two's complement format.

If you know for certain that the result of the subtraction will
always be 24-bits or less, then you could just throw away the
upper byte. Example:

Max positive value for 24-bits = 0x7FFFFF (8388607 decimal)

Max negative value for 24-bits = 0x800000 (-8388608 decimal)

In 32-bit format, those values are 0x007FFFFF and 0xFF800000,
so you can see that the upper byte can be discarded, and you
will still have the correct number in 24-bit format.


Why don't you want 2's complement format ? It allows the full
24-bits to be used. You get the widest range of numbers with it.

[nazoa]

Thanks for the suggestion, I understand. Yeah, the thing is that the data is being sent to a PC that is not expecting 2s complement. I guess that I can get around that with some extra code.

Thanks again.
Nils