limiting to "no less than" zero

[Darrel Taylor]

STATUS is a register in the PIC that contains the results (status) of arithmetic operations.
Depening on the chip family, it may also have Watchdog Timer status and FSR/page bits.

STATUS.0 is the CARRY flag.
After an addition, if the result is larger than 255 the carry bit will be set (1).
After a subtraction, if the result is less than 0 (BORROW), the carry bit will be cleared (0).

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

[Byte_Butcher]

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

OK, curiosity got me so I tried it with WORD values and it still seems to work...


x = 32768
Y = 32769
x = x + y
Result: X = 1, STATUS.0 = 1
'--------------------------------

x = 32768
Y = 32765
x = x + y
Result: X = 65533, STATUS.0 = 0
'-------------------------------------

x = 32768
Y = 32765
x = x - y
Result: X = 3, STATUS.0 = 1
'--------------------------------------

x = 32765
Y = 32771
x = x - y
Result: X = 65530, STATUS.0 = 0
'------------------------------------

I thought it was only 'sposed to work with 8 bit numbers?


Steve

[Darrel Taylor]

Try it with ...

X = 32768
Y = 65523

[Byte_Butcher]

Try it with ...

X = 32768
Y = 65523

Nope, that didn't work.
OK, 8 bit math then....

Thanks!

[Charles Linquis]

Since PBP handles the 16 (31) bit math, and certainly knows the results of the 8 bit operation, the writers of PBP *could* provide us with an overflow/underflow flag (PSTATUS?). If they really wanted to.