setting values and percentages

[richard]

result 50*4095/100 is 2047
result (4095/100)*50 is 2000

the calc follows typical bodmas rules (brackets, of , divide .....) see precedence in manual

the only 'suprising' bit is that the indermediate result (50*4095) is 204750 way too big for a word var. the pbp compiler code does indeed use 32 bit intermediate results.
the */ and ** operators allow you access these 32 bit results (providing you follow the rules of course)

[richard]

however
if you have a word var say numx
and x =4095
then 50*numx/100 the result is 81
the word var causes a intermediate stage overflow (bits 0-15 of 204750 =8142)
I still say integer math is tricky and results need to be double checked

[EarlyBird2]

however
if you have a word var say numx
and x =4095
then 50*numx/100 the result is 81
the word var causes a intermediate stage overflow (bits 0-15 of 204750 =8142)

I do not understand this.
What is the relationship between numx and x?
Why does an intermediate stage overflow occur and what is an intermediate stage overflow?

[EarlyBird2]

I do not understand this.

Why does an intermediate stage overflow occur and what is an intermediate stage overflow?

Could you explain please?

[richard]

What is the relationship between numx and x?

poor typing
if you have a word var say numx
and numx =4095
then 50*numx/100 the result is 81
the word var causes a intermediate stage overflow (bits 0-15 of 204750 =8142)
I still say integer math is tricky and results need to be double checked

intermediate overflow
50*4095 =204750 max value for word 65535 therefore an overflow occurs and the result becomes 8142

[EarlyBird2]

intermediate overflow
50*4095 =204750 max value for word 65535 therefore an overflow occurs and the result becomes 8142

But the result is not stored in numx it is stored in another word variable called result?

[richard]

simply stated
if there is a var in a multilstage calculation then any intermediate results must be able to be stored in that var , its just the way it works.
try it yourself
the */ ,** ops are the only workaround

[richard]

you might have missed the point

50*4095/100 is 2047 this works (all constants)

numx=4095
50*numx/100 is 81 this does not

numy=50
numy*4095/100 is 81 this does not

numz=100
50*4095/numz is 81 this does not

[EarlyBird2]

Now I understand what you are saying. It does seem odd to me that there is a 32 bit register for maths calculation which can not be used if there is a word or byte variable in the calculation because the word/byte variable could overflow. A bit more thinking is required on my part, I think.

Does this mean that using ** and */ uses 32 bit and * only 16 bit?