Hi,
When I create a variable I must use an entire byte. I want to be able to define a variable as a bit so I do not waste so much memory.
How do you do it?
Thanks
Ron
Defining a variable that is only 1 bit in size
-
Sean
- Valued Contributor
- Posts: 548
- Joined: Tue Jun 26, 2007 11:23 am
- Has thanked: 6 times
- Been thanked: 44 times
Hi Ron,
The best way to store single bits of data (or small combinations of bits) is to use the AND, OR and XOR functions in a calculation block. These commands can be used to manipulate individual bits within a byte or integer variable.
If you create a variable called Flags:
A bit can be set by ORing it with a 1
Flags = Flags OR 4 will force bit 2 to be set
A more convenient notation, using the bit shift function, might be:
Flags = Flags OR (1 << 2)
Clearing the same bit can be done by ANDing it with a 0;
Flags = Flags AND NOT(1 << 2)
The bit can be toggled by XORing it with a 1;
Flags = Flags XOR (1 << 2)
The bit can be tested by ANDing it with a 1;
IF Flags AND (1 << 2)
It is not worth doing this if you are only dealing with a single variable of this type, as it will still be stored as a byte (the minimum memory size the controller will handle). But if you have several single bit variables they can be packed into a single byte or integer, using a different bit for each flag, by using these techniques.
The savings in data memory will be offset by a slightly larger program and slight reduction in operating speed due to the additional operations
The best way to store single bits of data (or small combinations of bits) is to use the AND, OR and XOR functions in a calculation block. These commands can be used to manipulate individual bits within a byte or integer variable.
If you create a variable called Flags:
A bit can be set by ORing it with a 1
Flags = Flags OR 4 will force bit 2 to be set
A more convenient notation, using the bit shift function, might be:
Flags = Flags OR (1 << 2)
Clearing the same bit can be done by ANDing it with a 0;
Flags = Flags AND NOT(1 << 2)
The bit can be toggled by XORing it with a 1;
Flags = Flags XOR (1 << 2)
The bit can be tested by ANDing it with a 1;
IF Flags AND (1 << 2)
It is not worth doing this if you are only dealing with a single variable of this type, as it will still be stored as a byte (the minimum memory size the controller will handle). But if you have several single bit variables they can be packed into a single byte or integer, using a different bit for each flag, by using these techniques.
The savings in data memory will be offset by a slightly larger program and slight reduction in operating speed due to the additional operations