#14561 - Geoff - Thu Jan 08, 2004 6:49 pm
| source wrote: |
#define BIT0 1
if(counter & BIT0) { //tests if the counter is an odd number
|
'counter' is an unsigned integer that increments once every frame. Is this doing what I think it's doing (simply testing if counter is odd) or is it doing something else too? Like changing counter?
#14563 - Lord Graga - Thu Jan 08, 2004 7:50 pm
yes, it checks if it's odd.
#14564 - LOst? - Thu Jan 08, 2004 8:05 pm
| Geoff wrote: |
| source wrote: |
#define BIT0 1
if(counter & BIT0) { //tests if the counter is an odd number
|
'counter' is an unsigned integer that increments once every frame. Is this doing what I think it's doing (simply testing if counter is odd) or is it doing something else too? Like changing counter? |
You forget to increament it each time:
| Quote: |
int counter;
if (++counter & 1); |
#14570 - sajiimori - Thu Jan 08, 2004 8:24 pm
lol, he didn't post his entire source you know! You may as well tell him that he forgot to write a main().
#14595 - Geoff - Fri Jan 09, 2004 2:21 am
Awesome. I used to be very sloppy and test if(counter%2==1) but I've since learned that that's slowwwww.
#14596 - poslundc - Fri Jan 09, 2004 2:40 am
You can replace "% n" with "& (n-1)" whenever n is an even power of two.
This is extremely important in GBA programming, since as you observed there is no hardware divide, which means the modulus operator (%) is emulated in software and is slow as heck. But the bitwise-and operator (&) requires only one ALU instruction.
It also makes it extremely convenient that so many aspects of the GBA (sprite/tile widths, background sizes, palette sizes, etc.) are even powers of two.
Dan.
#14599 - LOst? - Fri Jan 09, 2004 3:11 am
| Geoff wrote: |
| Awesome. I used to be very sloppy and test if(counter%2==1) but I've since learned that that's slowwwww. |
Yea, one thing for sure. Don't use modulo with the GBA :P
#14608 - tom - Fri Jan 09, 2004 10:19 am
btw, gcc is smart enough to implement "% n" by "& (n-1)" if n is a power of two.