Commodore VIC-20 User Manual page 225

Chapter 6 Graphics
211
The bit within that byte is the remainder of the division by 8 we did to
find the column. To calculate the right bit, use
460 PL=7168+Y+64*COL
470 BIT=7-(X-COL*8)
We subtracted the remainder from 7 because our high-resolution
columns are numbered from left to right, but the bits in a byte are numbered
from right to left.
Just knowing the number of the bit is not enough. Because it will be
using POKE to change the display, your program must calculate the number
that corresponds to that bit. Remember that each bit in the byte represents a
power of 2.
Bit Number
I
71
6
I
5
I
4
I
3
I
2
11
I
0
I
Value for POKE 128 64 32 16 8 4 2 1
Since this is the case, you can easily convert from the bit number to the
POKE value using the exponentiation operator.
480 PV=21'BIT
Your program cannot simply POKE blindly into the byte it has found
because there are seven other bits there that you don't want to disturb. To
change just one bit, you must PEEK the byte to be changed, modify only the
correct bit, and then POKE it back. To change only a single bit, you can use
the AND and OR operators. For example, to set a bit to 0 (making its dot
the background color), you can use a variation of the masking technique
that was used to isolate the bits for joystick switches.
500 POKE PL,PEEK(Pl) AND NOT PI,'
Notice the use of the NOT operator. When we were isolating the
joystick switches we used the value of the bit directly. That produced a mask
in which the bit we wanted was a 1, and all the others were Os. This time we
are using a mask in which the bit we are interested in is a 0 and all the others
are Is. The AND will force that bit to 0 and leave the others undisturbed.
Suppose we want to turn off bit 3 in location 7432, which currently contains
a value of 43. We could use a BASIC statement like this.