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changed the order of the sections to improve flow

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Alexander Sisco 2025-02-04 08:57:23 -08:00
parent 3faad6e17b
commit 657fd6aff0
1 changed files with 60 additions and 50 deletions

110
exercises/110_bit_manipulation3.zig
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@ -1,5 +1,5 @@
// ----------------------------------------------------------------------------
// Setting, Clearing, and Toggling Bits
// Toggling, Setting, and Clearing Bits
// ----------------------------------------------------------------------------
//
// Another exciting thing about Zig is its suitability for embedded
@ -69,6 +69,58 @@ const testing = std.testing;
pub fn main() !void {
var PORTB: u4 = 0b0000; // only 4 bits wide for simplicity
//
// Let's first take a look at toggling bits.
//
// ------------------------------------------------------------------------
// Toggling bits with XOR:
// ------------------------------------------------------------------------
// XOR stands for "exclusive or". We can toggle bits with the ^ (XOR)
// bitwise operator, like so:
//
//
// In order to output a 1, the logic of an XOR operation requires that the
// two input bits are of different values. Therefore, 0 ^ 1 and 1 ^ 0 will
// both yield a 1 but 0 ^ 0 and 1 ^ 1 will output 0. XOR's unique behavior
// of outputing a 0 when both inputs are 1s is what makes it different from
// the OR operator; it also gives us the ability to toggle bits by putting
// 1s into our bitmask.
//
// - 1s in our bitmask operand, can be thought of as causing the
// corresponding bits in the other operand to flip to the opposite value.
// - 0s cause no change.
//
// The 0s in our bitmask preserve these values
// -XOR op- ---expanded--- in the output.
// _______________/
// / /
// 0110 1 1 0 0
// ^ 1111 0 1 0 1 (bitmask)
// ------ - - - -
// = 1001 1 0 0 1 <- This bit was already cleared.
// \_______\
// \
// We can think of these bits having flipped
// because of the presence of 1s in those columns
// of our bitmask.
print("Toggle pins with XOR on PORTB\n", .{});
print("-----------------------------\n", .{});
PORTB = 0b1100;
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("^ {b:0>4} // (bitmask)\n", .{0b0101});
PORTB ^= (1 << 1) | (1 << 0); // What's wrong here?
checkAnswer(0b1001, PORTB);
newline();
PORTB = 0b1100;
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("^ {b:0>4} // (bitmask)\n", .{0b0011});
PORTB ^= (1 << 1) & (1 << 0); // What's wrong here?
checkAnswer(0b1111, PORTB);
// Now let's take a look at setting bits with the | operator.
//
// ------------------------------------------------------------------------
// Setting bits with OR:
// ------------------------------------------------------------------------
@ -129,6 +181,10 @@ pub fn main() !void {
newline();
// So now we've covered how to toggle and set bits. What about clearing
// them? Well, this is where Zig throws us a curve ball. Don't worry we'll
// go through it step by step.
// ------------------------------------------------------------------------
// Clearing bits with AND and NOT:
// ------------------------------------------------------------------------
@ -165,10 +221,10 @@ pub fn main() !void {
//
// 2. The second step in creating our bit mask is to invert the bits
// ~0100 -> 1011
// we can write this as:
// in C we would write this as:
// ~(1 << 2) -> 1011
//
// But if we try to compile ~(1 << 2), we'll get an error:
// But if we try to compile ~(1 << 2) in Zig, we'll get an error:
// unable to perform binary not operation on type 'comptime_int'
//
// Before Zig can invert our bits, it needs to know the number of
@ -225,56 +281,10 @@ pub fn main() !void {
newline();
newline();
//
// ------------------------------------------------------------------------
// Toggling bits with XOR:
// Conclusion
// ------------------------------------------------------------------------
// XOR stands for "exclusive or". We can toggle bits with the ^ (XOR)
// bitwise operator, like so:
//
//
// In order to output a 1, the logic of an XOR operation requires that the
// two input bits are of different values. Therefore, 0 ^ 1 and 1 ^ 0 will
// both yield a 1 but 0 ^ 0 and 1 ^ 1 will output 0. XOR's unique behavior
// of outputing a 0 when both inputs are 1s is what makes it different from
// the OR operator; it also gives us the ability to toggle bits by putting
// 1s into our bitmask.
//
// - 1s in our bitmask operand, can be thought of as causing the
// corresponding bits in the other operand to flip to the opposite value.
// - 0s cause no change.
//
// The 0s in our bitmask preserve these values
// -XOR op- ---expanded--- in the output.
// _______________/
// / /
// 0110 1 1 0 0
// ^ 1111 0 1 0 1 (bitmask)
// ------ - - - -
// = 1001 1 0 0 1 <- This bit was already cleared.
// \_______\
// \
// We can think of these bits having flipped
// because of the presence of 1s in those columns
// of our bitmask.
print("Toggle pins with XOR on PORTB\n", .{});
print("-----------------------------\n", .{});
PORTB = 0b1100;
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("^ {b:0>4} // (bitmask)\n", .{0b0101});
PORTB ^= (1 << 1) | (1 << 0); // What's wrong here?
checkAnswer(0b1001, PORTB);
newline();
PORTB = 0b1100;
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("^ {b:0>4} // (bitmask)\n", .{0b0011});
PORTB ^= (1 << 1) & (1 << 0); // What's wrong here?
checkAnswer(0b1111, PORTB);
// While the examples in this exercise have used only 4-bit wide variables,
// working with 8 bits is no different. Here's a an example where we set
// every other bit beginning with the two's place: