// ----------------------------------------------------------------------------
// Quiz Time: Toggling, Setting, and Clearing Bits
// ----------------------------------------------------------------------------
//
// Another exciting thing about Zig is its suitability for embedded
// programming. Your Zig code doesn't have to remain on your laptop; you can
// also deploy your code to microcontrollers! This means you can write Zig to
// drive your next robot or greenhouse climate control system! Ready to enter
// the exciting world of embedded programming? Let's get started!
//
// ----------------------------------------------------------------------------
// Some Background
// ----------------------------------------------------------------------------
//
// A common activity in microcontroller programming is setting and clearing
// bits on input and output pins. This lets you control LEDs, sensors, motors
// and more! In a previous exercise (097_bit_manipulation.zig) you learned how
// to swap two bytes using the ^ (XOR - exclusive or) operator. This quiz will
// test your knowledge of bit manipulation in Zig while giving you a taste of
// what it's like to control registers in a real microcontroller. Included at
// the end are some helper functions that demonstrate how we might make our
// code a little more readable.
//
// Below is a pinout diagram for the famous ATmega328 AVR microcontroller used
// as the primary microchip on popular microcontroller platforms like the
// Arduino UNO.
//
// ============ PINOUT DIAGRAM FOR ATMEGA328 MICROCONTROLLER ============
// _____ _____
// | U |
// (RESET) PC6 --| 1 28 |-- PC5
// PD0 --| 2 27 |-- PC4
// PD1 --| 3 26 |-- PC3
// PD2 --| 4 25 |-- PC2
// PD3 --| 5 24 |-- PC1
// PD4 --| 6 23 |-- PC0
// VCC --| 7 22 |-- GND
// GND --| 8 21 |-- AREF
// |-- PB6 --| 9 20 |-- AVCC
// |-- PB7 --| 10 19 |-- PB5 --|
// | PD5 --| 11 18 |-- PB4 --|
// | PD6 --| 12 17 |-- PB3 --|
// | PD7 --| 13 16 |-- PB2 --|
// |-- PB0 --| 14 15 |-- PB1 --|
// | |___________| |
// \_______________________________/
// |
// PORTB
//
// Drawing inspiration from this diagram, we'll use the pins for PORTB as our
// mental model for this quiz on bit manipulation. It should be noted that
// in the following problems we are using ordinary variables, one of which we
// have named PORTB, to simulate modifying the bits of real hardware registers.
// But in actual microcontroller code, PORTB would be defined something like
// this:
// pub const PORTB = @as(*volatile u8, @ptrFromInt(0x25));
//
// This lets the compiler know not to make any optimizations to PORTB so that
// the IO pins are properly mapped to our code.
//
// NOTE : To keep things simple, the following problems are given using type
// u4, so applying the output to PORTB would only affect the lower four pins
// PB0..PB3. Of course, there is nothing to prevent you from swapping the u4
// with a u8 so you can control all 8 of PORTB's IO pins.
const std = @import("std");
const print = std.debug.print;
const testing = std.testing;
pub fn main() !void {
var PORTB: u4 = 0b0000; // only 4 bits wide for simplicity
// ------------------------------------------------------------------------
// Quiz
// ------------------------------------------------------------------------
// See if you can solve the following problems. The last two problems throw
// you a bit of a curve ball. Try solving them on your own. If you need
// help, scroll to the bottom of main to see some in depth explanations on
// toggling, setting, and clearing bits in Zig.
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);
newline();
print("Set pins with OR on PORTB\n", .{});
print("-------------------------\n", .{});
PORTB = 0b1001; // reset PORTB
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("| {b:0>4} // (bitmask)\n", .{0b0100});
PORTB = PORTB ??? (1 << 2); // What's missing here?
checkAnswer(0b1101, PORTB);
newline();
PORTB = 0b1001; // reset PORTB
print(" {b:0>4} // (reset state)\n", .{PORTB});
print("| {b:0>4} // (bitmask)\n", .{0b0100});
PORTB ??? (1 << 2); // What's missing here?
checkAnswer(0b1101, PORTB);
newline();
print("Clear pins with AND and NOT on PORTB\n", .{});
print("------------------------------------\n", .{});
PORTB = 0b1110; // reset PORTB
print(" {b:0>4} // (initial state of PORTB)\n", .{PORTB});
print("& {b:0>4} // (bitmask)\n", .{0b1011});
PORTB = PORTB & ???@as(u4, 1 << 2); // What character is missing here?
checkAnswer(0b1010, PORTB);
newline();
PORTB = 0b0111; // reset PORTB
print(" {b:0>4} // (reset state)\n", .{PORTB});
print("& {b:0>4} // (bitmask)\n", .{0b1110});
PORTB &= ~(1 << 0); // What's missing here?
checkAnswer(0b0110, PORTB);
newline();
newline();
}
// ************************************************************************
// IN-DEPTH EXPLANATIONS BELOW
// ************************************************************************
//
//
//
//
//
//
//
//
//
//
//
// ------------------------------------------------------------------------
// 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 outputting 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.
// _______________/
// / /
// 1100 1 1 0 0
// ^ 0101 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.
//
// Now let's take a look at setting bits with the | operator.
//
//
//
//
//
// ------------------------------------------------------------------------
// Setting bits with OR:
// ------------------------------------------------------------------------
// We can set bits on PORTB with the | (OR) operator, like so:
//
// var PORTB: u4 = 0b1001;
// PORTB = PORTB | 0b0010;
// print("PORTB: {b:0>4}\n", .{PORTB}); // output: 1011
//
// -OR op- ---expanded---
// _ Set only this bit.
// /
// 1001 1 0 0 1
// | 0010 0 0 1 0 (bitmask)
// ------ - - - -
// = 1011 1 0 1 1
// \___\_______\
// \
// These bits remain untouched because OR-ing with
// a 0 effects no change.
//
// ------------------------------------------------------------------------
// To create a bitmask like 0b0010 used above:
//
// 1. First, shift the value 1 over one place with the bitwise << (shift
// left) operator as indicated below:
// 1 << 0 -> 0001
// 1 << 1 -> 0010 <-- Shift 1 one place to the left
// 1 << 2 -> 0100
// 1 << 3 -> 1000
//
// This allows us to rewrite the above code like this:
//
// var PORTB: u4 = 0b1001;
// PORTB = PORTB | (1 << 1);
// print("PORTB: {b:0>4}\n", .{PORTB}); // output: 1011
//
// Finally, as in the C language, Zig allows us to use the |= operator, so
// we can rewrite our code again in an even more compact and idiomatic
// form: PORTB |= (1 << 1)
// 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:
// ------------------------------------------------------------------------
// We can clear bits with the & (AND) bitwise operator, like so:
// PORTB = 0b1110; // reset PORTB
// PORTB = PORTB & 0b1011;
// print("PORTB: {b:0>4}\n", .{PORTB}); // output -> 1010
//
// - 0s clear bits when used in conjunction with a bitwise AND.
// - 1s do nothing, thus preserving the original bits.
//
// -AND op- ---expanded---
// __________ Clear only this bit.
// /
// 1110 1 1 1 0
// & 1011 1 0 1 1 (bitmask)
// ------ - - - -
// = 1010 1 0 1 0 <- This bit was already cleared.
// \_______\
// \
// These bits remain untouched because AND-ing with a
// 1 preserves the original bit value whether 0 or 1.
//
// ------------------------------------------------------------------------
// We can use the ~ (NOT) operator to easily create a bitmask like 1011:
//
// 1. First, shift the value 1 over two places with the bit-wise << (shift
// left) operator as indicated below:
// 1 << 0 -> 0001
// 1 << 1 -> 0010
// 1 << 2 -> 0100 <- The 1 has been shifted two places to the left
// 1 << 3 -> 1000
//
// 2. The second step in creating our bitmask is to invert the bits
// ~0100 -> 1011
// in C we would write this as:
// ~(1 << 2) -> 1011
//
// 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
// bits it's being asked to invert.
//
// We do this with the @as (cast as) built-in like this:
// @as(u4, 1 << 2) -> 0100
//
// Finally, we can invert our new mask by placing the NOT ~ operator
// before our expression, like this:
// ~@as(u4, 1 << 2) -> 1011
//
// If you are offput by the fact that you can't simply invert bits like
// you can in languages such as C without casting to a particular size
// of integer, you're not alone. However, this is actually another
// instance where Zig is really helpful because it protects you from
// difficult to debug integer overflow bugs that can have you tearing
// your hair out. In the interest of keeping things sane, Zig requires
// you simply to tell it the size of number you are inverting. In the
// words of Andrew Kelley, "If you want to invert the bits of an
// integer, zig has to know how many bits there are."
//
// For more insight into the Zig team's position on why the language
// takes the approach it does with the ~ operator, take a look at
// Andrew's comments on the following github issue:
// https://github.com/ziglang/zig/issues/1382#issuecomment-414459529
//
// Whew, so after all that what we end up with is:
// PORTB = PORTB & ~@as(u4, 1 << 2);
//
// We can shorten this with the &= combined AND and assignment operator,
// which applies the AND operator on PORTB and then reassigns PORTB. Here's
// what that looks like:
// PORTB &= ~@as(u4, 1 << 2);
//
// ------------------------------------------------------------------------
// Conclusion
// ------------------------------------------------------------------------
//
// While the examples in this quiz have used only 4-bit wide variables,
// working with 8 bits is no different. Here's an example where we set
// every other bit beginning with the two's place:
// var PORTD: u8 = 0b0000_0000;
// print("PORTD: {b:0>8}\n", .{PORTD});
// PORTD |= (1 << 1);
// PORTD = setBit(u8, PORTD, 3);
// PORTD |= (1 << 5) | (1 << 7);
// print("PORTD: {b:0>8} // set every other bit\n", .{PORTD});
// PORTD = ~PORTD;
// print("PORTD: {b:0>8} // bits flipped with NOT (~)\n", .{PORTD});
// newline();
//
// // Here we clear every other bit beginning with the two's place.
//
// PORTD = 0b1111_1111;
// print("PORTD: {b:0>8}\n", .{PORTD});
// PORTD &= ~@as(u8, 1 << 1);
// PORTD = clearBit(u8, PORTD, 3);
// PORTD &= ~@as(u8, (1 << 5) | (1 << 7));
// print("PORTD: {b:0>8} // clear every other bit\n", .{PORTD});
// PORTD = ~PORTD;
// print("PORTD: {b:0>8} // bits flipped with NOT (~)\n", .{PORTD});
// newline();
// ----------------------------------------------------------------------------
// Here are some helper functions for manipulating bits
// ----------------------------------------------------------------------------
// Functions for setting, clearing, and toggling a single bit
fn setBit(comptime T: type, byte: T, comptime bit_pos: T) !T {
return byte | (1 << bit_pos);
}
test "setBit" {
try testing.expectEqual(setBit(u8, 0b0000_0000, 3), 0b0000_1000);
}
fn clearBit(comptime T: type, byte: T, comptime bit_pos: T) T {
return byte & ~@as(T, (1 << bit_pos));
}
test "clearBit" {
try testing.expectEqual(clearBit(u8, 0b1111_1111, 0), 0b1111_1110);
}
fn toggleBit(comptime T: type, byte: T, comptime bit_pos: T) T {
return byte ^ (1 << bit_pos);
}
test "toggleBit" {
var byte = toggleBit(u8, 0b0000_0000, 0);
try testing.expectEqual(byte, 0b0000_0001);
byte = toggleBit(u8, byte, 0);
try testing.expectEqual(byte, 0b0000_0000);
}
// ----------------------------------------------------------------------------
// Some additional functions for setting, clearing, and toggling multiple bits
// at once with a tuple because, hey, why not?
// ----------------------------------------------------------------------------
//
fn createBitmask(comptime T: type, comptime bits: anytype) !T {
comptime var bitmask: T = 0;
inline for (bits) |bit| {
if (bit >= @bitSizeOf(T)) return error.BitPosTooLarge;
if (bit < 0) return error.BitPosTooSmall;
bitmask |= (1 << bit);
}
return bitmask;
}
test "creating bitmasks from a tuple" {
try testing.expectEqual(createBitmask(u8, .{0}), 0b0000_0001);
try testing.expectEqual(createBitmask(u8, .{1}), 0b0000_0010);
try testing.expectEqual(createBitmask(u8, .{2}), 0b0000_0100);
try testing.expectEqual(createBitmask(u8, .{3}), 0b0000_1000);
//
try testing.expectEqual(createBitmask(u8, .{ 0, 4 }), 0b0001_0001);
try testing.expectEqual(createBitmask(u8, .{ 1, 5 }), 0b0010_0010);
try testing.expectEqual(createBitmask(u8, .{ 2, 6 }), 0b0100_0100);
try testing.expectEqual(createBitmask(u8, .{ 3, 7 }), 0b1000_1000);
try testing.expectError(error.BitPosTooLarge, createBitmask(u4, .{4}));
}
fn setBits(byte: u8, bits: anytype) !u8 {
const bitmask = try createBitmask(u8, bits);
return byte | bitmask;
}
test "setBits" {
try testing.expectEqual(setBits(0b0000_0000, .{0}), 0b0000_0001);
try testing.expectEqual(setBits(0b0000_0000, .{7}), 0b1000_0000);
try testing.expectEqual(setBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);
try testing.expectEqual(setBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);
try testing.expectEqual(setBits(0b0000_0000, .{ 2, 3, 4, 5 }), 0b0011_1100);
try testing.expectError(error.BitPosTooLarge, setBits(0b1111_1111, .{8}));
try testing.expectError(error.BitPosTooSmall, setBits(0b1111_1111, .{-1}));
}
fn clearBits(comptime byte: u8, comptime bits: anytype) !u8 {
const bitmask: u8 = try createBitmask(u8, bits);
return byte & ~@as(u8, bitmask);
}
test "clearBits" {
try testing.expectEqual(clearBits(0b1111_1111, .{0}), 0b1111_1110);
try testing.expectEqual(clearBits(0b1111_1111, .{7}), 0b0111_1111);
try testing.expectEqual(clearBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);
try testing.expectEqual(clearBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);
try testing.expectEqual(clearBits(0b1111_1111, .{ 0, 1, 6, 7 }), 0b0011_1100);
try testing.expectError(error.BitPosTooLarge, clearBits(0b1111_1111, .{8}));
try testing.expectError(error.BitPosTooSmall, clearBits(0b1111_1111, .{-1}));
}
fn toggleBits(comptime byte: u8, comptime bits: anytype) !u8 {
const bitmask = try createBitmask(u8, bits);
return byte ^ bitmask;
}
test "toggleBits" {
try testing.expectEqual(toggleBits(0b0000_0000, .{0}), 0b0000_0001);
try testing.expectEqual(toggleBits(0b0000_0000, .{7}), 0b1000_0000);
try testing.expectEqual(toggleBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);
try testing.expectEqual(toggleBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);
try testing.expectEqual(toggleBits(0b0000_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_0000);
try testing.expectEqual(toggleBits(0b0000_1111, .{ 0, 1, 2, 3 }), 0b0000_0000);
try testing.expectEqual(toggleBits(0b0000_0000, .{ 0, 2, 4, 6 }), 0b0101_0101);
try testing.expectError(error.BitPosTooLarge, toggleBits(0b1111_1111, .{8}));
try testing.expectError(error.BitPosTooSmall, toggleBits(0b1111_1111, .{-1}));
}
// ----------------------------------------------------------------------------
// Utility functions
// ----------------------------------------------------------------------------
fn newline() void {
print("\n", .{});
}
fn checkAnswer(expected: u4, answer: u4) void {
if (expected != answer) {
print("*************************************************************\n", .{});
print("= {b:0>4} <- INCORRECT! THE EXPECTED OUTPUT IS {b:0>4}\n", .{ answer, expected });
print("*************************************************************\n", .{});
} else {
print("= {b:0>4}", .{answer});
}
newline();
}