From 3faad6e17b03f13e91d4b93a525e6608c72c8ace Mon Sep 17 00:00:00 2001 From: Alexander Sisco <36649949+devspeare@users.noreply.github.com> Date: Sun, 2 Feb 2025 21:01:25 -0800 Subject: [PATCH] Suggesting a third exercise for bit manipulation --- build.zig | 32 ++ exercises/110_bit_manipulation3.zig | 443 ++++++++++++++++++++++++++++ 2 files changed, 475 insertions(+) create mode 100644 exercises/110_bit_manipulation3.zig diff --git a/build.zig b/build.zig index e528552..9774833 100644 --- a/build.zig +++ b/build.zig @@ -1208,6 +1208,38 @@ const exercises = [_]Exercise{ \\Max difference (new fn): 0.014 , }, + .{ .main_file = "110_bit_manipulation3.zig", .output = + \\Set pins with OR on PORTB + \\------------------------- + \\ 1001 // (initial state of PORTB) + \\| 0100 // (bitmask) + \\= 1101 + \\ + \\ 1001 // (reset state) + \\| 0100 // (bitmask) + \\= 1101 + \\ + \\Clear pins with AND and NOT on PORTB + \\------------------------------------ + \\ 1110 // (initial state of PORTB) + \\& 1011 // (bitmask) + \\= 1010 + \\ + \\ 0111 // (reset state) + \\& 1110 // (bitmask) + \\= 0110 + \\ + \\ + \\Toggle pins with XOR on PORTB + \\----------------------------- + \\ 1100 // (initial state of PORTB) + \\^ 0101 // (bitmask) + \\= 1001 + \\ + \\ 1100 // (initial state of PORTB) + \\^ 0011 // (bitmask) + \\= 1111 + }, .{ .main_file = "999_the_end.zig", .output = diff --git a/exercises/110_bit_manipulation3.zig b/exercises/110_bit_manipulation3.zig new file mode 100644 index 0000000..930f78a --- /dev/null +++ b/exercises/110_bit_manipulation3.zig @@ -0,0 +1,443 @@ +// ---------------------------------------------------------------------------- +// Setting, Clearing, and Toggling 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? This exercise is designed to +// give you a taste of what it's like to control registers in a +// microcontroller. Let's get started! +// +// 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. In this +// exercise, we'll take a closer look at bit manipulation and how we can write +// code that sets and clears specific bits as we would if we were programming +// the pins on a real microcontroller. Included at the end of this exercise 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 exercise in bit manipulation. It should be noted that +// in the following examples 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 examples 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 + // + // ------------------------------------------------------------------------ + // 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 (bit mask) + // ------ - - - - + // = 1011 1 0 1 1 + // \___\_______\ + // \ + // These bits remain untouched because OR-ing with + // a 0 effects no change. + // + // ------------------------------------------------------------------------ + // To create a bit mask 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) + + 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(); + + // ------------------------------------------------------------------------ + // 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 conjuction 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 (bit mask) + // ------ - - - - + // = 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 bit mask 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 bit mask is to invert the bits + // ~0100 -> 1011 + // we can write this as: + // ~(1 << 2) -> 1011 + // + // But if we try to compile ~(1 << 2), 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); + // + + 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(); + + + // + // ------------------------------------------------------------------------ + // 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); + + // 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: + + // 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(); +} +