0.1

.github/workflows/main.yml

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+name: CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: install software
+      run: |
+        sudo apt-get -qq install llvm
+        ls -lt /usr/bin/llvm-objcopy*
+
+        wget --quiet --output-document=- https://github.com/github/hub/releases/download/v2.12.3/hub-linux-amd64-2.12.3.tgz | tar zx
+        mv hub-linux-* hub
+        ./hub/bin/hub --version
+
+        ZIG=$(wget --quiet --output-document=- https://ziglang.org/download/index.json | jq --raw-output '.master."x86_64-linux".tarball')
+        wget --quiet --output-document=- $ZIG | tar Jx
+        mv zig-linux-x86_64-* zig
+        echo zig version $(./zig/zig version)
+    - name: build
+      run: |
+        REPO=$(basename $GITHUB_REPOSITORY)
+        ./zig/zig build -Darmv6
+        cp -a $REPO-armv6.img boot/
+        ./zig/zig build -Darmv7
+        cp -a $REPO-armv7.img boot/
+    - name: release draft
+      env:
+        GITHUB_USER: $GITHUB_ACTOR
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        REPO=$(basename $GITHUB_REPOSITORY)
+        RELEASE_TAG=$(grep '^const release_tag =' src/main.zig | sed 's/";//' | sed 's/^.*"//')
+        RELEASE_ASSET=$REPO-$RELEASE_TAG.zip
+        pushd boot
+        echo $RELEASE_TAG > RELEASE.md
+        echo >> RELEASE.md
+        cat ../release-message.md >> RELEASE.md
+        zip -r $RELEASE_ASSET .
+        ../hub/bin/hub release create --draft --prerelease --file RELEASE.md --attach $RELEASE_ASSET $RELEASE_TAG
+        popd

.gitignore

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+*.bin
+*.dat
+*.elf
+*.img
+*.zip
+zig-cache/

README.md

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+zig logo is displayed
+
+Successfully tested on rpi3b, rpi3b+
+
+Not yet working on armv6 raspberry pi models

boot/config.txt

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+# https://www.raspberrypi.org/documentation/configuration/config-txt/
+
+disable_commandline_tags=1
+kernel_old=1
+dtparam=audio=on
+disable_splash=1
+boot_delay=0
+cec_osd_name=Zig!
+
+[rpi0]
+kernel=zig-bare-metal-raspberry-pi-armv6.img
+[rpi1]
+kernel=zig-bare-metal-raspberry-pi-armv6.img
+[rpi2]
+kernel=zig-bare-metal-raspberry-pi-armv7.img
+[rpi3]
+kernel=zig-bare-metal-raspberry-pi-armv7.img
+[rpi4]
+kernel=zig-bare-metal-raspberry-pi-armv7.img

build.zig

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+const std = @import("std");
+const Builder = std.build.Builder;
+const builtin = @import("builtin");
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const want_armv6 = b.option(bool, "armv6", "Build armv6 instead of armv7 (armv7 is default)") orelse false;
+    const want_armv7 = b.option(bool, "armv7", "Build armv7 instead of armv6 (armv7 is default)") orelse false;
+
+    const exec_name = "zig-bare-metal-raspberry-pi";
+    const exe = b.addExecutable(exec_name, "src/main.zig");
+    exe.setOutputDir("zig-cache");
+    exe.setBuildMode(mode);
+
+    var arch: builtin.Arch = undefined;
+    var subarch: u32 = undefined;
+    var kernel_name: []const u8 = undefined;
+    if (want_armv6) {
+        arch = builtin.Arch{ .arm = builtin.Arch.Arm32.v6 };
+        subarch = 6;
+        kernel_name = exec_name ++ "-armv6.img";
+    } else {
+        arch = builtin.Arch{ .arm = builtin.Arch.Arm32.v7 };
+        subarch = 7;
+        kernel_name = exec_name ++ "-armv7.img";
+    }
+    const os = builtin.Os.freestanding;
+    const environ = builtin.Abi.eabihf;
+    exe.setTarget(arch, builtin.Os.freestanding, environ);
+    exe.addBuildOption(u32, "subarch", subarch);
+
+    const linker_script = "src/linker.ld";
+    exe.setLinkerScriptPath(linker_script);
+
+    const run_objcopy = b.addSystemCommand([_][]const u8{
+        "llvm-objcopy-6.0", exe.getOutputPath(),
+        "-O", "binary",
+        kernel_name,
+    });
+    run_objcopy.step.dependOn(&exe.step);
+
+    b.default_step.dependOn(&run_objcopy.step);
+}

release-message.md

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+# Requires:
+* a raspberry pi model 3b or 3b+ with a power supply
+* a computer that can write an sd card
+* an sd card that you can erase - its contents will be destroyed
+* an hdmi tv and an hdmi cable
+
+# Steps:
+* with the computer
+    * format the sd card as FAT32
+        * this destroys the current contents of the sd card
+    * download the zip file
+    * unzip it to the sd card
+* insert the sd card into the pi
+* connect the pi to the tv using the hdmi cable
+* turn on the tv
+* apply power to the pi
+* you should see the zig logo displayed along a changing banner of colored lines

src/arm_assembly_code.zig

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+pub const PERIPHERAL_BASE = if (build_options.subarch >= 7) 0x3F000000 else 0x20000000;
+
+var already_panicking: bool = false;
+pub fn panicf(comptime fmt: []const u8, args: ...) noreturn {
+    @setCold(true);
+    if (already_panicking) {
+        hang("\npanicked during kernel panic");
+    }
+    already_panicking = true;
+
+    serial.log("panic: " ++ fmt, args);
+    hang("panic completed");
+}
+
+pub fn io(offset: u32) *volatile u32 {
+    return @intToPtr(*volatile u32, PERIPHERAL_BASE + offset);
+}
+
+pub fn ioStruct(comptime StructType: type, offset: u32) *volatile StructType {
+    return @intToPtr(*volatile StructType, PERIPHERAL_BASE + offset);
+}
+
+// Loop count times in a way that the compiler won't optimize away.
+pub fn delay(count: usize) void {
+    var i: usize = 0;
+    while (i < count) : (i += 1) {
+        asm volatile("mov r0, r0");
+    }
+}
+
+pub fn hang(comptime format: []const u8, args: ...) noreturn {
+    serial.log(format, args);
+    while (true) {
+        if (build_options.subarch >= 7) {
+            v7.wfe();
+        }
+    }
+}
+
+pub const v7 = struct {
+    pub inline fn mpidr() u32 {
+        var word = asm("mrc p15, 0, %[word], c0, c0, 5"
+            : [word] "=r" (-> usize));
+        return word;
+    }
+
+    pub inline fn wfe() void {
+        asm volatile("wfe");
+    }
+};
+
+pub fn sp() u32 {
+    var word = asm("mov %[word], sp"
+        : [word] "=r" (-> usize));
+    return word;
+}
+
+pub fn cpsr() u32 {
+    var word = asm("mrs %[word], cpsr"
+        : [word] "=r" (-> usize));
+    return word;
+}
+
+pub fn spsr() u32 {
+    var word = asm("mrs %[word], spsr"
+        : [word] "=r" (-> usize));
+    return word;
+}
+
+pub fn sctlr() u32 {
+    var word = asm("mrc p15, 0, %[word], c1, c0, 0"
+        : [word] "=r" (-> usize));
+    return word;
+}
+
+pub fn scr() u32 {
+    var word = asm("mrc p15, 0, %[word], c1, c1, 0"
+        : [word] "=r" (-> usize));
+    return word;
+}
+
+pub fn dsbSt() void {
+    if (build_options.subarch >= 7) {
+        asm volatile("dsb st");
+    } else {
+        asm volatile("mcr p15, 0, r0, c7, c10, 4"
+            :
+            :
+            : "r0");
+    }
+}
+
+pub fn setVectorBaseAddressRegister(address: u32) void {
+    asm volatile("mcr p15, #0, %[address], cr12, cr0, 0"
+        :
+        : [address] "{r0}" (address)
+    );
+}
+
+// The linker will make the address of these global variables equal
+// to the value we are interested in. The memory at the address
+// could alias any uninitialized global variable in the kernel.
+extern var __bss_start: u8;
+extern var __bss_end: u8;
+extern var __end_init: u8;
+
+pub fn setBssToZero() void {
+    @memset((*volatile [1]u8)(&__bss_start), 0, @ptrToInt(&__bss_end) - @ptrToInt(&__bss_start));
+}
+
+const build_options = @import("build_options");
+const serial = @import("serial.zig");

src/gpio.zig

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+pub fn setAlt5(pin_number: u32) void {
+      setPinPull(pin_number, Pull.None);
+      setPinFunction(pin_number, GPIO_FUNCTION_ALT5);
+}
+
+pub fn initOutputPinWithPullNone(pin_number: u32) void {
+      setPinPull(pin_number, Pull.None);
+      setPinFunction(pin_number, GPIO_FUNCTION_OUT);
+}
+
+pub fn setPinOutputBool(pin_number: u32, onOrOff: bool) void {
+    if (onOrOff) {
+        pins_set.write(pin_number, 1);
+    } else {
+        pins_clear.write(pin_number, 1);
+    }
+}
+
+fn setPinPull(pin_number: u32, pull: Pull) void {
+    GPPUD.* = @enumToInt(pull);
+    arm.delay(150);
+    pins_pull.write(pin_number, 1);
+    arm.delay(150);
+    GPPUD.* = @enumToInt(Pull.None);
+    pins_pull.write(pin_number, 0);
+}
+
+fn setPinFunction(pin_number: u32, function: u32) void {
+    pins_function.write(pin_number, function);
+}
+
+pub fn ioArrayOf(base: u32, field_size: u32, length: u32) type {
+    var IoArray = struct {
+        const Self = @This();
+
+        fn write(self: Self, index: u32, value: u32) void {
+            const field_mask = u32(1) << @intCast(u5, field_size - 1);
+            rangeCheck(index, length - 1);
+            rangeCheck(value, field_mask);
+            const fields_per_word = 32 / field_size;
+            const register = @intToPtr(*volatile u32, base + (index / fields_per_word) * 4);
+            const shift = @intCast(u5, (index % fields_per_word) * field_size);
+            var word = register.*;
+            word &= ~(field_mask << shift);
+            word |= value << shift;
+            register.* = word;
+        }
+    };
+    return IoArray;
+}
+
+fn rangeCheck(x: u32, max: u32) void {
+    if (x > max) {
+        panicf("{} exceeds max {}", x, max);
+    }
+}
+
+const pins_set: ioArrayOf(GPSET0, 1, GPIO_MAX_PIN) = undefined;
+const pins_clear: ioArrayOf(GPCLR0, 1, GPIO_MAX_PIN) = undefined;
+const pins_pull: ioArrayOf(GPPUDCLK0, 1, GPIO_MAX_PIN) = undefined;
+const pins_function: ioArrayOf(GPFSEL0, 3, GPIO_MAX_PIN) = undefined;
+
+const GPIO_MAX_PIN = 53;
+
+const GPFSEL0 = PERIPHERAL_BASE + 0x200000;
+const GPSET0 = PERIPHERAL_BASE + 0x20001C;
+const GPCLR0 = PERIPHERAL_BASE + 0x200028;
+const GPPUD = arm.io(0x200094);
+const GPPUDCLK0 = PERIPHERAL_BASE + 0x200098;
+
+const Pull = enum {
+    None,
+    Down,
+    Up,
+};
+
+const GPIO_FUNCTION_FIELD_SIZE: u32 = 3;
+const GPIO_FUNCTION_OUT: u32 = 1;
+const GPIO_FUNCTION_ALT5: u32 = 2;
+
+const arm = @import("arm_assembly_code.zig");
+const panicf = arm.panicf;
+const PERIPHERAL_BASE = arm.PERIPHERAL_BASE;