adding reference impementation for reading files from fat

build.zig

@@ -18,16 +18,40 @@ pub fn build(b: *std.Build) void {
     var floppy = createFloppy(b, "main_floppy.img");
     floppy.run = formatFloppyFat12(b, floppy);
 
+    var installToFloppy = installImgToFloppy(b, floppy, initramfs, &.{
+        NamedFile{ .file = .{ .runFile = kernel }, .name = "::kernel.bin" },
+        NamedFile{ .file = .{ .simpleFile = b.path("src/reference_implementations/test.txt") }, .name = "::test.txt" },
+    });
+
     const img_install = b.addInstallBinFile(floppy.obj, "main_floppy.img");
-    img_install.step.dependOn(&installImgToFloppy(b, floppy, initramfs, kernel).step);
+    img_install.step.dependOn(&installToFloppy);
 
     b.getInstallStep().dependOn(&img_install.step);
 
+    const target = b.standardTargetOptions(.{});
+    const fat_reference_mod = b.createModule(.{
+        .root_source_file = b.path("src/reference_implementations/fat.zig"),
+        .target = target,
+        .link_libc = true,
+    });
+
     const run = b.addSystemCommand(&.{ "qemu-system-i386", "-fda" });
     run.addFileArg(floppy.obj);
 
     const run_step = b.step("run", "run the os in qemu");
     run_step.dependOn(&run.step);
+
+    const fat_reference = b.addExecutable(.{
+        .root_module = fat_reference_mod,
+        .name = "fat_reference_implementation",
+        .use_lld = false,
+        .use_llvm = false,
+    });
+
+    const fat_install = b.addInstallArtifact(fat_reference, .{});
+
+    const build_references = b.step("references", "build the reference implementations");
+    build_references.dependOn(&fat_install.step);
 }
 
 const AddNasmFilesOptions = struct {
@@ -43,6 +67,11 @@ const NasmFile = struct {
 
 const Floppydisk = NasmFile;
 
+const NamedFile = struct {
+    file: union(enum) { runFile: NasmFile, simpleFile: std.Build.LazyPath },
+    name: []const u8,
+};
+
 fn createFloppy(b: *std.Build, name: []const u8) Floppydisk {
     const dd = b.addSystemCommand(&.{ "dd", "if=/dev/zero" });
     const Floppy = dd.addPrefixedOutputFileArg("of=", name);
@@ -63,7 +92,7 @@ fn formatFloppyFat12(b: *std.Build, floppy: Floppydisk) *std.Build.Step.Run {
     return mkfs;
 }
 
-fn installImgToFloppy(b: *std.Build, floppy: Floppydisk, bootloader: NasmFile, kernel: NasmFile) *std.Build.Step.Run {
+fn installImgToFloppy(b: *std.Build, floppy: Floppydisk, bootloader: NasmFile, additional_files: []const NamedFile) std.Build.Step {
     const dd = b.addSystemCommand(&.{"dd"});
     dd.addPrefixedFileArg("if=", bootloader.obj);
     dd.addPrefixedFileArg("of=", floppy.obj);
@@ -71,14 +100,22 @@ fn installImgToFloppy(b: *std.Build, floppy: Floppydisk, bootloader: NasmFile, k
     dd.step.dependOn(&floppy.run.step);
     dd.step.dependOn(&bootloader.run.step);
 
-    const mcopy = b.addSystemCommand(&.{"mcopy"});
-    mcopy.addPrefixedFileArg("-i", floppy.obj);
-    mcopy.addFileArg(kernel.obj);
-    mcopy.addArg("::kernel.bin");
-    mcopy.step.dependOn(&dd.step);
-    mcopy.step.dependOn(&kernel.run.step);
-
-    return mcopy;
+    var mcopy_step = std.Build.Step.init(.{ .name = "mcopy", .owner = b, .id = .custom });
+    mcopy_step.dependOn(&dd.step);
+    for (additional_files) |f| {
+        const mcopy = b.addSystemCommand(&.{"mcopy"});
+        mcopy.addPrefixedFileArg("-i", floppy.obj);
+        switch (f.file) {
+            .runFile => |file| {
+                mcopy.addFileArg(file.obj);
+                mcopy_step.dependOn(&file.run.step);
+            },
+            .simpleFile => |file| mcopy.addFileArg(file),
+        }
+        mcopy.addArg(f.name);
+        mcopy_step.dependOn(&mcopy.step);
+    }
+    return mcopy_step;
 }
 
 // adapted from https://codeberg.org/raddari/zig-nasm-lib.git

src/reference_implementations/fat.zig

@@ -0,0 +1,117 @@
+const std = @import("std");
+
+const Errors = error{ InvalidSyntax, ReadSectors, FileNotFound };
+
+var buffer: [1024]u8 = undefined;
+
+var g_Fat: []u8 = undefined;
+
+const BootSector = extern struct {
+    BootJumpInstruction: [3]u8 align(1),
+    OemIdentifier: [8]u8 align(1),
+    BytesPerSector: u16 align(1),
+    SectorsPerCluster: u8 align(1),
+    ReservedSectors: u16 align(1),
+    FatCount: u8 align(1),
+    DirEntryCount: u16 align(1),
+    TotalSectors: u16 align(1),
+    MediaDescriptorType: u8 align(1),
+    SectorsPerFat: u16 align(1),
+    SectorsPerTrack: u16 align(1),
+    Heads: u16 align(1),
+    HiddenSectors: u32 align(1),
+    LargeSectorCount: u32 align(1),
+
+    // ebr
+    DriveNumber: u8 align(1),
+    _Reserved: u8 align(1),
+    Signature: u8 align(1),
+    VolumeId: u32 align(1),
+    VolumeLaber: [11]u8 align(1),
+    SystemId: [8]u8 align(1),
+};
+
+var g_BootSector: BootSector = undefined;
+
+const DirectoryEntry = extern struct {
+    Name: [11]u8 align(1),
+    Attributes: u8 align(1),
+    _Reserved: u8 align(1),
+    CreatedTimeTenths: u8 align(1),
+    CreatedTime: u16 align(1),
+    CreatedDate: u16 align(1),
+    AccessedDate: u16 align(1),
+    FirstClusterHigh: u16 align(1),
+    ModifiedTime: u16 align(1),
+    ModifiedDate: u16 align(1),
+    FirstClusterLow: u16 align(1),
+    Size: u32 align(1),
+};
+
+var g_RootDirectory: []DirectoryEntry = undefined;
+
+fn readStruct(reader: *std.fs.File.Reader, T: type) !T {
+    var ret: [@sizeOf(T)]u8 = undefined;
+    _ = try reader.*.read(&ret);
+    return @bitCast(ret);
+}
+
+fn readBootSector(disk: *std.fs.File) !void {
+    var reader = disk.*.reader(&buffer);
+    g_BootSector = try readStruct(&reader, BootSector);
+}
+
+fn readSectors(disk: *std.fs.File, lba: u32, count: u32, bufferOut: *[]anyopaque) !void {
+    const dest: []u8 = try std.heap.smp_allocator.alloc(u8, g_BootSector.BytesPerSector * count);
+    defer std.heap.smp_allocator.free(dest);
+    var stream = disk.readerStreaming(&buffer);
+    try stream.seekTo(lba * g_BootSector.BytesPerSector);
+    if (try stream.readStreaming(dest) != g_BootSector.BytesPerSector * count) return Errors.ReadSectors;
+    @memcpy(@as(*[]u8, @ptrCast(bufferOut)).*, dest);
+}
+
+fn readFat(disk: *std.fs.File) !void {
+    g_Fat = try std.heap.smp_allocator.alloc(u8, g_BootSector.SectorsPerFat * g_BootSector.BytesPerSector);
+    errdefer std.heap.smp_allocator.free(g_Fat);
+    try readSectors(disk, g_BootSector.ReservedSectors, g_BootSector.SectorsPerFat, &g_Fat);
+}
+
+fn readRootDirectory(disk: std.fs.File) !void {
+    const lba: u32 = g_BootSector.ReservedSectors + g_BootSector.SectorsPerFat + g_BootSector.FatCount;
+    const size: u32 = @sizeOf(DirectoryEntry) * g_BootSector.DirEntryCount;
+    const sectors: u32 = (size / g_BootSector.BytesPerSector);
+    if (@rem(size, g_BootSector.BytesPerSector > 0))
+        sectors += 1;
+
+    g_RootDirectory = try std.heap.smp_allocator.alloc(DirectoryEntry, sectors * g_BootSector.BytesPerSector);
+    errdefer std.heap.smp_allocator.free(g_RootDirectory);
+    try readSectors(disk, lba, sectors, &g_RootDirectory);
+}
+
+fn findFile(name: []const u8) !*DirectoryEntry {
+    for (g_RootDirectory, 0..) |Entry, i|
+        if (std.mem.eql(u8, Entry.Name, name))
+            return &g_RootDirectory[i];
+    return Errors.FileNotFound;
+}
+
+pub fn main() !void {
+    if (std.os.argv.len < 3) {
+        std.log.err("syntax: {s} <disk image> <file name>", .{std.os.argv[0]});
+        return Errors.InvalidSyntax;
+    }
+
+    var disk = try std.fs.cwd().openFileZ(std.os.argv[1], .{ .mode = .read_only });
+
+    _ = &disk;
+    try readBootSector(&disk);
+
+    try readFat(&disk);
+    defer std.heap.smp_allocator.free(g_Fat);
+
+    try readRootDirectory(disk);
+    defer std.heap.smp_allocator.free(g_RootDirectory);
+
+    const fileEntry = try findFile(std.os.argv[2]);
+    _ = fileEntry;
+}

src/reference_implementations/test.txt

@@ -0,0 +1,3 @@
+testing file
+aabaacaadaaeaafaagaahaaiaajaakaalaamaanaaoaapaaqaaraasaataauaavaawaaxaayaaz
+tstngfl