diff --git a/src/lib.zig b/src/lib.zig
index da3d787..1dd7639 100644
--- a/src/lib.zig
+++ b/src/lib.zig
@@ -1,9 +1,105 @@
 const builtin = @import("builtin");
-extern "c" fn _errno() *c_int;
-extern "c" fn ptrace(c_int, c_int, usize, usize) c_long;
 extern "c" fn IsDebuggerPresent() bool;
+extern "c" fn __errno_location() *c_int;
+
+pub const KB = 1024;
+pub const MB = 1024 * 1024;
+pub const GB = 1024 * 1024 * 1024;
+
+pub fn assert(ok: bool) void {
+    if (!ok) {
+        @branchHint(.unlikely);
+        unreachable;
+    }
+}
+
 fn errno() Error {
-    return @enumFromInt(_errno().*);
+    return @enumFromInt(__errno_location().*);
+}
+
+fn align_forward(T: type, value: T, alignment: T) T {
+    assert(alignment != 0);
+    const mask = alignment - 1;
+    const result = (value + mask) & ~mask;
+    return result;
+}
+
+pub fn align_forward_u64(value: u64, alignment: u64) u64 {
+    return align_forward(u64, value, alignment);
+}
+
+pub fn align_forward_u32(value: u32, alignment: u32) u32 {
+    return align_forward(u32, value, alignment);
+}
+
+const ValueFromFlag = enum {
+    sub,
+    cmov,
+};
+const value_from_flag_kind = ValueFromFlag.sub;
+
+pub fn value_from_flag(value: anytype, flag: anytype) @TypeOf(value) {
+    const flag_int: @TypeOf(value) = switch (@TypeOf(flag)) {
+        comptime_int => b: {
+            if (flag != 1 and flag != 0) {
+                unreachable;
+            }
+            break :b flag;
+        },
+        bool => @intFromBool(flag),
+        u1 => flag,
+        else => @compileError("Unhandled type: " ++ @typeName(@TypeOf(flag))),
+    };
+
+    const result = switch (value_from_flag_kind) {
+        .cmov => {
+            @compileError("TODO");
+        },
+        .sub => value & (@as(@TypeOf(value), 0) -% flag_int),
+    };
+    return result;
+}
+
+test "value_from_flag" {
+    const std = @import("std");
+    const expect = std.testing.expect;
+
+    try expect(value_from_flag(1, 1) == 1);
+    try expect(value_from_flag(2, true) == 2);
+    try expect(value_from_flag(3, false) == 0);
+    try expect(value_from_flag(3, true) == 3);
+    try expect(value_from_flag(3, 1) == 3);
+
+    try expect(value_from_flag(0xffff, 1) == 0xffff);
+    try expect(value_from_flag(0xffff, 0) == 0);
+    try expect(value_from_flag(0xffff, true) == 0xffff);
+    try expect(value_from_flag(0xffff, false) == 0);
+
+    try expect(value_from_flag(0xffffffff, 1) == 0xffffffff);
+    try expect(value_from_flag(0xffffffff, 0) == 0);
+    try expect(value_from_flag(0xffffffff, true) == 0xffffffff);
+    try expect(value_from_flag(0xffffffff, false) == 0);
+
+    try expect(value_from_flag(0xffffffffffffffff, 1) == 0xffffffffffffffff);
+    try expect(value_from_flag(0xffffffffffffffff, 0) == 0);
+    try expect(value_from_flag(0xffffffffffffffff, true) == 0xffffffffffffffff);
+    try expect(value_from_flag(0xffffffffffffffff, false) == 0);
+
+    const a: u32 = 1235;
+    const b_true: bool = true;
+    const b_false: bool = false;
+    const u_true: u1 = 1;
+    const u_false: u1 = 0;
+    try expect(value_from_flag(a, b_true) == a);
+    try expect(value_from_flag(a, b_false) == 0);
+    try expect(value_from_flag(a, u_true) == a);
+    try expect(value_from_flag(a, u_false) == 0);
+
+    const b: u64 = 0xffffffffffffffff;
+    try expect(value_from_flag(b, b_true) == b);
+    try expect(value_from_flag(b, b_false) == 0);
+    try expect(value_from_flag(b, u_true) == b);
+    try expect(value_from_flag(b, u_false) == 0);
 }
 
 pub const Error = enum(c_int) {
@@ -11,13 +107,15 @@ pub const Error = enum(c_int) {
     PERM = 1,
 };
 
+pub const u64_max = ~@as(u64, 0);
+
 pub const os = struct {
     pub extern "c" fn exit(c_int) noreturn;
     pub fn is_being_debugged() bool {
         var result = false;
-        switch (builtin.os) {
+        switch (builtin.os.tag) {
             .linux => {
-                if (ptrace(0, 0, 0, 0) == -1) {
+                if (linux.ptrace(0, 0, 0, 0) == -1) {
                     result = errno() == Error.PERM;
                 }
             },
@@ -27,4 +125,148 @@ pub const os = struct {
 
         return result;
     }
+
+    const linux = struct {
+        const PROT_READ: u32 = 1 << 0;
+        const PROT_WRITE: u32 = 1 << 1;
+        const PROT_EXEC: u32 = 1 << 2;
+
+        const MAP_PRIVATE: u32 = 1 << 1;
+        const MAP_ANONYMOUS: u32 = 1 << 5;
+        const MAP_NORESERVE: u32 = 1 << 14;
+        const MAP_POPULATE: u32 = 1 << 15;
+
+        extern "c" fn ptrace(c_int, c_int, usize, usize) c_long;
+        extern "c" fn mmap(usize, usize, u32, u32, c_int, isize) *align(4096) anyopaque;
+        extern "c" fn mprotect(*anyopaque, usize, u32) c_int;
+
+        fn protection_flags(protection: ProtectionFlags) u32 {
+            const result = value_from_flag(linux.PROT_READ, protection.read) | value_from_flag(linux.PROT_WRITE, protection.write) | value_from_flag(linux.PROT_EXEC, protection.execute);
+            return result;
+        }
+    };
+
+    const ProtectionFlags = packed struct {
+        read: u1,
+        write: u1,
+        execute: u1,
+    };
+    const MapFlags = packed struct {
+        private: u1,
+        anonymous: u1,
+        no_reserve: u1,
+        populate: u1,
+    };
+
+    pub fn reserve(base: u64, size: u64, protection: ProtectionFlags, map: MapFlags) *align(4096) anyopaque {
+        switch (builtin.os.tag) {
+            .windows => @compileError("TODO"),
+            else => {
+                const protection_flags: u32 = value_from_flag(linux.PROT_READ, protection.read) | value_from_flag(linux.PROT_WRITE, protection.write) | value_from_flag(linux.PROT_EXEC, protection.execute);
+                const map_flags = value_from_flag(linux.MAP_ANONYMOUS, map.anonymous) | value_from_flag(linux.MAP_PRIVATE, map.private) | value_from_flag(linux.MAP_NORESERVE, map.no_reserve) | switch (builtin.os.tag) {
+                    .linux => value_from_flag(linux.MAP_POPULATE, map.populate),
+                    else => 0,
+                };
+                const address = linux.mmap(base, size, protection_flags, map_flags, -1, 0);
+                if (@intFromPtr(address) == u64_max) {
+                    @branchHint(.unlikely);
+                    unreachable;
+                }
+                return address;
+            },
+        }
+    }
+
+    fn commit(address: *anyopaque, size: u64, protection: ProtectionFlags) void {
+        switch (builtin.os.tag) {
+            .windows => @compileError("TODO"),
+            else => {
+                const protection_flags = linux.protection_flags(protection);
+                const result = linux.mprotect(address, size, protection_flags);
+                if (result != 0) {
+                    unreachable;
+                }
+            },
+        }
+    }
+};
+
+pub const Arena = struct {
+    reserved_size: u64,
+    position: u64,
+    os_position: u64,
+    granularity: u64,
+    reserved: [32]u8 = [1]u8{0} ** 32,
+
+    const minimum_position = @sizeOf(Arena);
+
+    const Initialization = struct {
+        reserved_size: u64,
+        granularity: u64,
+        initial_size: u64,
+    };
+
+    pub fn initialize(initialization: Initialization) *Arena {
+        const protection_flags = os.ProtectionFlags{
+            .read = 1,
+            .write = 1,
+            .execute = 0,
+        };
+        const map_flags = os.MapFlags{
+            .private = 1,
+            .anonymous = 1,
+            .no_reserve = 1,
+            .populate = 0,
+        };
+        const arena: *Arena = @ptrCast(os.reserve(0, initialization.reserved_size, protection_flags, map_flags));
+        os.commit(arena, initialization.initial_size, .{
+            .read = 1,
+            .write = 1,
+            .execute = 0,
+        });
+
+        arena.* = .{
+            .reserved_size = initialization.reserved_size,
+            .os_position = initialization.initial_size,
+            .position = minimum_position,
+            .granularity = initialization.granularity,
+        };
+
+        return arena;
+    }
+
+    const default_size = 4 * GB;
+    const minimum_granularity = 4 * KB;
+
+    pub fn initialize_default(initial_size: u64) *Arena {
+        const arena = initialize(.{
+            .reserved_size = default_size,
+            .granularity = minimum_granularity,
+            .initial_size = initial_size,
+        });
+        return arena;
+    }
+
+    pub fn allocate_bytes(arena: *Arena, size: u64, alignment: u64) *u8 {
+        const aligned_offset = align_forward_u64(arena.position, alignment);
+        const aligned_size_after = aligned_offset + size;
+
+        if (aligned_size_after > arena.os_position) {
+            const target_committed_size = align_forward_u64(aligned_size_after, arena.granularity);
+            const size_to_commit = target_committed_size - arena.os_position;
+            const commit_pointer = @as(*anyopaque, @ptrFromInt(@intFromPtr(arena) + arena.os_position));
+            os.commit(commit_pointer, size_to_commit, .{
+                .read = 1,
+                .write = 1,
+                .execute = 0,
+            });
+            arena.os_position = target_committed_size;
+        }
+
+        const result = @as(*u8, @ptrFromInt(@intFromPtr(arena) + aligned_offset));
+        arena.position = aligned_size_after;
+        assert(arena.position <= arena.os_position);
+
+        return result;
+    }
 };
diff --git a/src/main.zig b/src/main.zig
index 00c3838..3e515a5 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -11,6 +11,8 @@ pub fn panic(message: []const u8, stack_trace: ?*anyopaque, return_address: ?usi
 }
 
 pub fn main() callconv(.C) c_int {
+    const arena = lib.Arena.initialize_default(2 * lib.MB);
+    _ = arena.allocate_bytes(1024, 1);
     return 0;
 }
 
@@ -21,3 +23,7 @@ comptime {
         });
     }
 }
+
+test {
+    _ = lib;
+}