diff --git a/src/header.zig b/src/header.zig
index 9872271..6a6169a 100644
--- a/src/header.zig
+++ b/src/header.zig
@@ -1,118 +1,105 @@
 const std = @import("std");
-const shell = @import("shell.zig");
+const native_endian = @import("builtin").target.cpu.arch.endian();
+const mem = std.mem;
+const max_shells = @import("shell.zig").max_shells;
 
-const HeaderSize = @divExact(@bitSizeOf(Header), 8);
-const Magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
-const Version = 0;
-const Bom = 0x1234;
+const header_size = @sizeOf(Header);
+const magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
+const version = 0;
 
-pub const SectionLength = 64;
+const Endian = enum(u8) {
+    big,
+    little,
 
-const InvalidHeader = error{
-    InvalidMagic,
-    InvalidVersion,
-    InvalidBom,
-    TooManyShells,
-    InvalidOffset,
+    fn native() Endian {
+        return switch (native_endian) {
+            .Little => Endian.little,
+            .Big => Endian.big,
+        };
+    }
 };
 
-const Header = packed struct {
-    magic: [4]u8,
-    version: u8,
-    bom: u16,
+pub const section_length_bits = 6;
+pub const section_length = 1 << section_length_bits;
+
+pub const InvalidHeader = error{
+    InvalidMagic,
+    InvalidVersion,
+    InvalidEndianess,
+    TooManyShells,
+};
+
+pub const Header = packed struct {
+    magic: [4]u8 = magic,
+    version: u8 = version,
+    endian: Endian = Endian.native(),
+    nblocks_shell_blob: u8,
     num_shells: u8,
-    num_users: u32,
     num_groups: u32,
-    offset_bdz_uid2user: u32,
-    offset_bdz_groupname2group: u32,
-    offset_bdz_username2user: u32,
-    offset_idx: u32,
-    offset_groups: u32,
-    offset_users: u32,
-    offset_groupmembers: u32,
-    offset_additional_gids: u32,
+    num_users: u32,
+    nblocks_bdz_gid: u32,
+    nblocks_bdz_groupname: u32,
+    nblocks_bdz_uid: u32,
+    nblocks_bdz_username: u32,
+    nblocks_groups: u64,
+    nblocks_users: u64,
+    nblocks_groupmembers: u64,
+    nblocks_additional_gids: u64,
 
-    pub fn init(blob: [HeaderSize]u8) InvalidHeader!Header {
-        const self = @bitCast(Header, blob);
-        for (Magic) |item, index| {
-            if (self.magic[index] != item) return error.InvalidMagic;
-        }
-        if (self.version != 0) {
+    pub fn fromBytes(blob: [header_size]u8) InvalidHeader!Header {
+        const self = mem.bytesAsValue(Header, blob);
+
+        if (!mem.eql(magic, blob[0..4]))
+            return error.InvalidMagic;
+
+        if (self.version != 0)
             return error.InvalidVersion;
-        }
-        if (self.bom != Bom) {
-            return error.InvalidBom;
-        }
-        if (self.num_shells > shell.max_shells) {
-            return error.TooManyShells;
-        }
 
-        const offsets = [_]u32{
-            self.offset_bdz_uid2user,
-            self.offset_bdz_groupname2group,
-            self.offset_bdz_username2user,
-            self.offset_idx,
-            self.offset_groups,
-            self.offset_users,
-            self.offset_groupmembers,
-            self.offset_additional_gids,
-        };
-        for (offsets) |offset| {
-            if (offset & (SectionLength - 1) != 0) {
-                return error.InvalidOffset;
-            }
-        }
+        if (self.endian != Endian.native())
+            return error.InvalidEndianess;
+
+        if (self.num_shells > max_shells)
+            return error.TooManyShells;
 
         return self;
     }
 
-    pub fn asArray(self: *const Header) [HeaderSize]u8 {
-        return @bitCast([HeaderSize]u8, self.*);
+    pub fn asBytes(self: *const Header) [header_size]u8 {
+        return mem.asBytes(self);
     }
 };
 
 const testing = std.testing;
 
 test "Section length is a power of two" {
-    try testing.expect(std.math.isPowerOfTwo(SectionLength));
+    try testing.expect(std.math.isPowerOfTwo(section_length));
+}
+
+test "bit header size is equal to @sizeOf(Header)" {
+    try testing.expectEqual(@sizeOf(Header) * 8, @bitSizeOf(Header));
 }
 
 test "header pack, unpack and validation" {
-    const goodHeader = Header{
-        .magic = Magic,
-        .version = Version,
-        .bom = Bom,
-        .num_shells = 0,
-        .num_users = 0,
-        .num_groups = 0,
-        .offset_bdz_uid2user = 0,
-        .offset_bdz_groupname2group = 0,
-        .offset_bdz_username2user = 0,
-        .offset_idx = 0,
-        .offset_groups = 0,
-        .offset_users = 0,
-        .offset_groupmembers = 0,
-        .offset_additional_gids = 0,
-    };
+    //const goodHeader = Header{};
 
-    const gotHeader = try Header.init(goodHeader.asArray());
-    try testing.expectEqual(goodHeader, gotHeader);
+    //const gotHeader = try Header.init(goodHeader.asArray());
+    //try testing.expectEqual(goodHeader, gotHeader);
 
-    {
-        var header = goodHeader;
-        header.magic[0] = 0;
-        try testing.expectError(error.InvalidMagic, Header.init(header.asArray()));
-    }
+    //{
+    //    var header = goodHeader;
+    //    header.magic[0] = 0;
+    //    try testing.expectError(error.InvalidMagic, Header.init(header.asArray()));
+    //}
 
-    {
-        var header = goodHeader;
-        header.bom = 0x3412;
-        try testing.expectError(error.InvalidBom, Header.init(header.asArray()));
-    }
+    //{
+    //    var header = goodHeader;
+    //    header.bom = 0x3412;
+    //    try testing.expectError(error.InvalidBom, Header.init(header.asArray()));
+    //}
 
-    {
-        var header = goodHeader;
-        header.offset_bdz_uid2user = 65;
-        try testing.expectError(error.InvalidOffset, Header.init(header.asArray()));
-    }
+    //{
+    //    var header = goodHeader;
+    //    header.offset_bdz_uid2user = 65;
+    //    try testing.expectError(error.InvalidOffset, Header.init(header.asArray()));
+    //}
 }
diff --git a/src/sections.zig b/src/sections.zig
index aab0859..98db2cb 100644
--- a/src/sections.zig
+++ b/src/sections.zig
@@ -23,7 +23,10 @@ const PackedGroup = @import("group.zig").PackedGroup;
 const ShellSections = @import("shell.zig").ShellWriter.ShellSections;
 const ShellReader = @import("shell.zig").ShellReader;
 const ShellWriter = @import("shell.zig").ShellWriter;
+const Header = @import("header.zig").Header;
 const max_shells = @import("shell.zig").max_shells;
+const section_length_bits = @import("header.zig").section_length_bits;
+const section_length = @import("header.zig").section_length;
 const cmph = @import("cmph.zig");
 const bdz = @import("bdz.zig");
 
@@ -390,6 +393,13 @@ fn cmpGroup(_: void, a: Group, b: Group) bool {
     return a.gid < b.gid;
 }
 
+// nblocks returns how many blocks a particular slice will take.
+fn nblocks(arr: []const u8) u32 {
+    const upper = pad.roundUp(u38, section_length_bits, @intCast(u38, arr.len));
+    assert(upper & (section_length - 1) == 0);
+    return @truncate(u32, upper >> 6);
+}
+
 pub const AllSections = struct {
     allocator: Allocator,
 
@@ -407,6 +417,7 @@ pub const AllSections = struct {
     idx_groupname2group: []const u32,
     idx_uid2user: []const u32,
     idx_name2user: []const u32,
+    header: Header,
 
     pub fn init(
         allocator: Allocator,
@@ -456,6 +467,21 @@ pub const AllSections = struct {
         var idx_name2user = try bdzIdx([]const u8, allocator, bdz_username, unames, users.idx2offset);
         errdefer allocator.free(idx_name2user);
 
+        const header = Header{
+            .nblocks_shell_blob = undefined,
+            .num_shells = undefined,
+            .num_groups = undefined,
+            .num_users = undefined,
+            .nblocks_bdz_gid = undefined,
+            .nblocks_bdz_groupname = undefined,
+            .nblocks_bdz_uid = undefined,
+            .nblocks_bdz_username = undefined,
+            .nblocks_groups = undefined,
+            .nblocks_users = undefined,
+            .nblocks_groupmembers = undefined,
+            .nblocks_additional_gids = undefined,
+        };
+
         return AllSections{
             .allocator = allocator,
             .bdz_gid = bdz_gid,
@@ -475,6 +501,7 @@ pub const AllSections = struct {
             .idx_groupname2group = idx_groupname2group,
             .idx_uid2user = idx_uid2user,
             .idx_name2user = idx_name2user,
+            .header = header,
         };
     }
 
@@ -745,3 +772,18 @@ test "bdzIdx on str" {
     defer testing.allocator.free(result);
     try expectUsedHashes(testing.allocator, result);
 }
+
+test "nblocks" {
+    const T = struct { want: u32, arr: []const u8 };
+    const tests = &[_]T{
+        .{ .want = 0, .arr = &[_]u8{} },
+        .{ .want = 1, .arr = &[_]u8{ 1, 2, 42 } },
+        .{ .want = 1, .arr = &[_]u8{1} ** 63 },
+        .{ .want = 1, .arr = &[_]u8{1} ** 64 },
+        .{ .want = 2, .arr = &[_]u8{1} ** 65 },
+    };
+
+    for (tests) |tt| {
+        try testing.expectEqual(tt.want, nblocks(tt.arr));
+    }
+}