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Tiny C++ boot loader

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# About

"I wrote a bootloader in C++, [I AM A CHAMPION!](https://www.youtube.com/watch?v=j4miM_sZU-k)"

Here is a pre-compiled version: https://goliath32.com/hausboot/hausboot.tar.xz

It includes a disk image, scripts to run it and the individual components
as separate binaries.

The abomination in this repository consists of:

- An MBR boot sector that chain loads a sector form a bootable partition.
- A VBR (partition boot sector) designed to fit into a FAT32 super block,
  chain loads a second stage from hidden sectors.
- A second stage that loads a kernel binary from a FAT32 file system into
  high memory, enters protected mode with A20 enabled and runs the kernel.
- A tiny [multiboot compliant](https://en.wikipedia.org/wiki/Multiboot_specification)
  kernel that prints out some info and then [HALTs](https://www.youtube.com/watch?v=tRgwr_4l6BE).

The MBR & VBR were initially written as a [proof-of-concept](https://www.youtube.com/watch?v=vq7NFBm2oRw)
and "told you so" to show off that a boot sector could be written in
what's mostly high-level C++.

The second stage FAT32 kernel loader is the result of [unhealthy obsession
to see this through](https://www.youtube.com/watch?v=QFKzaXW8_Ek), no matter
how stupid the whole thing is.

This is *not* intended, nor in any way, shape or form [fit for any real world use](https://www.youtube.com/watch?v=QfN1GRqKXpM).

## Background

TODO: make the weird ramblings less incoherent and turn it into a blog post?

I learned to program with QBASIC and 8086 assembly under MS-DOS. I later
learned how a CPU works from the gate level up, before I learned C. Over the
years, I used C++ on-again-off-again, having to re-learn it every time
since 2011, because the standards commedy keeps changing the darn thing.

On your typical beginner forums, there were many who ambitiously wanted to
write a computer game, or an OS. For both there are many dedicated websites,
with tutorials. If you go down the OS route, you end up being told that OS
development is *all about* the nookies and crannies of the x86 architecture.
Instead of learning about scheduling, memory management or filesystems, you
learn about the intricacies of the protected mode and the A20 gate.

Among other things, you are told that boot loaders are hand written in assembly.
You cannot write a boot sector in C, C is too big and FAT. Booting
happens [below C level](https://source.denx.de/u-boot/u-boot).

Even after many years of C programming, a real eye-opener for me was attending
a (now sadly discontinued) masters course on compiler construction. Turns out,
compilers are *really good*. Even my toy optimizing compiler I wrote that
semester could churn out surprisingly efficient assembly. And I'm pretty sure
the gcc people are a lot smarter than me, in addition to a 30 year head start.

To a degree, it changed my way of thinking about programming. Source code should
first and foremost be written *for people to read*. Source code should
convey *high level meaning*, both to people, as well as to the compiler. Bit
twiddling hacks are an utter waste of time and make the job harder for the
compiler as well. Keeping down asymptotic complexity is your job, optimizing the
coefficient is the compilers job (but you should really be thinking in terms of
data structures anyway, rather than algorithms).

Programming languages really are a *formal language* based *calculus* that
describes *meaning*. A compiler is essentially a term-rewriting system and you
are operating with the semantics of that *calculus*. Adages like "C is just an
assembly pre-processor" are not just nonsense, but outright dangerous. The chasm
between C semantics and machine semantics are what gives rise to fear and
misunderstanding around the dreaded undefined-behavior-boogy-man that HN loves
to gossip about so much.

Pondering further on that, I concluded that an expressive, strong type system
is really key to expressing such high level meaning and semantics. A modern,
high-level language should IMO not have bit manipulation *at all*. Flag fields
are really just composite data types in disguise, model them as such and gain
type safety! I'd go as far as claim that memory-safety issues are themselves
really just a *symptom* of flaws in the type system.

**Anyway**, revisiting C++ once again, it made me realize that you can model
all of those high level type semantics really well with classes. Thinking about
classes as user defined *types* is an immensely powerful concept, and C++ even
allows you to micro manage memory layout if need be. If leveraged, C++
effectively hands you tools of Ada like proportions, you just need to use them.

Thinking about C++ and compiler construction gave me the shower-thought
realization that it should absolutely be possible to write a boot sector
in C++. And after all, C++ is a "systems programming language", right? :-P

I found the idea especially funny, because much of the criticism I heard/read
about C++ over the years pertained to C++ programs being big, bloated resource
hogs. Also I found the idea to be an interesting challenge/exercise. Which is
what brings us here.

## How to Build This

You need a *very* recent version of g++, as well as the 32 bit version
of `libstdc++` if you are on x86_64 (if you are on "Apple Silicon": sorry...).

The project uses the [Meson](https://mesonbuild.com/) build system, to compile
the sources, simply run:

```sh
meson setup ./build
cd build
meson compile
```

A sample disk image, along with a [Bochs](https://en.wikipedia.org/wiki/Bochs)
config file are generated in `test/` in the build directory.

To run it in bochs simply run:

```sh
bochs -q -f /path/to/bochsrc.txt
```

Alternatively, you can try running it in Qemu:

```sh
qemu-system-i386 -drive format=raw,file=/path/to/disk.img
```

With any luck, it might work on your machine as well :-). I have only tested it
with Bochs and Qemu on two Fedora installations and an OpenSuSE machine so far.

# Boot Strapping Process

## Master Boot Record (MBR)

When you turn on an (legacy BIOS) PC, we are basically transported back to
the 1980s again. The CPU pretends to be a 16 bit mode 8086 that can address
up to 1M of memory using segmentation.

After reset, the 8086 starts execution at FFFF:0000, or some such, i.e. at
linear address 0xFFFF0. The IBM PC hardware has a ROM chip mapped at that region
that contains the BIOS. The lower 640k of address space are left to the
operating system, including stuff like memory mapped hardware (e.g. video RAM).
This is called "conventional memory" for some reason (as opposed to
*unconventional* memory?).

The boot strap code in the BIOS simply looks for a floppy or hard drive that
has the magic signature 0xAA55 at the end of the first sector. This magic value
indicates a bootable drive. The BIOS simply copies the first sector to the
magic address 0x7C00 and jumps into it.

This would typically contain an OS boot loader in hand written assembly that
fetches a bigger executable from the same disk. This would possibly be a
second stage loader that can read the file system and load the OS kernel.

Eventually somebody figured, it would be a jolly idea to have partitioned
hard disks on the IBM too! This was implemented by storing a partition table
at the end of the first sector. Instead of the original boot strap program,
the first sector now contains the "Master Boot Record" that is
loaded & executed by the BIOS. It relocates itself (typically to `0x0600`;
that's what Microsoft went with), inspects the partition table and chain
loads the actual boot loader from the first sector of a partition marked
as bootable.

The BIOS code was left untouched, and doesn't need to know about partitions.
It's completely an OS feature, but the partition boot loader needs to be aware
of it, hence we started calling those "Volume Boot Record" (VBR).

The "standard" layout of the partition table is simply what DOS et al settled
on, using 4 entries, each 16 byte. Later schemes like GPT with UUID partition
labels and additional, *extended* partitions are built on top of that in a
backwards compatible way, which is why you can still only have up to 4 regular,
non-extended partitions there.

But with a typical, old DOS MBR, this leaves us with 446 bytes of payload
area for the MBR program. For comparison: if we used 80-column IBM cards,
that would be roughly 6 or 7 punched cards (depending on the format).

In the directory `mbr`, there is C++ source for a program that fits in this
space and does the task described above. The bulk of the implementation is
actually in C++ classes defined in headers in the `include` directory.

## Volume Boot Record (VBR)

The volume boot record sits in the first sector of the partition and is
chain-loaded by the MBR.

It typically shares the sector with the super block of a filesystem. For
instance, FAT32 uses the first 90 bytes for filesystem information. The
first 3 bytes hold a jump instruction, so we don't accidentally try to
execute the filesystem information.

Subtracting the super block and 2 byte boot signature, that leaves a FAT32 VBR
with 420 bytes of payload for a program (6 punched cards).

So what the FAT32 VBR does, is chain-load a much larger program, stored in
reserved sectors between the super block and the first FAT. We are guaranteed
at least 32 reserved sectors, but this count includes the first sector itself,
the special FS information sector, and a backup copy of the first sector (can
be disabled).

If we disable the backup copy and relocate the FS information sector, we can
max that out to 30 sectors, or a *whopping 15k* for our second stage
boot loader.

In the `vbr` directory, there is a C++ program that should fit into
that 420 byte region, and chain loads the second stage. The `installfat`
program from the `tools` directory contains C++ source for a program that
modifies a FAT image as described and installs the VBR, as well as the
second stage binaries.

## Second Stage Boot Loader

The second stage boot loader has enough breathing space to do some more hardware
initialization and information gathering (e.g. getting the memory layout and
enabling the [A20 line](https://en.wikipedia.org/wiki/A20_line)).

It implements an actual FAT32 reader, reads a config file from disk that tells
it from where to load the kernel.

The kernel is copied to high memory, the BSS is zeroed out, and a multiboot
information structure is setup up, before finally switching into protected
mode and calling the kernel.

The code for this sits in `stage2`, but once again, most of the magic happens
in the headers included from `include`. In the `tools` directory, there is a
program called `fatedit` that can fumble files into a FAT32 image, so we don't
have to mount the silly thing.

The FAT32 parser is currently limited to short names only.

The multiboot code is also fairly limited. It does not support parsing ELF files
and insists that the kernel provides memory layout information instead.

## The FAT filesystem

The FAT (**f**latulent, **a**rchaic **t**rash) filesystem was the original,
native filesystem for DOS (**d**ead **o**perating **s**ystem) and has over the
years become the lingua franca of filesystems, supported by practically every
relevant OS, typically used on external storage media that can be exchanged
between them. At it's core it is idiotically simple, but full of quirks and
semi backwards compatible extensions.

For those interested: [Microsoft published a white paper on it](https://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/fatgen103.doc)

It mainly consists of a super block and a "file allocation table" that manages
the data blocks on the disk. Because power can fail during writes, there are
typically 2 of those.

The data blocks are called *clusters*, indexing starts after the allocation
table (the first one has index 2 for some reason).

Data belonging to a file is stored using a linked list of clusters. The links
are stored in the allocation table, which is simply an array of cluster indices.
Basically if you know the cluster index that you are currently looking at, you
use that as an index into the array and the value stored there is the index of
the next cluster where you should continue reading.

There are special sentinel values to indicate that there is no next cluster (the
one you are holding is the last one). Unused clusters are marked with special
value 0 in the table.

A directory is basically a file that contains a sequence of entry records,
storing a name, the first cluster where the entry data begins, how big it is,
and some flags (e.g. this is not a file but actually a directory). The super
block tells us the cluster index of the root directory, so we can recursively
scan our way through the tree.

The main difference between FAT12, FAT16 and FAT32 is the number of bits used for
the cluster indices in the table, i.e. 12, 16 and "28 out of 32" respectively.
Also, FAT12 and FAT16 had some special case handling for the root directory.
This probably has boot loader related reasons, FAT32 instead simply has a
reserved region between the VBR and the first FART, where we can park a second
stage boot loader.

You should note, that you **cannot choose** which FAT type you are using! It
depends *entirely* on **the number of clusters**. If your disk has up to 4085
clusters, you are using FAT12, for up to 65525 clusters, you are using FAT16,
and for higher numbers, FAT32.

You cannot format a floppy with FAT16 or a 10M hard disk with FAT32. What you
can do, is dial up the number of sectors per cluster to fudge the cluster count
below a threshold and use a *smaller* FAT for a *bigger* disk.

A disk must have *at least* some circa 32M to be FAT32 formatted. With 1 sector
per cluster (giving us 65536 clusters). For 4k clusters it would be somewhere
around 256M. In both cases, you should bump it up a little further tough. The
Microsoft spec says, you should stay away from the threshold numbers, because
different implementations are subtly broken in different ways.

## Multiboot

The Multiboot specificiation is something the GNU people came up with. It
describes a bootloader-to-kernel binary interface.

The kernel binary contains a header with a magic signature and some flags in
the first few kilobytes of the file. The boot loader reads it to find out where
to load the kernel binary to and what extra information it wants. The kernel
is a 32 bit executable, it *can be* an ELF file, or manually specify the file
layout in the multiboot header.

The boot loader sets up 32 bit protected mode for the kernel, enables the A20
line and passes a big information structure to the kernel, containing e.g. the
memory layout, kernel command line or the initrd location in memory.

To get the memory layout, the boot loader would typically use BIOS
[interrupt 15h, AX=E820](https://en.wikipedia.org/wiki/E820). In fact, the
multiboot memory map structure is pretty much identical to what this returns,
except that it tacks on an additional size field.

By the way: The Linux kernel **does not** use multiboot. It has a real-mode
setup stub that does (among other things) the protected mode and A20 setup.
It also reads the `E820` memory map on its own.

There is a information structure in the `bzImage` at fixed offset `0x01F1`.
The boot loader would load the first `N` sectors (whatever the info struct says)
to low memory and the rest of the kernel into high-memory.

# Minor Things I Learned Along the Way

## Building 16 bit code with gcc

gcc supports 16 bit code in a really passive aggressive way. Turns out, you can
use 32 bit registers from 16 bit code by using the `0x66` op-code prefix. Once
you are in 32 bit mode, the meaning of the prefix is inverted (i.e. use it to
access only the lower 16 bit instead of the full register).

When told to generate 16 bit code, gcc will basically generate 32 bit code with
instructions prefixed to run in 16 bit real mode. You need to take this into
account, when calling from assembly into gcc generated code or vice versa.

This also means that current gcc theoretically cannot target anything pre i386.

Also, gcc will happily poop out XMM/SSE instructions with a `0x66` prefix when
it needs to move data around. Set the proper `-march=i386` flag, to make sure
it doesn't do that.

Furthermore, accessing memory past 64k is a PITA. There is an `0x67` opcode
prefix that allows you to load/store using a 32 bit register. On Qemu, this
works just fine, but on real hardware, this this will trigger a protection
fault if you try to access memory past the 64k current segment boundary.

## Computed Goto

gcc allows you to do this:

```C++
somelabel:
	auto *wtf = &&somelabel;

goto *wtf;
```

the intended use is to hack together jump tables for state machines by hand.
But it also allows pointer arithmetic on the, um "label pointer" and even
funkier stuff:

```C++
	goto *((void *)0x7c00);
```

I initially had the MBR written in C++ entirely, including the self relocating,
but ultimately replace the entry/exit code with an assembly stub instead. It
was very fidgety and relied on the compiler generating *specific* code, e.g.
not turning an absolute jump to a relative one. This is also, why the relocating
code couldn't just *call* main either. The compiler would prefer a more
compact, relative call, or if main was static, just inline it.

## Disassembling a raw Binary

I had to resort to this a few times to double check:

```sh
objdump -D -m i8086 -b binary ./mbr/mbr.bin
```

## Debugging Low Level Code with gdb and Qemu

Qemu has built in support for the gdbserver protocol:

```sh
qemu-system-i386 -drive format=raw,file=./disk.img -S -s
```

You can then connect to it from within gdb:

```sh
$ gdb
...Dr. Stallman, of MIT, has pointed out that bobbadah bobbadah hoe daddy
yanga langa furjeezama bing jingle oh yeah...
(gdb) target remote localhost:1234
...bla bla bla...What machine is this? I don't have debug symbols, lol...
(gdb) set architecture i8086
The target architecture is set to "i8086".
(gdb) break *0x7c00
Breakpoint 1 at 0x7c00
(gdb) c
Continuing.

Breakpoint 1, 0x00007c00 in ?? ()
```

You can dump registers and disassemble memory ranges, and so on.

## Debugging Low Level Code with Bochs

Bochs needs to be compiled with debugger mode enabled for this. Typical Linux
Distros have a separate package for that, with a separate binary that can
launch you into a debug shell:

```sh
$ bochs-debugger -q -f bochsrc.txt
... bla bla bla ...
(0) [0x0000fffffff0] f000:fff0 (unk. ctxt): jmpf 0xf000:e05b          ; ea5be000f0
<bochs:1> lbreak 0x7c00
<bochs:2> c
... bla bla bla ...
(0) Breakpoint 1, 0x0000000000007c00 in ?? ()
Next at t=16300062
(0) [0x000000007c00] 0000:7c00 (unk. ctxt): xor ax, ax                ; 31c0
<bochs:3>
```

You can single step with `s`, an optional number argument steps over several
instructions, disassembly and address are shown.

For break points, `break` expects a `segment:offset` address, while `lbreak`
expects a linear address, `c` continues until the next break point.

You can dump registers with `r`:

```sh
<bochs:3> r
CPU0:
rax: 00000000_0000aa55
rbx: 00000000_00000000
rcx: 00000000_00090000
rdx: 00000000_00000080
rsp: 00000000_0000ffd6
rbp: 00000000_00000000
rsi: 00000000_000e0000
rdi: 00000000_0000ffac
r8 : 00000000_00000000
r9 : 00000000_00000000
r10: 00000000_00000000
r11: 00000000_00000000
r12: 00000000_00000000
r13: 00000000_00000000
r14: 00000000_00000000
r15: 00000000_00000000
rip: 00000000_00007c00
eflags 0x00000082: id vip vif ac vm rf nt IOPL=0 of df if tf SF zf af pf cf
```

or inspect/hexdump memory regions with `x /<count><unit> <address>`:

```sh
<bochs:12> x /16b 0x7df0
[bochs]:
0x0000000000007df0 <bogus+       0>:	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x0000000000007df8 <bogus+       8>:	0x00	0x00	0x00	0x00	0x00	0x00	0x55	0xaa
```

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