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-rw-r--r--00/README.md383
-rw-r--r--00/README.txt147
-rwxr-xr-x00/hexcompilebin632 -> 632 bytes
-rw-r--r--README.md99
-rw-r--r--README.txt25
-rwxr-xr-xbootstrap.sh39
-rw-r--r--instructions.txt6
7 files changed, 525 insertions, 174 deletions
diff --git a/00/README.md b/00/README.md
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+++ b/00/README.md
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+# stage 00
+
+This directory contains the file `hexcompile`, a handwritten executable. It
+takes input file `A` containing space/newline/[any character]-separated
+hexadecimal numbers and outputs them as bytes to the file `B`. On 64-bit Linux,
+try running `./hexcompile` from this directory (I've already provided an `A`
+file), and you will get a file named `B` containing the text `Hello, world!`.
+This stage is needed so that you can use your favorite text editor to write
+executables by hand (which have bytes outside of ASCII/UTF-8). I wrote it with
+a program called hexedit, which can be found on most Linux distributions. Only
+64-bit Linux is supported, because each OS/architecture combination would need
+its own separate executable. The executable is 632 bytes long, and you could
+definitely make it smaller if you wanted to, especially if you didn't limit it
+to the set of instructions I've decided on. Let's take a look at what's inside
+(`od -t x1 -An hexcompile`):
+
+```
+7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
+02 00 3e 00 01 00 00 00 78 00 40 00 00 00 00 00
+40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 40 00 38 00 01 00 00 00 00 00 00 00
+01 00 00 00 07 00 00 00 78 00 00 00 00 00 00 00
+78 00 40 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 02 00 00 00 00 00 00 00 02 00 00 00 00 00 00
+00 10 00 00 00 00 00 00 48 b8 74 02 40 00 00 00
+00 00 48 89 c7 48 b8 00 00 00 00 00 00 00 00 48
+89 c6 48 89 c2 48 b8 02 00 00 00 00 00 00 00 0f
+05 48 89 c5 48 b8 76 02 40 00 00 00 00 00 48 89
+c7 48 b8 41 00 00 00 00 00 00 00 48 89 c6 48 b8
+a4 01 00 00 00 00 00 00 48 89 c2 48 b8 02 00 00
+00 00 00 00 00 0f 05 48 89 ef 48 b8 68 02 40 00
+00 00 00 00 48 89 c6 48 b8 03 00 00 00 00 00 00
+00 48 89 c2 48 b8 00 00 00 00 00 00 00 00 0f 05
+48 89 c3 48 b8 03 00 00 00 00 00 00 00 48 39 d8
+0f 8f 37 01 00 00 48 b8 68 02 40 00 00 00 00 00
+48 89 c3 48 8b 03 48 89 c3 48 89 c7 48 b8 ff 00
+00 00 00 00 00 00 48 21 d8 48 89 c6 48 b8 39 00
+00 00 00 00 00 00 48 89 c3 48 89 f0 48 39 d8 0f
+8f 1e 00 00 00 48 b8 30 00 00 00 00 00 00 00 48
+f7 d8 48 89 f3 48 01 d8 e9 26 00 00 00 00 00 00
+00 00 00 48 b8 a9 ff ff ff ff ff ff ff 48 89 f3
+48 01 d8 e9 0b 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 48 89 c2 48 b8 ff 00 00 00 00 00 00 00
+48 89 c3 48 89 f8 48 c1 e8 08 48 21 d8 48 93 48
+b8 39 00 00 00 00 00 00 00 48 93 48 39 d8 0f 8f
+1f 00 00 00 48 89 c3 48 b8 d0 ff ff ff ff ff ff
+ff 48 01 d8 e9 2a 00 00 00 00 00 00 00 00 00 00
+00 00 00 48 89 c3 48 b8 a9 ff ff ff ff ff ff 48
+01 d8 e9 0c 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 48 89 c7 48 89 d0 48 c1 e0 04 48 89 fb
+48 09 d8 48 93 48 b8 68 02 40 00 00 00 00 00 48
+93 48 89 03 48 89 de 48 b8 04 00 00 00 00 00 00
+00 48 89 c7 48 b8 01 00 00 00 00 00 00 00 48 89
+c2 0f 05 e9 8f fe ff ff 00 00 00 00 00 48 b8 3c
+00 00 00 00 00 00 00 0f 05 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 41 00 42 00
+```
+
+Okay, that doesn't tell us much. I'll annotate it below. You might notice that
+all the numbers are backwards, e.g. `3e 00` for the number 0x003e (62 decimal).
+This is because almost all modern architectures (including x86-64) are
+little-endian, meaning that the *least significant byte* goes first, and the
+most significant byte goes last. There are various reasons why this is easier to
+deal with, but I won't explain that here.
+
+## ELF header
+This header has a bunch of metadata about the executable.
+
+- `7f 45 4c 46` Special identifier saying that this is an ELF file (ELF is the
+format of almost all Linux executables)
+- `02` 64-bit
+- `01` Little-endian
+- `01` ELF version 1 (there is no version 2 yet)
+- `00 00 00 00 00 00 00 00 00` Reserved (not important yet, but may be in a later
+version of ELF)
+- `02 00` Object type = executable file (not a dynamic library/etc.)
+- `3e 00` Architecture x86-64
+- `01 00 00 00` Version 1 of ELF, again
+- `78 00 40 00 00 00 00 00` **Entry point of the executable** = 0x400078 (explained later)
+- `40 00 00 00 00 00 00 00` Program header table offset in bytes from start of file (see below)
+- `00 00 00 00 00 00 00 00` Section header table offset (we're not using sections)
+- `00 00 00 00` Flags (not important)
+- `40 00` The size of this header, in bytes = 64
+- `38 00` Size of the program header (see below) = 56
+- `01 00` Number of program headers = 1
+- `00 00` Size of each section header (unused)
+- `00 00` Number of section headers (unused)
+- `00 00` Index of special .shstrtab section (unused)
+
+## program header
+The program header describes a segment of data that is loaded into memory when
+the program starts. Normally, you would have more than one of these, maybe
+one for code, one for read-only data, and one for read-write data, but to
+simplify things we've only got one, which we'll use for any code and any data
+we need. This means it'll have to be read-enabled, write-enabled, and
+execute-enabled. Normally people don't do this, for security, but we won't worry
+about that (don't compile any untrusted code with any compiler from this series!)
+Without further ado, here's the contents of the program header:
+
+- `01 00 00 00` Segment type 1 (this should be loaded into memory)
+- `07 00 00 00` Flags = RWE (readable, writeable, and executable)
+- `78 00 00 00 00 00 00 00` Offset in file = 120
+- `78 00 40 00 00 00 00 00` Virtual address = 0x400078
+
+**wait a minute, what's that?**
+
+We just specified the *virtual address* of this segment. This is the virtual
+memory address that the segment will be loaded to. Virtual memory means that
+memory addresses in our program do not actually correspond to where the memory
+is physically stored in RAM. There are many reasons for it, including allowing
+different processes to have overlapping memory addresses, making sure that some
+memory can't be read/written/executed, etc. You can read more about it
+elsewhere.
+- `00 00 00 00 00 00 00 00` Physical address (not applicable)
+- `00 02 00 00 00 00 00 00` Size of this segment in the executable file = 512
+bytes
+- `00 02 00 00 00 00 00 00` Size of this segment when loaded into memory = also
+512 bytes
+- `00 10 00 00 00 00 00 00` Segment alignment = 4096 bytes
+
+That last field, segment alignment, is needed, because on default-settings Linux
+each page (block) of memory is 4096 bytes long, and has to start at an address
+that is a multiple of 4096. Our program needs to be loaded into a memory page,
+so its *virtual address* needs to be a multiple of 4096. We're using `0x400000`.
+But wait! Didn't we use `0x400078` for the virtual address? Well, yes but that's
+because the *data in the file* is loaded to address `0x400078`. The actual page
+of memory that the OS will allocate for our code will start at `0x400000`. The
+reason we need to start `0x78` bytes in is that Linux expects the data *in the
+file* to be at the same position in the page as when it will be loaded, and it
+appears at offset `0x78` in our file. Don't worry if you didn't understand all
+of that.
+
+## the code
+
+Now we get to the actual code in our executable (well there's a bit of data here
+too). We specified `0x400078` as the *entry point* of our executable, which
+means that the program will start executing from there. That virtual address
+corresponds to the start of the code right here:
+
+The first thing we want to do is open our input file, `A`:
+
+- `48 b8 74 02 40 00 00 00 00 00` `mov rax, 0x400274`
+- `48 89 c7` `mov rdi, rax`
+- `48 b8 00 00 00 00 00 00 00 00` `mov rax, 0`
+- `48 89 c6` `mov rsi, rax`
+- `48 89 c2` `mov rdx, rax`
+- `48 b8 02 00 00 00 00 00 00 00` `mov rax, 2`
+- `0f 05` `syscall`
+
+These instructions execute syscall `2` with arguments `0x400274`, `0`, `0`.
+If you're familiar with C code, this is `open("A", O_RDONLY, 0)`.
+A syscall is the mechanism which lets software ask the kernel to do things.
+[Here](https://filippo.io/linux-syscall-table/) is a nice table of syscalls you
+can look through if you're interested.
+Syscall #2, on Linux, is `open`. It's used to open a file. On Linux, you can
+read about it by running `man 2 open`.
+The first argument, `0x400274`, is a pointer to some data at the very end of
+this segment (scroll down). Specifically, it holds the byte `41` (ASCII `A`),
+followed by `00` (null byte). This indicates the name of the file, "A". The
+second argument (`O_RDONLY`, or 0) specifies that we will be reading from this
+file. The third is only really needed when creating new files, but I've just
+set it to 0, why not.
+
+This call gives us back a *file descriptor*, used later to read from the file,
+in register `rax`.
+
+- `48 89 c5` `mov rbp, rax` Store the file descriptor for later
+
+Now we'll open the output file
+
+- `48 b8 76 02 40 00 00 00 00 00` `mov rax, 0x400276`
+- `48 89 c7` `mov rdi, rax`
+- `48 b8 41 00 00 00 00 00 00 00` `mov rax, 0x41`
+- `48 89 c6` `mov rsi, rax`
+- `48 b8 a4 01 00 00 00 00 00 00` `mov rax, 0o644`
+- `48 89 c2` `mov rdx, rax`
+- `48 b8 02 00 00 00 00 00 00 00` `mov rax, 2`
+- `0f 05` `syscall`
+
+These instructions execute the syscall `open("B", O_WRONLY|O_CREAT, 0644)`. This
+is similar to our first one, but with some important differences. First, the
+second argument specifies both that we are writing to a file `0x01`, and that we
+want to create the file if it doesn't exist `0x40`. Secondly, the third
+argument specifies the permissions that the file should be created with (`644` -
+user read/write, group read). This here isn't particularly important to how the
+program works.
+
+- `48 89 ef` `mov rdi, rbp`
+- `48 b8 68 02 40 00 00 00 00 00` `mov rax, 0x400268`
+- `48 89 c6` `mov rsi, rax`
+- `48 b8 03 00 00 00 00 00 00 00` `mov rax, 3`
+- `48 89 c2` `mov rdx, rax`
+- `48 b8 00 00 00 00 00 00 00 00` `mov rax, 0`
+- `0f 05` `syscall`
+
+Here we call syscall #0 (`read`) to read from a file. The arguments are:
+- `fd (rdi) = rbp` read from the file descriptor we stored away earlier
+- `buf (rsi) = 0x400268` output to a part of this segment I've left empty
+- `count (rdx) = 3` read 3 bytes
+
+The number of bytes *actually* read (taking into account the fact that we might
+have reached the end of the file) is stored in `rax`.
+
+Note that we read the entire file 3 bytes at a time, which is a *terrible* idea
+for performance. syscalls take quite a while (3 microseconds or so, which would
+make this very slow for a several-megabyte file), so modern programs tend to
+read ~4KB at a time. But our programs will be small, and we don't care a lot
+about performance, so it's okay.
+
+- `48 89 c3` `mov rbx, rax`
+- `48 b8 03 00 00 00 00 00 00 00` `mov rax, 3`
+- `48 39 d8` `cmp rax, rbx`
+- `0f 8f 37 01 00 00` `jg 0x40024d`
+
+Together, these instructions say to jump to a different part of the code
+(explained later), if we ended up reading less than 3 bytes, i.e. we reached the
+end of the file. Note that rather than specifying the *address* to jump to, we
+specify the *relative address* (it's relative to the address of the first byte
+after the jump instruction). In other words, we're adding `0x137` to the program
+counter, `rip`. This has many reasons including saving space.
+
+- `48 b8 68 02 40 00 00 00 00 00` `mov rax, 0x400268`
+- `48 89 c3` `mov rbx, rax`
+- `48 8b 03` `mov rax, qword [rbx]`
+
+This copies out 8 bytes of the data that was just read into the 64-bit register
+rax. We only read 3 bytes of data from the file, but the rest will just be
+zeros (because that's what we put at offset `0x268` of the file).
+
+- `48 89 c3` `mov rbx, rax`
+- `48 89 c7` `mov rdi, rax`
+
+Here we copy away this data for later use.
+
+- `48 b8 ff 00 00 00 00 00 00 00` `mov rax, 0xff`
+- `48 21 d8` `and rax, rbx`
+
+This grabs the first byte of data we read and stores it in `rax`. This will be
+the code of the first ASCII character of the hexadecimal number in our input
+file.
+
+- `48 89 c6` `mov rsi, rax`
+- `48 b8 39 00 00 00 00 00 00 00` `mov rax, 0x39 ('9')`
+- `48 89 c3` `mov rax, rbx`
+- `48 89 f0` `mov rax, rsi`
+- `48 39 d8` `cmp rax, rbx`
+- `0f 8f 1e 00 00 00` `jg 0x400173`
+
+These instructions compare that character code against the character code for
+`9`. If it's greater, then it's one of the hex digits `a` through `f`, which are
+handled separately later.
+
+- `48 b8 30 00 00 00 00 00 00 00` `mov rax, 0x30 ('0')`
+- `48 f7 d8` `neg rax`
+- `48 89 f3` `mov rbx, rsi`
+- `48 01 d8` `add rax, rbx`
+
+Subtract the character code for `0` from the character code we read in, to get
+the *number* corresponding to the first hex digit in the pair.
+
+- `e9 26 00 00 00` `jmp 0x400193`
+
+Go to a different part of the program (we'll get there later).
+
+- `00 00 00 00 00 00`
+
+Unneeded 0 bytes I left in, to make room in case I needed it.
+
+Now we get to the `a`-`f` handling code:
+
+- `48 b8 a9 ff ff ff ff ff ff ff` `mov rax, -87`
+- `48 89 f3` `mov rbx, rsi`
+- `48 01 d8` `add rax, rbx`
+- `e9 0b 00 00 00` `jmp 0x400193`
+- `00 00 00 00 00 00 00 00 00 00 00` (unused)
+
+If our character code is one of `abcdef`, we add `-87` (subtract `87`) from it,
+to convert the character code to the numerical value of the digit. Here I
+decided to just set `rax` to the two's complement encoding for `-87`, but you
+could also use the `neg` instruction, like I did last time. <s>I just wanted to
+show two different ways of doing it</s> I thought of the better way the second
+time around.
+
+Now we get to `0x400193`, the common place we jumped to from both branches.
+
+- `48 89 c2` `mov rdx, rax`
+
+Store away the first digit in the pair into `rdx`.
+
+- `48 b8 ff 00 00 00 00 00 00 00` `mov rax, 0xff`
+- `48 89 c3` `mov rbx, rax`
+- `48 89 f8` `mov rax, rdi`
+- `48 c1 e8 08` `shr rax, 8`
+- `48 21 d8` `and rax, rbx`
+
+Now we extract the second character code we read from the file.
+The entire character code to number conversion is rewritten here, but slightly
+differently this time because I came up with some new ideas.
+
+- `48 93` `xchg rax, rbx`
+- `48 b8 39 00 00 00 00 00 00 00` `mov rax, 0x39 ('9')`
+- `48 93` `xchg rax, rbx`
+- `48 39 d8` `cmp rax, rbx`
+- `0f 8f 1f 00 00 00` `jg 0x4001e3 ('a'-'f' handling code)`
+- `48 89 c3` `mov rbx, rax`
+- `48 b8 d0 ff ff ff ff ff ff ff` `mov rax, -48`
+- `48 01 d8` `add rax, rbx`
+- `e9 2a 00 00 00` `jmp 0x400203`
+- `00 00 00 00 00 00 00 00 00 00` (unused)
+
+('a'-'f' handling)
+- `48 89 c3` `mov rbx, rax`
+- `48 b8 a9 ff ff ff ff ff ff` `mov rax, -87`
+- `48 01 d8` `add rax, rbx`
+- `e9 0c 00 00` `jmp 0x400203`
+- `00 00 00 00 00 00 00 00 00 00 00 00 00` (unused)
+
+(common code)
+- `48 89 c7` `mov rdi, rax`
+
+Okay now we've read the first hex digit into `rdx`, and the second into `rdi`.
+
+- `48 89 d0` `mov rax, rdx`
+- `48 c1 e0 04` `shl rax, 4`
+- `48 89 fb` `mov rbx, rsi`
+- `48 09 d8` `or rax, rbx`
+
+Okay, now we have the full hexadecimal number in `rax`!
+
+- `48 93` `xchg rax, rbx`
+- `48 b8 68 02 40 00 00 00 00 00` `mov rax, 0x400268`
+- `48 93` `xchg rax, rbx`
+- `48 89 03` `mov qword [rbx], rax`
+
+This stores the byte we want to write to the file at address `0x400268`. This is
+the same address we used to read in the input text; again, it's just part of
+this segment I've left blank.
+
+- `48 89 de` `mov rsi, rbx`
+- `48 b8 04 00 00 00 00 00 00 00` `mov rax, 4`
+- `48 89 c7` `mov rdi, rax`
+- `48 b8 01 00 00 00 00 00 00 00` `mov rax, 1`
+- `48 89 c2` `mov rdx, rax`
+- `0f 05` `syscall`
+
+Here we call syscall #1, `write`, with arguments:
+
+- `fd = 4` we could have stored away the file descriptor we got before for the
+output file, like we did with the input file, but I was out of easy-to-use
+registers! Instead, we can use the fact that Linux assigns file descriptors
+sequentially starting from 3 (0, 1, and 2 are standard input, output, and
+error), so we know our output file, the second file we opened, will have
+descriptor 4.
+- `buf = 0x400268` where we put our data
+- `count = 1` write 1 byte
+
+- `e9 8f fe ff ff` `jmp 0x4000d7`
+- `00 00 00 00 00` (unused)
+
+Now we go back to read in the next pair of digits! Finally...
+
+- `48 b8 3c 00 00 00 00 00 00 00` `mov rax, 0x3c`
+- `0f 05` `syscall`
+
+This is where we conditionally jumped to way back when we determined if we
+reached the end of the file. This just calls syscall #60, `exit`, to exit our
+program nicely. We didn't specify the exit code, but that's okay for our
+purposes.
+And we could close the files (syscall #3), to tell Linux we're done with them,
+but we don't need to. It'll close all our open file descriptors when our program
+exits.
+
+
+- `00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00` Unused bytes (I wasn't
+sure exactly how long the program would be)
+- `00 00 00 00 00 00 00 00` This is where we read/wrote the file data!
+- `41 00` Input file name, `"A"`
+- `42 00` Output file name, `"B"`
+
+That's quite a lot to take in for such a simple program, but here we are! We now
+have something that will let us write individual bytes with an ordinary text
+editor and get them translated into a binary file.
diff --git a/00/README.txt b/00/README.txt
deleted file mode 100644
index 19e57d3..0000000
--- a/00/README.txt
+++ /dev/null
@@ -1,147 +0,0 @@
---- stage 00 ---
-
-This directory contains the file 'hexcompile', a handwritten executable.
-It takes an input file A containing space/newline/[any character]-separated
-hexadecimal numbers and outputs them as bytes to the file B. On 64-bit Linux,
-try running ./hexcompile from this directory (I've already provided an A file),
-and you will get a file named B containing the text "Hello, world!".
-I made this program so that you can use your favorite text editor to write
-executables by hand (which have bytes outside of ASCII/UTF-8).
-I wrote it with a program called hexedit, which can be found on most Linux
-distributions. Only 64-bit Linux is supported, because each OS/architecture
-combination would need its own separate executable. The executable is 632 bytes
-long, and you could definitely make it smaller if you wanted to. Let's take a
-look at what's inside (see hexdump -C hexcompile):
-7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
-02 00 3e 00 01 00 00 00 78 00 40 00 00 00 00 00
-40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
-00 00 00 00 40 00 38 00 01 00 00 00 00 00 00 00
-01 00 00 00 07 00 00 00 78 00 00 00 00 00 00 00
-78 00 40 00 00 00 00 00 00 00 00 00 00 00 00 00
-00 02 00 00 00 00 00 00 00 02 00 00 00 00 00 00
-00 10 00 00 00 00 00 00 48 b8 74 02 40 00 00 00
-00 00 48 89 c7 48 b8 00 00 00 00 00 00 00 00 48
-89 c6 48 89 c2 48 b8 02 00 00 00 00 00 00 00 0f
-05 48 89 c5 48 b8 76 02 40 00 00 00 00 00 48 89
-c7 48 b8 41 00 00 00 00 00 00 00 48 89 c6 48 b8
-a4 01 00 00 00 00 00 00 48 89 c2 48 b8 02 00 00
-00 00 00 00 00 0f 05 48 89 c1 48 89 ef 48 b8 68
-02 40 00 00 00 00 00 48 89 c6 48 b8 03 00 00 00
-00 00 00 00 48 89 c2 48 b8 00 00 00 00 00 00 00
-00 0f 05 48 89 c3 48 b8 03 00 00 00 00 00 00 00
-48 39 d8 0f 8f 37 01 00 00 48 b8 68 02 40 00 00
-00 00 00 48 89 c3 48 8b 03 48 89 c3 48 89 c7 48
-b8 ff 00 00 00 00 00 00 00 48 21 d8 48 89 c6 48
-b8 39 00 00 00 00 00 00 00 48 89 c3 48 89 f0 48
-39 d8 0f 8f 1e 00 00 00 48 b8 30 00 00 00 00 00
-00 00 48 f7 d8 48 89 f3 48 01 d8 e9 26 00 00 00
-00 00 00 00 00 00 48 b8 a9 ff ff ff ff ff ff ff
-48 89 f3 48 01 d8 e9 0b 00 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c2 48 b8 ff 00 00 00 00
-00 00 00 48 89 c3 48 89 f8 48 c1 e8 08 48 21 d8
-48 93 48 b8 39 00 00 00 00 00 00 00 48 93 48 39
-d8 0f 8f 1f 00 00 00 48 89 c3 48 b8 d0 ff ff ff
-ff ff ff ff 48 01 d8 e9 2a 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c3 48 b8 a9 ff ff ff ff
-ff ff 48 01 d8 e9 0c 00 00 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c7 48 89 d0 48 c1 e0 04
-48 89 fb 48 09 d8 48 93 48 b8 68 02 40 00 00 00
-00 00 48 93 48 89 03 48 89 de 48 b8 04 00 00 00
-00 00 00 00 48 89 c7 48 b8 01 00 00 00 00 00 00
-00 48 89 c2 0f 05 e9 8f fe ff ff 00 00 00 00 00
-48 b8 3c 00 00 00 00 00 00 00 0f 05 00 00 00 00
-00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
-00 00 00 00 41 00 42 00
-
-Okay, that doesn't tell us much, I'll annotate it below. You might notice that
-all the numbers are backwards, e.g. 3e 00 for the number 0x003e (62 decimal).
-This is because almost all modern architectures (including x86-64) are
-little-endian, meaning that the *least significant byte* goes first, and the
-most significant byte goes last. There are various reasons why this is easier to
-deal with, which I won't explain here.
-
--- ELF header --
-This header has a bunch of metadata about the executable.
-
-7f 45 4c 46 - Special identifier saying that this is an ELF file (ELF is the
-format of almost all Linux executables)
-02 - 64-bit
-01 - Little-endian
-01 - ELF version 1 (there is no version 2 yet)
-00 00 00 00 00 00 00 00 00 - Reserved (not important yet, but may be in a later
-version of ELF)
-02 00 - This is an executable file (not a dynamic library/etc)
-3e 00 - Architecture x86-64
-01 00 00 00 - Version 1 of ELF (minor version or something)
-78 00 40 00 00 00 00 00 - **Entry point of the executable** = 0x400078 (explained later)
-40 00 00 00 00 00 00 00 - Program header table offset in bytes from start of file (see below)
-00 00 00 00 00 00 00 00 - Section header table offset (we're not using sections)
-00 00 00 00 - Flags (not important)
-40 00 - The size of this header, in bytes = 64
-38 00 - Size of the program header (see below) = 56
-01 00 - Number of program headers = 1
-00 00 - Size of each section header (unused)
-00 00 - Number of section headers (unused)
-00 00 - Index of special .shstrtab section (unused)
-
--- Program header --
-The program header describes a segment of data that is loaded into memory when
-the program starts. Normally, you would have more than one of these, one for
-code, one for read-only data, and one for read-write data, perhaps, but to
-simplify things we've only got one, which we'll use for any code and any data
-we need. This means it'll have to be read-enabled, write-enabled, *and*
-execute-enabled. Normally people don't do this, for security, but we won't worry
-about that (don't compile any untrusted code with any compiler from this series!)
-Without further ado, here's the contents of the program header:
-
-01 00 00 00 - Segment type 1 (this should be loaded into memory)
-07 00 00 00 - Flags = RWE (readable, writeable, and executable)
-78 00 00 00 00 00 00 00 - Offset in file = 120
-78 00 40 00 00 00 00 00 - Virtual address = 0x400078
-- Wait a minute, what's that? -
-We just specified the *virtual address* of this segment. This is the virtual
-memory address that the segment will be loaded to. Virtual memory means that
-memory addresses in our program do not actually correspond to where the memory
-is physically stored in RAM. There are many reasons for it, including allowing
-different processes to have overlapping memory addresses, making sure that some
-memory can't be read/written/executed, etc. You can read more about it
-elsewhere.
-00 00 00 00 00 00 00 00 - Physical address (not applicable)
-00 02 00 00 00 00 00 00 - Size of this segment in the executable file = 512
-bytes
-00 02 00 00 00 00 00 00 - Size of this segment when loaded into memory = also
-512 bytes
-00 10 00 00 00 00 00 00 - Segment alignment = 4096 bytes
-48 b8 74 02 40 00 00 00
-00 00 48 89 c7 48 b8 00 00 00 00 00 00 00 00 48
-89 c6 48 89 c2 48 b8 02 00 00 00 00 00 00 00 0f
-05 48 89 c5 48 b8 76 02 40 00 00 00 00 00 48 89
-c7 48 b8 41 00 00 00 00 00 00 00 48 89 c6 48 b8
-a4 01 00 00 00 00 00 00 48 89 c2 48 b8 02 00 00
-00 00 00 00 00 0f 05 48 89 c1 48 89 ef 48 b8 68
-02 40 00 00 00 00 00 48 89 c6 48 b8 03 00 00 00
-00 00 00 00 48 89 c2 48 b8 00 00 00 00 00 00 00
-00 0f 05 48 89 c3 48 b8 03 00 00 00 00 00 00 00
-48 39 d8 0f 8f 37 01 00 00 48 b8 68 02 40 00 00
-00 00 00 48 89 c3 48 8b 03 48 89 c3 48 89 c7 48
-b8 ff 00 00 00 00 00 00 00 48 21 d8 48 89 c6 48
-b8 39 00 00 00 00 00 00 00 48 89 c3 48 89 f0 48
-39 d8 0f 8f 1e 00 00 00 48 b8 30 00 00 00 00 00
-00 00 48 f7 d8 48 89 f3 48 01 d8 e9 26 00 00 00
-00 00 00 00 00 00 48 b8 a9 ff ff ff ff ff ff ff
-48 89 f3 48 01 d8 e9 0b 00 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c2 48 b8 ff 00 00 00 00
-00 00 00 48 89 c3 48 89 f8 48 c1 e8 08 48 21 d8
-48 93 48 b8 39 00 00 00 00 00 00 00 48 93 48 39
-d8 0f 8f 1f 00 00 00 48 89 c3 48 b8 d0 ff ff ff
-ff ff ff ff 48 01 d8 e9 2a 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c3 48 b8 a9 ff ff ff ff
-ff ff 48 01 d8 e9 0c 00 00 00 00 00 00 00 00 00
-00 00 00 00 00 00 48 89 c7 48 89 d0 48 c1 e0 04
-48 89 fb 48 09 d8 48 93 48 b8 68 02 40 00 00 00
-00 00 48 93 48 89 03 48 89 de 48 b8 04 00 00 00
-00 00 00 00 48 89 c7 48 b8 01 00 00 00 00 00 00
-00 48 89 c2 0f 05 e9 8f fe ff ff 00 00 00 00 00
-48 b8 3c 00 00 00 00 00 00 00 0f 05 00 00 00 00
-00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
-00 00 00 00 41 00 42 00
diff --git a/00/hexcompile b/00/hexcompile
index 0c462c1..379b7eb 100755
--- a/00/hexcompile
+++ b/00/hexcompile
Binary files differ
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..141047a
--- /dev/null
+++ b/README.md
@@ -0,0 +1,99 @@
+# boostrapping a (Linux x86-64) C compiler
+
+Compilers nowadays are written in languages like C, which themselves need to be
+compiled. But then, you need a C compiler to compile your C compiler! Of course,
+the very first C compiler was not written in C (because how would it be
+compiled?). Instead, it was slowly built up, starting from a very basic
+assembler, eventually reacing a full-scale compiler. This process is known as
+bootstrapping. In this repository, we'll explore how that's done. Each directory
+represents a new "stage" in the process. The first one, `00`, is a hand-written
+executable, and the last one will be a C compiler. Each directory has its own
+README explaining what's going on.
+
+You can run `bootstrap.sh` to run through and test every stage.
+
+## the basics
+
+In this series, I want to explain *everything* that's going on. I'm going to
+need to assume some passing knowledge about computers, but here's a quick
+overview of what you'll want to know before starting. I can't explain everything
+so you may need to do your own research. You don't need to understand each of
+these in full, just get a general idea at least:
+
+- what an operating system is
+- what memory is
+- what a programming language is
+- what a compiler is
+- what an executable file is
+- number bases -- if a number is preceded by 0x, 0o, or 0b in this series, that
+means hexadecimal/octal/binary respectively. So 0xff = FF hexadecimal = 255
+decimal.
+- what a CPU is
+- what a CPU architecture is
+- what a CPU register is
+- what a pointer is
+- bits, bytes, kilobytes, etc.
+- bitwise operations (not, or, and, xor, left shift, right shift)
+- 2's complement
+- null-terminated strings
+- how floating-point numbers work
+- maybe some basic Intel-style x86-64 assembly (you can probably pick it up on
+the way though)
+
+
+## instruction set
+
+x86-64 has a *gigantic* instruction set. The manual for it is over 2,000 pages
+long! So, it makes sense to select only a small subset of it to use for all the
+stages of our compiler. The set I've chosen can be found in `instructions.txt`.
+I think it achieves a pretty good balance between having few enough
+instructions to be manageable and having enough instructions to be useable.
+To be clear, you don't need to read that file to understand the series, at least
+not right away.
+
+## principles
+
+- as simple as possible
+
+Bootstrapping a compiler is not an easy task, so we're trying to make it as easy
+as possible. We don't even necessarily need a standard-compliant C compiler, we
+only need enough to compile someone else's C compiler, specifically TCC
+(https://bellard.org/tcc/) since that's a compiler with very few dependencies.
+
+- efficiency is not a concern
+
+We will create big and slow executables, and that's okay. It doesn't really
+matter if compiling TCC takes 8 as opposed to 0.01 seconds; once we compile TCC
+with itself, we'll get the same executable either way.
+
+## reflections on trusting trust
+
+In 1984, Ken Thompson wrote the well-known article
+[*Reflections on Trusting Trust*](http://users.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf).
+This is one of the things that inspired me to start this project. To summarize
+the article: it is possible to create a malicious C compiler which will
+replicate its own malicious functionalities (e.g. detecting password-checking
+routines to make them also accept another password the attacker knows) when used
+to compile other C compilers. For all we know, such a compiler was used to
+compile GCC, say, and so all programs around today could be compromised. Of
+course, this is practically definitely not the case, but it's still an
+interesting experiment to try to create a fully trustable compiler. This
+project can't necessarily even do that though, because the Linux kernel, which
+we depend on, is compiled from C, so we can't fully trust *it*. To *truly*
+create a fully trustable compiler, you'd need to manually write to a USB with a
+circuit, create an operating system from nothing (without even a text editor),
+and then follow this series, or maybe you don't even trust your CPU vendor...
+I'll leave that to someone else
+
+## license
+
+```
+This project is in the public domain. Any copyright protections from any law
+for this project are forfeited by the author(s). No warranty is provided for
+this project, and the author(s) shall not be held liable in connection with it.
+```
+
+## contributing
+
+If you notice a mistake/want to clarify something, you can submit a pull request
+via GitHub, or email `pommicket at pommicket.com`. Translations are welcome!
diff --git a/README.txt b/README.txt
deleted file mode 100644
index 7776f2e..0000000
--- a/README.txt
+++ /dev/null
@@ -1,25 +0,0 @@
---- boostrapping a (Linux x86-64) C compiler ---
-
-Compilers nowadays are written in languages like C, which themselves need to be
-compiled. But then, you need a C compiler to compile your C compiler! Of course,
-the very first C compiler was not written in C (because how would it be
-compiled?). Instead, it was slowly built up, starting from a very basic
-assembler, eventually reacing a full-scale compiler. This process is known as
-bootstrapping. In this repository, we'll explore how that's done. Each directory
-represents a new "stage" in the process. The first one, "00", is a hand-written
-executable, and the last one will be a C compiler. Each directory has its own
-README.txt explaining in full what's going on.
-
--- instruction set --
-x86-64 has a *gigantic* instruction set. The manual for it is over 2,000 pages
-long! So, it makes sense to select only a small subset of it to use for all the
-stages of our compiler. The set I've chosen can be found in instructions.txt (a
-work in progress). I think it achieves a pretty good balance between
-having few enough instructions to be manageable and having enough
-instructions to be useable.
-
--- license --
-
-This software is in the public domain. Any copyright protections from any law
-for this software are forfeited by the author(s). No warranty is provided for
-this software, and the author(s) shall not be held liable in connection with it.
diff --git a/bootstrap.sh b/bootstrap.sh
new file mode 100755
index 0000000..eb34e2f
--- /dev/null
+++ b/bootstrap.sh
@@ -0,0 +1,39 @@
+#!/bin/sh
+
+# check OS/architecture
+
+esc() {
+ : # comment out the following line to disable color output
+ printf '\33[%dm' "$1"
+}
+
+echo_red() {
+ esc 31
+ echo "$1"
+ esc 0
+}
+
+echo_green() {
+ esc 32
+ echo "$1"
+ esc 0
+}
+
+if uname -a | grep -i 'x86_64' | grep -i -q 'linux'; then
+ : # all good
+else
+ echo_red "Only 64-bit Linux is supported. This doesn't seem to be 64-bit Linux."
+ exit 1
+fi
+
+cd 00
+rm -f B
+./hexcompile A
+if [ "$(cat B)" != 'Hello, world!' ]; then
+ echo_red 'Stage 00 failed.'
+ exit 1
+fi
+rm -f B
+cd ..
+
+echo_green 'Done all stages!'
diff --git a/instructions.txt b/instructions.txt
index 8ab3568..140c9c6 100644
--- a/instructions.txt
+++ b/instructions.txt
@@ -1,7 +1,9 @@
-SYSCALL CALLING CONVENTION
-rdi rsi rdx r10 r8 r9
+Linux syscall calling convention:
+rax - syscall number
+rdi, rsi, rdx, r10, r8, r9 - arguments
return value placed in rax
+Instruction set:
mov rax, imm64
>48 b8 IMM64