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+--- 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