Add description of bootstrap stages to README.

README.md

@@ -1,6 +1,6 @@
 # stage0-uefi
 
-This is a port of stage0-posix (https://github.com/oriansj/stage0-posix) to UEFI.
+This is a port of stage0-posix (https://github.com/oriansj/stage0-posix) to UEFI. Its purpose is to start with a tiny binary seed that can be manually inspected and use it to build C toolchain and some extra tools.
 
 ## Usage
 
@@ -20,3 +20,101 @@ It is possible to create images without `kaem-optional-seed.efi` that contain on
 - Optional Data has to be `hex0 amd64\kaem-minimal.hex0 EFI\BOOT\BOOTX64.efi`.
 - Save boot entry and boot it. This will build kaem-minimal and exit.
 - Now boot normally from bootable media.
+
+## Stages
+
+### kaem-optional (Optional "shell")
+
+`kaem-optional` is a trivial shell that can read list of commands together with their command line arguments from a file and executes them. It also supports line comments but has no other features.
+
+### hex0
+
+`hex0` is fairly trivial to implement and for each pair of hexadecimals characters it outputs a byte. We have also added two types of line comments (# and ;) to create a well commented lines like
+
+```
+    # :loop_options [_start + 0x6F]
+    4839D3          ; cmp_rbx,rdx                 # Check if we are done
+    74 14           ; je !loop_options_done       # We are done
+    4883EB 02       ; sub_rbx, !2                 # --options
+```
+
+In the first steps we use initial `hex0` binary seed to rebuild `kaem-optional` and `hex0` from their source.
+
+`hex0` code is somewhat tedious to read and write as it is basically a well documented machine code. We have to manually calculate all jumps in the code.
+
+### hex1
+
+This is the last program that has to be written in `hex0` language. `hex1` is a simple extension of `hex0` and adds a single character labels and allows calculating 32-bit offsets from current position in the code to the label. `hex1` code might look like
+
+```
+:a #:loop_options
+    4839D3          ; cmp_rbx,rdx                 # Check if we are done
+    0F84 %b         ; je %loop_options_done       # We are done
+    4883EB 02       ; sub_rbx, !2                 # --options
+```
+
+### hex2
+
+`hex2` is our final hex language that adds support for labels of arbitrary length. It also allows accessing them via 8, 16, 32-bit relative addresses (!, @, %) and via 16-bit or 32-bit ($, &) absolute addresses though only the former addressing mode is used in `stage0-uefi`.
+
+```
+:loop_options
+    4839D3                ; cmp_rbx,rdx                        # Check if we are done
+    74 !loop_options_done ; je8 !loop_options_done             # We are done
+    4883EB 02             ; sub_rbx, !2                        # --options
+```
+
+### catm
+
+`catm` allows concatenating files via `catm.efi output_file input1 input2 ... inputN`. This allows us to split shared code into separate files. We will first use it to append `PE` header to `.hex2` files. Before this step PE header had to be included in the source file itself.
+
+### M0
+
+The `M0` assembly language is the simplest assembly language you can create that enables the creation of real world programs with practical application. It includes only a single keyword: `DEFINE` and leverages the language properties of `hex2` along with extending the behavior to populate immediate values of various sizes and formats.
+
+Thus `M0` code looks like
+
+```
+DEFINE cmp_rbx,rdx 4839D3
+DEFINE je 0F84
+DEFINE sub_rbx, 4881EB
+
+:loop_options
+    cmp_rbx,rdx                         # Check if we are done
+    je %loop_options_done               # We are done
+    sub_rbx, %2                         # --options
+```
+
+### cc_amd64
+
+The `cc_amd64` implements a subset of the C language designed in `M0` assembly. It is somewhat limitted subset of C but complete enough to make it easy to write a real C compiler written in the C subset that `cc_amd64` supports.
+
+At this stage we start using `M2libc` (https://github.com/oriansj/M2libc/) as our C library. In fact, `M2libc` ships two versions of C library. At this stage we use a single-file (`bootstrap.c`) C library that contains just enough to build `M2-Planet`.
+
+### M2-Planet
+
+This is the only C program that we build with `cc_amd64`. M2-Planet (https://github.com/oriansj/M2-Planet) supports a larger subset of C than `cc_amd64` and we are somewhat closer to C89 (it does not implement all C89 features but on the other hand it does have some C99 features). `M2-Planet` also includes a very basic preprocessor, so we can use stuff like `#define`, `#ifdef`.
+
+`M2-Planet` supports generating code for various architectures including `x86`, `amd64`, `armv7`, `aarch64`, `riscv32` and `riscv64`.
+
+`M2-Planet` is also capable of using full `M2libc` C library that has more features and optimizations compared to bootstrap version of `M2libc`.
+
+`M2libc` hides all UEFI specific bits inside it, so that applications written for POSIX (such as `M2-Planet`) can run without any source modifications.
+
+### C versions linker and assembler
+
+We then build C version of `hex2` (also called `hex2`) and C version of `M0` called `M1`. These are more capable than their platform specific hex counterparts and are fully cross-platform. Thus we can now have the whole toolchain written in C.
+
+### kaem
+
+We now build `kaem` which is a more capable version of `kaem-optional` and adds support for variables, environmental variables, conditionals, aliases. It also has various built-ins such as `cd` and `echo`.
+
+(NOTE that at the moment UEFI port of M2libc is still work in progress so not all features of `kaem` work).
+
+### M2-Planet (built against full M2libc)
+
+We can now rebuild `M2-Planet` so that it itself can benefit from full `M2libc`.
+
+### Work in progress
+
+Stage0-uefi is not yet complete and we still need to improve M2libc to reach the feature parity with `stage0-posix`.