Add readme file and make the example compile

README.md

@@ -0,0 +1,40 @@
+# A simple RISC-V emulator
+
+This is a toy emulator for RISC-V, made for educational purposes.
+The goal is to have a base rv32i instruction set (the bare minimum) plus a M-extension for division and multiplication. In theory, it should be enough to execute simple C programs compiled with GCC and sprinkled with a few linker scripts. Of course, no libc because there's no OS.
+
+## Compiling and running
+
+You'd need nix. Install it and run:
+
+```sh
+nix develop
+
+eval "$configurePhase"
+ninja
+```
+
+You should get the `rve` binary. Run it:
+
+```sh
+./rve
+```
+
+At the moment the program is hardcoded into the `main()` function directly in machine code. As a follow-up, I'm working on compiling programs with the gcc cross-toolchain, but it will require some time to fiddle with options.
+
+## Cross-toolchain and an example program in C
+
+There is an example program which you can compile, but not yet run. Do this:
+
+```sh
+cd example
+make
+```
+
+As a result, you'll get an `example` binary. To view the disassembly:
+
+```sh
+riscv32-none-elf-objdump -D example
+```
+
+As you can see, it's quite barebones, with the program starting at 0x1000. The reason it can't yet be run with the emulator is mostly in the absence of the boot section which initializes the stack and calls the main function. Stay tuned.

example/Makefile

@@ -1,2 +1,4 @@
 example: example.c
 	riscv32-none-elf-gcc -fno-builtin -fvisibility=hidden -nostdlib -nostartfiles -march=rv32im -mabi=ilp32 -o example example.c -T linker.ld
+	riscv32-none-elf-strip -R .riscv.attributes example
+	riscv32-none-elf-strip -R .comment example

example/example.c

@@ -1,9 +1,9 @@
-static int mem;
+static int mem = 56;
 
 int main() {
   int a = 42;
   int b = 5;
-  mem = a*b + 3;
+  mem = mem + a*b + 3;
 
   return 0;
 }