readme edits

02/README.md

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+# stage 02
+
+The compiler for this stage is in the file `in01`, an input for our previous compiler.
+The specifics of how this compiler works are in the comments in that file, but here I'll
+give an overview.
+Let's take a look at `in02`, an example input file for this compiler:
+```
+jm
+:-co   jump to code
+::hw
+'H
+'e
+'l
+'l
+'o
+',
+' 
+'w
+'o
+'r
+'l
+'d
+'!
+\n
+::he  end of hello world
+::co  start of code
+//
+// now we'll calculate the length of the hello world string
+// by subtracting hw from he.
+//
+im
+--he
+BA
+im
+--hw
+nA
++B
+DA   put length in rdx
+// okay now we can write it
+im
+##1.
+JA    set rdi to 1 (stdout)
+im
+--hw
+IA    set rsi to a pointer to "Hello, world!\n"
+im
+##1.  write
+sy
+im
+##0.  exit code 0
+JA
+im
+##3c. exit = syscall 0x3c
+sy
+```
+
+You can try adding more characters to the hello world message, and it'll just work;
+the length of the text is computed automatically!
+
+This time, commands are separated by newlines instead of semicolons.
+Each line begins with a 2-character command identifier. There are some special identifiers though:
+
+- `::` marks a *label*
+- `--` outputs a label's (absolute) address
+- `:-` outputs a label's relative address
+- `##` outputs a number
+
+All other commands work like they did in the previous compiler—if you scroll down in the
+`in01` source file, you'll see the full command table.
+
+## labels
+
+Labels are the most important new feature of this language.
+
+## two passes?
+
+## other features
+
+Now instead of writing out each of the 8 bytes making up a number,
+we can just write it in hexadecimal (e.g. `##3c.` for `3c 00 00 00 00 00 00 00`),
+and the compiler will automatically
+extend it to 8 bytes.
+This is especially nice because we don't need to write numbers backwards
+for little-endianness anymore!
+Numbers cannot appear at the end of a line (this was
+to make the compiler simpler to write), so I'm adding a `.` at the end of
+each one to avoid making that mistake.
+
+Anything after a command is treated as a comment;
+additionally `//` can be used for comments on their own lines.
+I decided to implement them as simply as possible:
+I just added the command `//` to the command table, which outputs the byte `0x90`—this
+means "do nothing" (`nop`) in x86-64.
+Note that this means that the following code will not work as expected:
+```
+im
+// load the value 0x333 into rax
+##333.
+```
+since `0x90` gets inserted between the "load immediate" instruction code, and the immediate.
+
+## limitations
+
+Many of the limitations of our previous compilers apply to this one. Also,
+if you use a label without defining it, it uses address 0, rather than outputting
+an error message. This could be fixed: if the value in the label table is 0, and if we are
+on the second pass, output an error message. This compiler was already tedious enough
+to implement, though! 
+But thanks to labels, for future compilers at least we won't have to calculate
+any jump offsets manually.