Question

I’m building an assembler for a Java-like bytecode. Essentially, it is a Python function which takes in a string, which was read from a text file containing opcodes, and writes the compiled binary data to a file. Here is an example of an input string string (from Example.txt):

class Example

Method add

meta
lva 4

code
0: ipush 1 0
1: ipush 2 2
2: iload 1
3: iload 2
4: iadd
5: istore 3
6: iload 3
7: ireturn

My task is to convert the Example.txt text file into it’s binary representation (machine code for my virtual machine). Each opcode (e.g. iadd) is exactly one byte long (i.e. a number in the range of 0 to 255). Some opcodes take in operands, which are one byte long too.

Saving only the method’s opcodes in a binary file is pretty easy. I’d parse each opcode into a number in the range of 0 to 255, and use array.append(op), and then array.tofile(handle).

However, I need to save not only the opcodes, but data, such as the name of the class, name of the method, etc.

How can I save both one-byte values and more complex data to a binary file?

I have envisioned something like this…

The file is divided into sections. The first section, for example, would be class Example. The next section is the one started by Method add.

Essentially, there are only two types of sections, the name section, which only contains class name, and the method section, which is then subsectioned into the meta and code sections.

Name sections start with 00 (in hex), and then need to present the name of the class (this is my problem). That 00 is essentially the hex representation of class. Method sections start with FF, and then they too, need to present their name in the next few bytes. Then, I could have 0F to mean “start of meta section” and F0 “start of code section” inside the primary Method section.

However, a problem remains. How do I name these sections? I had an idea of converting each character in e.g. “Example” to its hex representation, a single byte, and then saving those in the file after 00 or FF. This probably means that I will need some “end of section” symbol.

Is there a better/simpler/already implemented way of doing this?

Answer 1

You are asking a pretty sprawling question about design as well as basic Python usage. I’m not sure you’ll get a good answer until you limit the question a bit.

But let’s focus on the question of how to convert a string, e.g. “Example”, into an array of integers, so that you can append it to your array. Be sure you have a byte string; if you have a string of Unicode characters, be sure to encode it into an array of bytes first. (If you are using Python 2.x, your string is already an array of bytes.)

out = [0]  # your output array

s = "Example" # a byte string, not a Unicode string
l = list(s)   # convert to array of ints

out = out + l
print out

gives:

[0, 'E', 'x', 'a', 'm', 'p', 'l', 'e']