Miki Tebeka’s Clever Idea¶

When I was researching assemblers written in Python I came across a fascinating blog post describing a very clever trick for implementing an assembler in Python. Miki Tebeka realized that he could lean on Python itself for the usual tasks of parsing and lexing and he managed to create a custom assembler in just two days.

He used one class per instruction whereas I have a method and a function for each instruction. The assembler uses two passes and the method is for the first while the function is for the second. See the internals discussion for more information.

The basic trick is to define callables that mimic the names of assembler instructions and directives and then execfile() the “assembler” source file within a context that includes them. The callables then create the appropriate binary output.

Source Format¶

The source format is pure Python and can contain pretty much any code you want. (As one side-effect of this, no special macro facility is needed, instead you can simply define Python functions in your “assembler” source and call them normally to use them.)

Here’s a sample of the source format for Pigeon. It’s a simple routine to initialize the UART perhipheral of the ATmega328P:

label(UART_INIT)
ldi(r16, high(520)) # 2400 baud w/ 20Mhz osc
sts(UBRR0H, r16)
ldi(r16, low(520))  # See Datasheet
sts(UBRR0L, r16)
# The chip defaults to 8N1 so we won't
# set it here even though we should.
ldi(r16, (1 << TXEN0) | (1 << RXEN0)) # Enable transmit/receive
sts(UCSR0B, r16)
ret()

As you can see, it’s Python code yet it closely mimics assembly. As each function is executed it adds a description of the instruction to be assembled to a datastructure maintained by the pyavrasm.AVRAssembly object managing the assembly process.

As an example of the pythonic nature of the assembler source code consider the following “macro-ized” version of the above code:

def stsi(target, immediate):
    ldi(r16, immediate)
    sts(target, r16)

label(UART_INIT)
stsi(UBRR0H, high(520)) # 2400 baud w/ 20Mhz osc
stsi(UBRR0L, low(520))  # See Datasheet
# The chip defaults to 8N1 so we won't
# set it here even though we should.
stsi(UCSR0B, (1 << TXEN0) | (1 << RXEN0)) # Enable transmit/receive
ret()

You can see that the “macro” is just a Python function. The resulting binary code would be the same.

Show off a comparison of Pigeon Assembler’s source format, the Amtel ASM format, and gcc as format.