ASSEMBLER

I wrote a little multipass assembler to translate the lcc assembly output into an executable image.

The xr16 assembler reads one or more assembly files and emits both image and listing files. The lexical analyzer reads the source characters and recognizes tokens like the identifier _main. The parser scans tokens on each line and recognizes instructions and operands, such as register names and effective address expressions. The symbol table remembers labels and their addresses, and a fixup table remembers symbolic references.

In pass one, the assembler parses each line. Labels are added to the symbol table. Each instruction expands into one or more machine instructions. If an operand refers to a label, we record a fixup to it.

In pass two, we check all branch fixups. If a branch displacement exceeds 128 words, we rewrite it using a jump. Because inserting a jump may make other branches far, we repeat until no far branches remain.

Next, we evaluate fixups. For each one, we look up the target address and apply that to the fixup subject word. Lastly, we emit the output files.

I also wrote a simple instruction set simulator. It is useful for exercising both the compiler and the embedded application in a friendly environment.

Well, by now you are probably wondering if there is any hardware to this project. Indeed there is! First, let’s consider our target FPGA device.