SIsimias·Mar 16, 2017
I've never implemented an assembler but I did implement a few toy disassemblers and emulators and you'd need a bit more than a simple LUT for many architectures.
In particular some ISAs have "modular" opcodes so to speak, where a machine opcode contains several fields that can be mixed and matched. For instance the ARM instruction set has a 4 bit condition code in each instruction so "add" "addeq" (add if equal) and "addne" (add if not equal) are basically the same opcode with a different condition flag.
Furthermore not all instructions have necessarily the same layout, so your "LUT" must also contain enough information on how to pack the opcode and its operands correctly.
Lastly assembly opcodes don't necessarily map one-to-one with machine code, it's not rare that a single assembly directive would result in multiple machine opcodes.
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instance in MIPS assembly "li $t0, 0x42" will generate a single opcode (probably something like "addiu $t0, $0, 0x42") while "li $t0, 0x12345" will generate two ("lui $t0, 1; addiu $t0, $t0, 0x2345"). In ARM Thumb the "bl" instruction is actually encoded with two successive 16bit opcodes.
EDIT: actually I just remembered that I did implement a very basic MIPS I assembler, here's the codegen portion: https://github.com/simias/rustation/blob/master/src/assemble...
MIPS is a really simple ISA though, I suspect an assembler for x86 would be a lot more... more.