Overview

This project provides an assembler and virtual machine for the educational LC-3 16-bit archictechture, originally designed by Patt and Patel ¹.

Specification

This implementation covers most, but not all of the specification ². Including,

1. Branch instructions
2. Arithmetic instructions
3. Trap routines for: a. Polling based input and output

This implementation also incorporates some of the extensions introduced by F. Sieker ³, specifically pseudo-ops ⁴ (ZERO!, SET_COND! and COPY!) as well as the use of the RES opcode for built-in POP! and PUSH!.

Assembler

The assembler produces .obj and .sym files, which can then be fed into the virtual machine. The .obj files, which are machine code binaries, appear to be compatible with some other virtual machines ⁵⁶.

The assembler, invoked with lc3-asm-vm asm currently supports the following CLI flags:

--verbose-log : outputs internal details about assembly process

--case-sensitivive : makes labels case sensitive, closer to original spec

--no-sym-file : no <file>.obj.asm file is generated

Running the VM: Linker/Loader

This implementation introduces a linker to LC-3 assembly.

Currently the linker operates in two stages: (1) it resolves external addressess using a pre-existing .sym listing, (2) when a .obj file is loaded into the virtual machine, it can be loaded alongside other '.obj' files which contain the required addresses.

lc3-asm-vm asm lib.asm

produces lib.obj and lib.obj.sym.

lc3-asm-vm prog.asm --link lib.obj.sym

produces prog.obj with the correct external addresses. Then, at runtime ⁷, call

lc3-asm-vm prog.obj lib.obj

to start a virtual machine instance, load both object files into the memory and set the program counter to the start address of prog.obj.

Linker: assembly syntax

In assembly, labels are marked for import/export by putting the .IMPORT and .EXPORT directives immediately after the label.

E.g. in prog.asm

FUNCTION .IMPORT

and lib.asm

FUNCTION .EXPORT ADD R0, R1, #3

Getting Started

1. Install Rust.
2. Download or clone the repo.
3. Navigate to the root directory of the install.
4. Run cargo build
5. Run lc3-asm-vm...

Example: hello_world.asm

;
;	Hello World! in LC3 assembly.
;
		.ORIG	x3000
		LEA	R0, hello
		PUTS    
		LD	R0, endl
		OUT
		HALT
hello		.STRINGZ "Hello World!"
endl		.FILL	#10
		.END

To assemble this program, you would enter (ignoring file paths)

lc3-asm-vm asm hello_world.asm

Then to run it on the simulator or virtual machine:

lc3-asm-vm load hello_world.obj

Approach

lc3-asm-vm is written in Rust, and the only dependency outside of the Rust Standard Library is the console crate. However, as it stands, external libraries for, e.g., config file loading and CLI argument parsing would provide more functionality with less effort.

At the same time, I wrote this project with the goal of learning about CPU/IS architechture, Rust, assembly and assemblers/compilers. There are design decisions that could be improved on, especially concerning the tokenizer and parser. But the process of finding solutions, even if they are not the ideal solution, cements learning in a way that just implementing or leveraging existing solutions almost never can.

Footnotes

1. Designed by Y. N. Patt & S. Patel, first described in Introduction to Computing Systems Patt & Patel (2000). ↩

2. Interrupt based I/O, the RTI instruction and Program Priority Levels are not currently implemented. ↩

3. https://www.cs.colostate.edu/~fsieker/TestSemester/assignments/LC3CSU/doc/index.html ↩

4. With different conventions/terms. ↩

5. e.g. https://wchargin.com/lc3web/ ↩

6. But where POP! and PUSH! are used the resulting .obj file will lead to reserved opcode exceptions. ↩

7. Note that no error is raised if the user fails to load an external object file, which is a disadvantage of this particular approach. ↩