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lc3as.py

An assembler for the LC-3 fictitious computer.

The LC-3 is described in the book Introduction to Computing Systems: From Bits and Gates to C and Beyond (2nd edition) (3rd edition).

The ISA, which consists of only 15 instructions, is described in Appendix A, which is freely downloadable from the publisher's website.

The LC-3 is 16-bit, word-addressable, and little-endian.

Usage

$ ./lc3as.py --help
lc3as.py: an assembler for the LC-3 fictitious machine.

Assemble foo.s (creates foo.bin, binary format):
  lc3as.py foo.s

Assemble standard input into standard output (binascii format):
  cat foo.s | lc3as.py
  cat foo.s | lc3as.py --binascii

Assemble standard input into standard output (hex format):
  cat foo.s | lc3as.py --hex

Assemble standard input into standard output (forced binary format):
Warning: this will spew garbage into your terminal, so redirect the output:
  cat foo.s | lc3as.py --binary > foo.bin
  cat foo.s | lc3as.py --binary | hexdump -C

Stop after lexing foo.s into tokens (JSON output):
  lc3as.py --lex foo.s | jq .

Stop after parsing foo.s into statements (JSON output):
  lc3as.py --parse foo.s | jq .

Stop after generating the symbol table:
  lc3as.py --symbols foo.s | column -t

Assume input has already been parsed (JSON):
(Useful for developing your own custom assembler syntax)
  ./my-custom-frontend foo.s | lc3as.py --json-input --binary > foo.bin
  lc3as.py --parse foo.s | lc3as.py --json-input --binary > foo.bin

Display this help message:
  lc3as.py -h
  lc3as.py --help

Binary output

Binascii output

Hex output

Compare to the binary output:

Lexer output

The lexer's token output can be viewed as JSON:

Parser output

The parser's output can be viewed as JSON:

The lexer is regex-based, and the parser implementation is recursive-descent. This approach is described in Gary Bernhardt's A Compiler from Scratch.

Accepting JSON input

The fact that this assembler accepts JSON as input allows you to leverage lc3as.py as the backend for your own custom assembler frontend! Create your own custom assembler syntax!

Symbol table

The assembler can be halted after the 1st pass to view the symbol table:

Test failure output

If a test fails, diff is run to highlight the relevant difference in output:

Installing the official LC-3 assembler on Linux, macOS

The tests included with this script work by comparing the output of my assembler to the output of the official assembler.

Here is how I installed the official assembler on my Linux computer:

mkdir -p ~/dist
wget -O ~/dist/lc3tools-0.12.zip http://highered.mheducation.com/sites/dl/free/0072467509/104652/lc3tools_v12.zip
mkdir -p ~/tmp
cd ~/tmp
unzip ~/dist/lc3tools-0.12.zip
cd lc3tools
./configure
make
mkdir -p ~/opt/lc3tools-0.12/bin
cp -a lc3{as,convert,sim,sim-tk} ~/opt/lc3tools-0.12/bin/
mkdir -p ~/bin
cd ~/bin
ln -sf ~/opt/lc3tools-0.12/bin/* .

This will place symlinks to the LC-3 tools into ~/bin, which needs to be in your $PATH.

Before building the LC-3 tools, you may need to install some dependencies. On Debian:

sudo apt-get install flex wish libncurses-dev

Note: on your distro, wish may be provided by tk.

See also

pepaslabs/lc-3b-tools, a disassembler I wrote (in C99) for the LC-3b.