git @ Cat's Eye Technologies SixtyPical / semaphore-setup

Tree @semaphore-setup (Download .tar.gz)


Version 0.20. Work-in-progress, everything is subject to change.

SixtyPical is a low-level programming language supporting a sophisticated static analysis. Its reference compiler can generate efficient code for several 6502-based target platforms while catching many common mistakes at compile-time, reducing the time spent in debugging.

Quick Start

Make sure you have Python (2.7 or 3.5+) installed. Then clone this repository and put its bin directory on your executable search path. Then you can run:


If you have the VICE emulator suite installed, you can run

sixtypical --run-on=x64 eg/c64/hearts.60p

and it will compile the hearts.60p source code and automatically start it in the x64 emulator, and you should see:

Screenshot of result of running hearts.60p

You can try sixtypical --run-on on other sources in the eg directory tree, which contains more extensive examples, including an entire game(-like program); see eg/ for a listing.


SixtyPical aims to fill this niche:

  • You'd use assembly, but you don't want to spend hours debugging (say) a memory overrun that happened because of a ridiculous silly error.
  • You'd use C or some other "high-level" language, but you don't want the extra overhead added by the compiler to manage the stack and registers.

SixtyPical gives the programmer a coding regimen on par with assembly language in terms of size and hands-on-ness, but also able to catch many ridiculous silly errors at compile time.

Low level

Many of SixtyPical's primitive instructions resemble those of the MOS Technology 6502 — it is in fact intended to be compiled to 6502 machine code. However, it also provides some "higher-level" operations based on common 8-bit machine-language programming idioms, including

  • copying values from one register to another (via a third register when there are no underlying instructions that directly support it)
  • copying, adding, and comparing 16-bit values (done in two steps)
  • explicit tail calls
  • indirect subroutine calls

While a programmer will find these constructs convenient, their inclusion in the language is primarily to make programs easier to analyze.

Static analysis

The SixtyPical language defines an effect system, and the reference compiler abstractly interprets the input program to check that it conforms to it. It can detect common mistakes such as

  • you forgot to clear carry before adding something to the accumulator
  • a subroutine that you called trashes a register you thought it preserved
  • you tried to read or write a byte beyond the end of a byte array
  • you tried to write the address of something that was not a routine, to a jump vector

Efficient code

Unlike most conventional languages, in SixtyPical the programmer must manage memory very explicitly, selecting the registers and memory locations to store each piece of data in. So, unlike a C compiler such as cc65, a SixtyPical compiler doesn't need to generate code to handle calling conventions or register allocation. This results in smaller (and thus faster) programs.

The flagship demo, a minigame for the Commodore 64, compiles to a 930-byte .PRG file.

Target platforms

The reference implementation can analyze and compile SixtyPical programs to 6502 machine code formats which can run on several 6502-based 8-bit architectures:

For example programs for each of these, see eg/


SixtyPical is defined by a specification document, a set of test cases, and a reference implementation written in Python.

There are over 400 test cases, written in Falderal format for readability. In order to run the tests for compilation, dcc6502 needs to be installed.