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Version 0.18. Work-in-progress, everything is subject to change.

SixtyPical is a low-level programming language with advanced static analysis. Many of its primitive instructions resemble those of the 6502 CPU — in fact it is intended to be compiled to 6502 machine code — but along with these instructions are constructs which ease structuring and analyzing the code. The language 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, 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

Many of these checks are done with abstract interpretation, where we go through the program step by step, tracking not just the changes that happen during a specific execution of the program, but sets of changes that could possibly happen in any run of the program.

SixtyPical also provides some convenient operations based on machine-language programming idioms, such as

  • copying values from one register to another (via a third register when there are no underlying instructions that directly support it); this includes 16-bit values, which are copied in two steps
  • explicit tail calls
  • indirect subroutine calls

SixtyPical is defined by a specification document, a set of test cases, and a reference implementation written in Python 2. The reference implementation can analyze and compile SixtyPical programs to 6502 machine code, which can be run on several 6502-based 8-bit architectures:

  • Commodore 64
  • Commodore VIC-20
  • Atari 2600 VCS
  • Apple II

Quick Start

If you have the VICE emulator installed, from this directory, you can run

./ c64 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 the script on other sources in the eg directory tree, which contains more extensive examples, including an entire game(-like program); see eg/ for a listing.