You might be interested to know I’ve implemented a high-level language that compiles to Piet, yet supports functions, recursion and flow of control ;) Writing a compiler is actually pretty straightforward: each function is a strip of image, with some stub code hardcoded into the compiler (analogous to C++’s crt0.s) that sets up the initial stackframe and contains code to redirect flow of control to each function’s image area based on the value on the top of the stack. On entry to a function, more hardwired code pops the top value off the stack and branches to a location within the function based on that. On entry to a block, a value is pushed for every variable local to that block; the compiler keeps track of the local stackframe size so is able to generate the appropriate number of pops when leaving a block or exiting the function; this together with the table of local variables provides enough information to generate the rolls for fetching and storing variable values. To call a function, its arguments are pushed, followed by the index of the current function and return location within it, followed by the index of the target function and 0 for the entrypoint, and flow of control is transferred to the init stub.

See http://www.toothycat.net/wiki/wiki.pl?MoonShadow/Piet – scroll down a little way past the assembler example for links to the compiler and samples.