The main challenge of writing uLisp was enabling it to run on a processor like the ATmega328. Although this has 32 Kbytes of flash program memory, it only provides 2 Kbytes of RAM.
Most Lisp systems store their entire workspace in RAM, including symbols, system functions, user-defined functions, and data. Clearly this wasn't going to be possible in 2 Kbytes. The solution was to store as much as possible in the program memory, and only use RAM when absolutely necessary.
After experimenting with several alternatives I came up with the following solution:
- The names of the built-in system functions are stored in a lookup table in program memory.
- User-defined symbols are restricted to three letters or digits, so they can be packed into a single 16-bit word.
- The arithmetic uses signed integers, so they will fit into a single word.
- Every Lisp object is a two-word cell, representing either a symbol, number, or cons.
- Garbage collection marks objects by setting the top bit of the leftmost word. This is unused because the amount of RAM will never be more than 32 Kbytes.
In the early stages of designing the interpreter I found Nurullah Akkaya's article "A micro-manual for LISP Implemented in C" very useful .
For the garbage collector I found Bob Nystrom's article "Baby's First Garbage Collector" useful .
For help with making the interpreter tail-recursive I relied on the section "A Properly Tail-Recursive Interpreter" in Peter Norvig's classic book "Paradigms of Artificial Intelligence Programming" .
I am also grateful to the user clawson on AVR Freaks who showed me how to design a procedure name lookup table in program memory.