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Objects

The following description refers to the 16-bit versions of uLisp. The 32-bit versions are essentially the same, except that objects consist of two 32-bit cells.

All objects in uLisp consist of two 2-byte cells, so every object occupies 4 bytes. There are three fundamental types of object: symbol, number, or cons:

• A symbol contains the identifier SYMBOL in the left cell, and the symbol in the right cell.
• A number contains the identifier NUMBER in the left cell, and the 16-bit signed integer value in the right cell.
• Finally, a cons contains an address pointer in each cell pointing to other objects.

The type identifiers SYMBOL and NUMBER are defined by the type enum; a simplified version is:

enum type {ZERO=0, SYMBOL=2, NUMBER=6};

The low number addresses are never used, so we can uniquely identify the type from the left-hand cell.

The object type is defined by the following typedef:

typedef struct sobject {
  union {
    struct {
      sobject *car;
      sobject *cdr;
    };
    struct {
      unsigned int type;
      union {
        symbol_t name;
        int integer;
        int chars; // For strings
      };
    };
  };
} object;

The two pointers in a cons cell are called car and cdr, for historical reasons.

Setting up uLisp's workspace

The space used by uLisp is reserved by the declaration:

object Workspace[WORKSPACESIZE] OBJECTALIGNED;

The workspacesize depends on the amount of RAM available on the processor. It's 318 objects on an ATmega328.

Initially all the workspace is allocated to the freelist: a list of objects, linked together by cdr pointers. The last object in the list has a NULL cdr pointer:

The workspace is initialised by initworkspace():

void initworkspace () {
  Freelist = NULL;
  for (int i=WORKSPACESIZE-1; i>=0; i--) {
    object *obj = &Workspace[i];
    car(obj) = NULL;
    cdr(obj) = Freelist;
    Freelist = obj;
    Freespace++;
  }
}

Making objects

When allocating objects, cells are taken from the front of the freelist, and the freelist pointer is updated. For example, after creating the list:

(sq 6)

the freelist will be:

An object is allocated by calling myalloc():

object *myalloc () {
  if (Freespace == 0) error2(NIL, PSTR("no room"));
  object *temp = Freelist;
  Freelist = cdr(Freelist);
  Freespace--;
  return temp;
}

The myalloc() function is called by the routines that make an object of each type.

Number objects are created by number(), which takes an integer value:

object *number (int n) {
  object *ptr = myalloc();
  ptr->type = NUMBER;
  ptr->integer = n;
  return ptr;
}

Symbol objects are created by symbol(), which takes an unsigned integer containing a packed version of the symbol:

object *symbol (symbol_t name) {
  object *ptr = myalloc();
  ptr->type = SYMBOL;
  ptr->name = name;
  return ptr;
}

Finally cons() takes two objects to be pointed to by the car and cdr of the cons:

object *cons (object *arg1, object *arg2) {
  object *ptr = myalloc();
  ptr->car = arg1;
  ptr->cdr = arg2;
  return ptr;
}