Boxed Values

LispBM can encode values of 4 different types into the car or cdr of a cons-cell. How this works can be found in the earler walkthrough of lispBM. The 4 types that can be encoded are, symbol, character, integer and unsigned integer. All of these types are 28Bit.

To be able to express computations on 32Bit values, integer, unsigned integer and float as well as arrays, a set of boxed types were introduced.

The boxed values, except arrays, are stored in cons cells but have an extra level of indirection in the form of a typed pointer. Since the lisp heap is small there are unused bits on the most significant side. The 4 most significant bits are used to encode different typed pointers. Currently the following kinds of typed pointers are in use:

#define PTR_TYPE_CONS        0x10000000u
#define PTR_TYPE_BOXED_I     0x20000000u
#define PTR_TYPE_BOXED_U     0x30000000u
#define PTR_TYPE_BOXED_F     0x40000000u
#define PTR_TYPE_ARRAY       0xD0000000u

As an example, a float value is encoded as a cons cell where all the 32Bits of the car are used for the floating point value. The cdr of this cell contail a special symbol DEF_REPR_BOXED_F_TYPE. The special symbols used in the cdr position of boxed values are listed below.

#define DEF_REPR_ARRAY_TYPE     0x20FFFF
#define DEF_REPR_BOXED_I_TYPE   0x21FFFF
#define DEF_REPR_BOXED_U_TYPE   0x22FFFF
#define DEF_REPR_BOXED_F_TYPE   0x23FFFF

Only the cdr field of a cons cell hold a GC bit so there is no problem in using all 32Bits of the car if only it can be identified properly.

Arrays are a special case and not fully implemented yet. Currently the only thing that is an array in lispBM is a text string. Most of the plumbing is there for implementing arrays of any other lispBM value type, it just needs to be hacked up. Arrays are encoded in a very similar way to boxed values, just that instead of a 32Bit value in the car slot there is an arbitrary pointer. The array storage itself is allocated using malloc. When garbage collection frees a cons cell with the special symbol DEF_REPR_ARRAY_TYPE, it also calls free on the pointer in the car position.