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Every object in the runtime system is represented by a 4 bytes pointer. The lowest 3 bits of this pointer decide its type, and the others 29 bits its content. The content can be either an immediate value or a pointer to a memory area in the heap. In order to assure that an address in the heap have the lowest 3 bits set to 0 so that they don’t interfere with the tag every object must be allocated at 8 byte boundaries. Fixnums are treated specially beacuse they use only 2 bits as a tag.
| Type | Content | Tag |
|---|---|---|
| Fixnum | 30-bit integer | 00 |
| Cons cell | 29-bit adrdess | 001 |
| Vector | 29-bit address | 101 |
| Closure | 29-bit address | 010 |
| Symbol | 29-bit address | 011 |
| Extended | 29-bit adress | 110 |
Objects with the tag 111 are treated as fixed values:
| Name | Content |
|---|---|
| Nil | 0000 0000 0000 0000 0000 0000 0010 1111 |
| T | 0000 0000 0000 0000 0000 0000 0110 1111 |
| Unbound Value | 0000 0000 0000 0000 0000 0000 1111 1111 |
Characters are 8-bit ASCII characters.
| Character | 0000 0000 0000 0000 <8-bit ASCII value> 0000 1111 |
Objects tagged with 110 are treated as extended objects: the first 4 bytes of the memory area referenced by the address content contain an extended type tag. The extended type tag is then followed by the contents of the object
| Type | Tag | Followed by |
|---|---|---|
| String | <30-bit fixnum> 00 | 8-bit characters, as many as specified by the fixnum in the tag |
| Float | 0000 0000 0000 0000 0000 0000 1100 1111 | 8-byte double precision floating point number |
Cons cell: 8 bytes divided in 2 4-byte parts: the car and the cdr, which can have any value
| size | 4 byte | 4 byte |
|---|---|---|
| content | car | cdr |
Vector: 4 bytes telling the length of the vector followed by the 4-byte elements
| size | 4 byte | 4 byte | 4 byte | … | 4 byte |
|---|---|---|---|---|---|
| content | length | element 0 | element 1 | … | element n-1 |
Closure: 3 mandatory fields holding the address of the code of the function, the number of the other fields (symbol + closed variables), a symbol representing the function for printing purposes, and then the references to the closed variables.
| size | 4 byte | 4 byte | 4 byte | 4 byte | … | 4 byte |
|---|---|---|---|---|---|---|
| content | number of closed vars + 1 | code address | function name | 1st closed variable | … | n-th closed variable |
Symbol: string representing the symbol, a global value associated with it (eventually the unbound value), and a pointer to its property list
| size | 4 byte | 4 byte | 4 byte |
|---|---|---|---|
| content | pointer to a string | global value associated with the symbol | p-list |