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The type system
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.
Simple Types Tag Table
| 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:
Fixed Values Table
| 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 Configuration
| 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
Extended Objects Tag Table
| 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 |
Structures
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 |
