13.23.3 The relocation vector

The relocation vector is a Scheme vector referenced by the meta data of a code object. In this documentation we refer to the code object associated to a relocation vector as the “subject code object”. The relocation vector can be empty. The relocation vector is divided into records, each some words long:

   0     1     2     3     4     5     6     7
|-----|-----|-----|-----|-----|-----|-----|-----| reloc vector
|...........|.................|.................|
 1st record      2nd record        3rd record

the first word in each record contains a fixnum which must be interpreted as bit field (this fixnum is never zero). The layout of the first machine word in a record is:

• On 32-bit platforms:

       bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
  ... |----|----|----|----|----|----|----|----|
  ........................|.........|.........| 1st word
    number of bytes        record tag fixnum tag
  

• On 64-bit platforms:

       bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
  ... |----|----|----|----|----|----|----|----|
  ...................|.........|..............| 1st word
    number of bytes   record tag   fixnum tag
  

The 2 least significant bits of the fixnum’s data represent a type tag for the record:

#b00

Record referencing a vanilla Scheme object.

#b01

Record referencing the address of a C function.

#b10

Record referencing an offset in the memory block of a Scheme object.

#b11

Record referencing an entry point in a code object.

the remaining most significant bits of the fixnum’s data area represent a number of bytes, a displacement in the data area of the subject code object selecting a target word:

 meta data        machine code
|---------|----------------+---+-------| code object
          |................|...|
             displacement   target word

The purpose of each record in the relocation vector is to provide operands from which a value for the target word can be computed and stored in the middle of machine code; this way the machine code can easily use it. When the subject code object is created: the values must be computed and stored. Every time the garbage collector moves the subject code object: the values must be recomputed and updated. Whenever the subject code object is serialised into a FASL file: the relocation vector is serialised along with the code.

To allow inspection and comprehension of the records, the function code-reloc-vector->sexp transforms a relocation vector into a human readable symbolic expression (see code-reloc-vector->sexp).