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Note: In all cases below, bits are numbered starting at 1 and beginning with the lowest-order bit.
There exist two different kinds of data objects in the CHICKEN system: immediate and non-immediate objects.
Immediate objects are represented by a single machine word, which is usually of 32 bits length, or 64 bits on 64-bit architectures. The immediate objects come in four different flavors:
fixnums, that is, small exact integers, where bit 1 is set to 1. This gives fixnums a range of 31 bits for the actual numeric value (63 bits on 64-bit architectures).
characters, where bits 1-4 are equal to C_CHARACTER_BITS. The Unicode code point of the character is encoded in bits 9 to 32.
booleans, where bits 1-4 are equal to C_BOOLEAN_BITS. Bit 5 is one for #t and zero for #f.
other values: the empty list, the value of unbound identifiers, the undefined value (void), and end-of-file. Bits 1-4 are equal to C_SPECIAL_BITS; bits 5 to 8 contain an identifying number for this type of object. The following constants are defined: C_SCHEME_END_OF_LIST C_SCHEME_UNDEFINED C_SCHEME_UNBOUND C_SCHEME_END_OF_FILE
Collectively, bits 1 and 2 are known as the immediate mark bits. When bit 1 is set, the object is a fixnum, as described above, and bit 2 is part of its value. When bit 1 is clear but bit 2 is set, it is an immediate object other than a fixnum. If neither bit 1 nor bit 2 is set, the object is non-immediate, as described below.
Non-immediate objects are blocks of data represented by a pointer into the heap. The pointer's immediate mark bits (bits 1 and 2) must be zero to indicate the object is non-immediate; this guarantees the data block is aligned on a 4-byte boundary, at minimum. Alignment of data words is required on modern architectures anyway, so we get the ability to distinguish between immediate and non-immediate objects for free.
The first word of the data block contains a header, which gives information about the type of the object. The header has the size of a machine word, usually 32 bits (64 bits on 64 bit architectures).
Bits 1 to 24 contain the length of the data object, which is either the number of bytes in a string (or byte-vector) or the the number of elements for a vector or for a structure type.
Bits 25 to 28 contain the type code of the object.
Bits 29 to 32 contain miscellaneous flags used for garbage collection or internal data type dispatching. These flags are:
- Flag used for forwarding garbage collected object pointers.
- Flag that specifies whether this data object contains raw bytes (a string or byte-vector) or pointers to other data objects.
- Flag that specifies whether this object contains a special non-object pointer value in its first slot. An example for this kind of objects are closures, which are a vector-type object with the code-pointer as the first item.
- Flag that specifies whether the data area of this block should be aligned on an 8-byte boundary (floating-point numbers, for example).
The actual data follows immediately after the header. Note that block-addresses are always aligned to the native machine-word boundary. Scheme data objects map to blocks in the following manner:
pairs: vector-like object (type bits C_PAIR_TYPE), where the car and the cdr are contained in the first and second slots, respectively.
vectors: vector object (type bits C_VECTOR_TYPE).
strings: byte-vector object (type bits C_STRING_TYPE).
procedures: special vector object (type bits C_CLOSURE_TYPE). The first slot contains a pointer to a compiled C function. Any extra slots contain the free variables (since a flat closure representation is used).
flonums: a byte-vector object (type bits C_FLONUM_BITS). Slots one and two (or a single slot on 64 bit architectures) contain a 64-bit floating-point number, in the representation used by the host systems C compiler.
symbols: a vector object (type bits C_SYMBOL_TYPE). Slots one and two contain the toplevel variable value and the print-name (a string) of the symbol, respectively.
ports: a special vector object (type bits C_PORT_TYPE). The first slot contains a pointer to a file- stream, if this is a file-pointer, or NULL if not. The other slots contain housekeeping data used for this port.
structures: a vector object (type bits C_STRUCTURE_TYPE). The first slot contains a symbol that specifies the kind of structure this record is an instance of. The other slots contain the actual record items.
pointers: a special vector object (type bits C_POINTER_TYPE). The single slot contains a machine pointer.
tagged pointers: similar to a pointer (type bits C_TAGGED_POINTER_TYPE), but the object contains an additional slot with a tag (an arbitrary data object) that identifies the type of the pointer.
Data objects may be allocated outside of the garbage collected heap, as long as their layout follows the above mentioned scheme. But care has to be taken not to mutate these objects with heap-data (i.e. non-immediate objects), because this will confuse the garbage collector.
For more information see the header file chicken.h.
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