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The first step in garbage collection is to mark all heap objects in use. Each heap cell has a bit reserved for this purpose. For pairs (cons cells) the lowest order bit (0) of the CDR is used. For other types, bit 8 of the CAR is used. The GC bits are never set except during garbage collection. Special C macros are defined in scm.h to allow easy manipulation when GC bits are possibly set. CAR, TYP3, and TYP7 can be used on GC marked cells as they are.

Returns the CDR of a cons cell, even if that cell has been GC marked.

Returns the 16 bit type code of a cell.

We need to (recursively) mark only a few objects in order to assure that all accessible objects are marked. Those objects are sys_protects[] (for example, dynwinds), the current C-stack and the hash table for symbols, symhash.

The function gc_mark() is used for marking SCM cells. If obj is marked, gc_mark() returns. If obj is unmarked, gc_mark sets the mark bit in obj, then calls gc_mark() on any SCM components of obj. The last call to gc_mark() is tail-called (looped).

The function mark_locations is used for marking segments of C-stack or saved segments of C-stack (marked continuations). The argument len is the size of the stack in units of size (STACKITEM).

Each longword in the stack is tried to see if it is a valid cell pointer into the heap. If it is, the object itself and any objects it points to are marked using gc_mark. If the stack is word rather than longword aligned (#define WORD_ALIGN), both alignments are tried. This arrangement will occasionally mark an object which is no longer used. This has not been a problem in practice and the advantage of using the c-stack far outweighs it.

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