At any given time only a part of the memory reserved for list structures will actually be in use for storing S-expressions. The remaining registers are arranged in a single list called the free-storage list. A certain register, FREE, in the program contains the location of the first register in this list. When a word is required to form some additional list structure, the first word on the free-storage list is taken and the number in register FREE is changed to become the location of the second word on the free-storage list. No provision need be made for the user to program the return of registers to the free-storage list.
This return takes place automatically whenever the free-storage list has been exhausted during the running of a LISP program. The program that retrieves the storage is called the garbage collector.
Any piece of list structure that is accessible to programs in the machine is considered an active list and is not touched by the garbage collector. The active lists are accessible to the program through certain fixed sets of base registers, such as the registers in the list of atomic symbols, the registers that contain partial results of the LISP computation in progress, etc. The list structures involved may be arbitrarily long but each register that is active must be connected to a base register through a car-cdr chain of registers. Any register that cannot be so reached is not accessible to any program and is nonactive, therefore its contents are no longer of interest.
The nonactive, i.e., inaccessible, registers are reclaimed for the free-storage list by the garbage collector as follows. First, every active register that can be reached through a car-cdr chain is marked by setting its sign negative. Whenever a negative register is reached in a chain during this process, the garbage collector knows that the rest of the list involving that register has already been marked. Then the garbage collector does a linear sweep of the free-storage area, collecting all registers with a positive sign into a new free-storage list, and restoring the original signs of the active registers.
Sometimes list structure points to full words such as BCD print names and numbers. The garbage collector cannot mark these words because the sign bit may be in use. The garbage collector must also stop tracing because the pointers in the address and decrement of a full word are not meaningful.
These problems are solved by putting full words in a reserved section of memory called full-word space. The garbage collector stops tracing as soon as it leaves the free-storage space. Marking in full-word space is accomplished by a bit table.