A running memory image of Ficl consists of one or more FICL_SYSTEMs,
each of which owns exactly one dictionary (FICL_DICT),
and one or more virtual machines (FICL_VM). Each VM owns two stacks
(FICL_STACK) - one for parameters (the parameter stack)
and one for return addresses (the return stack).
Ficl is a permissive, untyped language by nature,
so its fundamental unit of storage is a CELL: a chunk of memory
large enough to hold an address or a scalar type.
FICL_SYSTEM
The system structure associates one or more virtual machines with a dictionary. All FICL_SYSTEMS
include a link pointer that is used to keep track of every allocated system so that memory
can be freed by ficlTermSystem. Each system contains a list of virtual machines associated with
it. Each system has at least one virtual machine. In a typical implementation, there is one virtual
machine per native OS thread, and there may be several VMs sharing a single FICL_SYSTEM, or one
FICL_SYSTEM per VM if the implementation needs to support multiple user sessions in a robust way.
A FICL_SYSTEM also includes a special dictionary for local variable support (if enabled
by FICL_WANT_LOCALS) and another for environment variable support. Environment variables
describe the configuration of the system in conformance with American National Standard Forth
(ANS Forth).
FICL_DICT
A dictionary manages a fixed-size block of contiguous memory. It serves two roles: to keep track
of allocated memory, and to collect symbol tables called wordlists. Each dictionary contains at
least one wordlist. The dictionary organized memory (perhaps this is too kind) as an array of
CELLs that grows from low memory to high memory within fixed limits determined by the
FICL_DEFAULT_DICT parameter in sysdep.h.
A wordlist is the controlling structure of a Ficl symbol table. Each wordlist is a hash table
containing pointers to FICL_WORDs. Each FICL_WORD associates a pointer to code with one or more
CELLs of the dictionay. Each word usually has a name as well, but this is not required. It is
possible to create anonymous words using :NONAME.
Each word's code pointer determines that word's runtime behavior, and by implication the purpose
of its payload data. Some words interpret their payload as a list of Ficl words, and execute them.
This is how new behaviors of the language are defined. Other words view their payload field as
a location in which one or more CELLs can be stored (VARIABLEs, for example). At runtime, such
words push the address of their payload area onto the parameter stack.
FICL_VM
The virtual machine collects state related to execution of Ficl words. Each VM includes
registers used by the inner interpreter, some state variables (AKA user variables) such as
the current numeric base, and a jmpbuf.
A VM has a pointer to the FICL_SYSTEM of which it is a part. It also has a pointer to an incoming
text string that it is interpreting. There are VM methods that excute a word given its address
(xt), and ones that interpret a text string.
FICL_STACK
Each VM owns a parameter stack, a return stack, and if float support is enabled, a float parameter
stack. Parameters, return addresses, and floats are all CELL sized, and values may be
moved back and forth among stacks using various Ficl words for that purpose.
