Forthress

Summary

Forthress is a Forth dialect made for fun and educational purposes. Forthress is written in NASM using bootstrap technique. It means that the main interpreter/compiler loop (outer loop) is written in Forthress. The inner interpreter (see next in src/forthress.asm) is written in assembly, and so are some words.

Most of the language traits are fairy close to the classic Forth dialects. Several things have to be mentioned about Forthress:

• It uses Indirect Threaded Code
• Strings are null-terminated
• XT stands for execution token, an address immediately following word header
• Word header has zero-bytes around the name. Here is the example for dup:

link (8 bytes)	zero (1)	name (variable)	zero (1)	flags (1)	implementation
0x0000000000000000	0	d u p	0	0	dup_impl

Forthress was written as an exercise and an example of how one can create a working Forth interpreter which bootstraps itself.

Forthress is also created as an example for my course book "Low-Level Programming: C, Assembly, and Program Execution on Intel x86-64 Architecture".

Predefined words

• drop ( a -- )

• swap ( a b -- b a )

• dup ( a -- a a )

• rot ( a b c -- b c a )

• Arithmetic:

  • + ( y x -- [ x + y ] )
  • * ( y x -- [ x * y ] )
  • / ( y x -- [ x / y ] )
  • % ( y x -- [ x mod y ] )
  • - ( y x -- [x - y] )
  • < ( y x -- [x < y] )

• Logic:

  • not ( a -- a' ) a' = 0 if a != 0 a' = 1 if a == 0

  • = ( a b -- c ) c = 1 if a == b c = 0 if a != b

  • land ( a b -- a && b ) Logical and

  • lor ( a b -- a || b ) Logical or

• Bitwise

  • and ( a b -- a & b ) Bitwise and
  • or ( a b -- a | b ) Bitwise or

• ' Read word, find its XT, place on stack (or zero if no such word).

Example:

' dup . ( will output dup's address ) 

colon "info", info

• count ( str -- len ) Accepts a null-terminated string, calculates its length.

• printc ( str cnt -- ) Prints a certain amount of characters from string.

• . Drops element from stack and sends it to stdout.

• .S Shows stack contents. Does not pop elements.

• init Stores the data stack base. It is useful for .S.

• docol This is the implementation of any colon-word. The XT itself is not used, but the implementation (i_docol) is.

• exit Exit from colon word.

• r> Push from return stack into data stack.

• >r Pop from data stack into return stack.

• r@ Non-destructive copy from the top of return stack to the top of data stack.

• find ( str -- header_addr ) Accepts a pointer to a string, returns pointer to the word header in dictionary.

• cfa ( word_addr -- xt ) Converts word header start address to the execution token

• emit ( c -- ) Outputs a single character to stdout

• word ( addr -- len ) Reads word from stdin and stores it starting at address addr. Word length is pushed into stack

• number ( str -- len num ) Parses an integer from string.

• prints ( addr -- ) Prints a null-terminated string.

• bye Exits Forthress

• syscall ( call_num a1 a2 a3 a4 a5 a6 -- new_rax new_rdx) Executes syscall The following registers store arguments (according to ABI) rdi , rsi , rdx , r10 , r8 and r9

• branch Jump to a location. Location is absolute. That means that using it interactively is quasi-impossible; however, using it as a low-level primitive to implement if and similar constructs is much more convenient.

  Branch is a compile-only word.

• 0branch Jump to a location if TOS = 0. Location is calculated in a similar way.

  Branch0 is a compile-only word.

• lit Pushes a value immediately following this XT.

• inbuf Address of the input buffer (is used by interpreter/compiler).

• mem Address of user memory.

• last_word Header of last word address.

• state, state State cell address. The state cell stores either 1 (compilation mode) or 0 (interpretation mode).

• here Points to the last cell of the word currently being defined .

• execute ( xt -- ) Execute word with this execution token on TOS.

• @ ( addr -- value ) Fetch value from memory.

• ! ( addr val -- ) Store value by address.

• !c ( addr char -- ) Store one byte by address.

• @c ( addr -- char ) Read one byte starting at addr.

• , ( x -- ) Add x to the word being defined.

• c, ( c -- ) Add a single byte to the word being defined.

• create ( flags name -- ) Create an entry in the dictionary name is the new name. Only immediate flag is implemented ATM.

• : Read word from current input stream and start defining it.

• ;" End the current word definition

• interpret Forthress interpreter/compiler. Uses in_fd internally to know what to interpret.

• interpret-fd (fd -- ) Interpret everything read from file descriptor fd.

Extras

• trap default implementation of a word that will be executed on SIGSEGV.
• trap_dispatch selects the most recent trap version.

Constants

• dp Address of a cell storing the end of global data segment.
• mem Address of the start of global data segment.
• state compile (1) or interpret (0)
• here Current position in current word. Used in compile mode by immediate words.
• in_fd The file descriptor from which we are currently reading words.

Bootstrap

Forthress interpreter uses following words (in order of appearance):

dup
find
branch0 
cfa
state
fetch
lit
minus,
fetch_char
not
swap
drop
comma
exit
execute
number 
state
here
equals
prints
bye 

Compatibility

Linux/LXSS only (it relies on system calls). I don't think we should support more systems because this is an educational project first, and multiple preprocessor directives will clutter it to death.

Code overview

• src/forthress.asm defines the entry point, most important constants, inner interpreter, memory regions etc.
• src/macro.inc is an utility file which stores macro definitions to sweeten the words definition.
• src/words.inc is the assembly file containing all predefined words.
• src/util.asm is built into a separate static library containing input and output utility functions to read strings or numbers from arbitrary descriptor and output them to arbitrary descriptor. Forthress is using Linux system calls directly to deal with I/O and does not rely on any library (such as libc).