STATE Is the interpreter executing code (0) or compiling a word (non-zero)?
LATEST Points to the latest (most recently defined) word in the dictionary.
- HERE When compiling, compiled words go here.
+ HERE Points to the next free byte of memory. When compiling, compiled words go here.
_X These are three scratch variables, used by some standard dictionary words.
_Y
_Z
the top of the return stack.
*/
- defcode "DSP@",4,,DSPFETCH
- mov %esp,%eax
- push %eax
- NEXT
-
- defcode "DSP!",4,,DSPSTORE
- pop %esp
- NEXT
-
defcode ">R",2,,TOR
pop %eax // pop parameter stack into %eax
PUSHRSP %eax // push it on to the return stack
lea 4(%ebp),%ebp // pop return stack and throw away
NEXT
+/*
+ PARAMETER (DATA) STACK ----------------------------------------------------------------------
+
+ These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter
+ stack for us, and it is accessed through %esp.
+*/
+
+ defcode "DSP@",4,,DSPFETCH
+ mov %esp,%eax
+ push %eax
+ NEXT
+
+ defcode "DSP!",4,,DSPSTORE
+ pop %esp
+ NEXT
+
/*
INPUT AND OUTPUT ----------------------------------------------------------------------
+ These are our first really meaty/complicated FORTH primitives. I have chosen to write them in
+ assembler, but surprisingly in "real" FORTH implementations these are often written in terms
+ of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures
+ the implementation. After all, you may not understand assembler but you can just think of it
+ as an opaque block of code that does what it says.
+
+ Let's discuss input first.
+ The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack).
+ So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space)
+ is pushed on the stack.
+ In FORTH there is no distinction between reading code and reading input. We might be reading
+ and compiling code, we might be reading words to execute, we might be asking for the user
+ to type their name -- ultimately it all comes in through KEY.
+ The implementation of KEY uses an input buffer so a certain size (defined at the end of the
+ program). It calls the Linux read(2) system call to fill this buffer and tracks its position
+ in the buffer using a couple of variables, and if it runs out of input buffer then it refills
+ it automatically. The other thing that KEY does is if it detects that stdin has closed, it
+ exits the program, which is why when you hit ^D the FORTH system cleanly exits.
*/
#include <asm-i386/unistd.h>
mov $__NR_exit,%eax // syscall: exit
int $0x80
+/*
+ By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout.
+ This implementation just uses the write system call. No attempt is made to buffer output, but
+ it would be a good exercise to add it.
+*/
+
defcode "EMIT",4,,EMIT
pop %eax
call _EMIT
.bss
2: .space 1 // scratch used by EMIT
+/*
+ Back to input, WORD is a FORTH word which reads the next full word of input.
+
+ What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on).
+ Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it
+ calculates the length of the word it read and returns the address and the length as
+ two words on the stack (with address at the top).
+
+ Notice that WORD has a single internal buffer which it overwrites each time (rather like
+ a static C string). Also notice that WORD's internal buffer is just 32 bytes long and
+ there is NO checking for overflow. 31 bytes happens to be the maximum length of a
+ FORTH word that we support, and that is what WORD is used for: to read FORTH words when
+ we are compiling and executing code. The returned strings are not NUL-terminated, so
+ in some crazy-world you could define FORTH words containing ASCII NULs, although why
+ you'd want to is a bit beyond me.
+
+ WORD is not suitable for just reading strings (eg. user input) because of all the above
+ peculiarities and limitations.
+
+ Note that when executing, you'll see:
+ WORD FOO
+ which puts "FOO" and length 3 on the stack, but when compiling:
+ : BAR WORD FOO ;
+ is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling
+ and immediate mode, and you'll understand why.
+*/
+
defcode "WORD",4,,WORD
call _WORD
push %ecx // push length
// overwrite this buffer. Maximum word length is 32 chars.
5: .space 32
- defcode "EMITSTRING",10,,EMITSTRING
- mov $1,%ebx // 1st param: stdout
- pop %ecx // 2nd param: address of string
- pop %edx // 3rd param: length of string
-
- mov $__NR_write,%eax // write syscall
- int $0x80
+/*
+ . (also called DOT) prints the top of the stack as an integer. In real FORTH implementations
+ it should print it in the current base, but this assembler version is simpler and can only
+ print in base 10.
- NEXT
+ Remember that you can override even built-in FORTH words easily, so if you want to write a
+ more advanced DOT then you can do so easily at a later point, and probably in FORTH.
+*/
defcode ".",1,,DOT
pop %eax // Get the number to print into %eax
call _EMIT
ret
- // Parse a number from a string on the stack -- almost the opposite of . (DOT)
- // Note that there is absolutely no error checking. In particular the length of the
- // string must be >= 1 bytes.
+/*
+ Almost the opposite of DOT (but not quite), SNUMBER parses a numeric string such as one returned
+ by WORD and pushes the number on the parameter stack.
+
+ This function does absolutely no error checking, and in particular the length of the string
+ must be >= 1 bytes, and should contain only digits 0-9. If it doesn't you'll get random results.
+
+ This function is only used when reading literal numbers in code, and shouldn't really be used
+ in user code at all.
+*/
defcode "SNUMBER",7,,SNUMBER
pop %edi
pop %ecx
jnz 1b
ret
+/*
+ DICTIONARY LOOK UPS ----------------------------------------------------------------------
+
+ We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure.
+
+ The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the
+ dictionary. What it actually returns is the address of the dictionary header, if it finds it,
+ or 0 if it didn't.
+
+ So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer:
+
+ pointer to this
+ |
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+
+ See also >CFA which takes a dictionary entry pointer and returns a pointer to the codeword.
+
+ FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why.
+*/
+
defcode "FIND",4,,FIND
pop %edi // %edi = address
pop %ecx // %ecx = length
xor %eax,%eax // Return zero to indicate not found.
ret
- defcode ">CFA",4,,TCFA // DEA -> Codeword address
+/*
+ FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall
+ that FORTH definitions are compiled into lists of codeword pointers).
+
+ In the example below, WORD DOUBLE FIND >CFA
+
+ FIND returns a pointer to this
+ | >CFA converts it to a pointer to this
+ | |
+ V V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+
+ Notes:
+
+ Because names vary in length, this isn't just a simple increment.
+
+ In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but
+ that is not true in most FORTH implementations where they store a back pointer in the definition
+ (with an obvious memory/complexity cost). The reason they do this is that it is useful to be
+ able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions.
+*/
+
+ defcode ">CFA",4,,TCFA
pop %edi
call _TCFA
push %edi
andl $~3,%edi
ret
- defcode "CHAR",4,,CHAR
- call _WORD // Returns %ecx = length, %edi = pointer to word.
- xor %eax,%eax
- movb (%edi),%al // Get the first character of the word.
- push %eax // Push it onto the stack.
- NEXT
+/*
+ COMPILING ----------------------------------------------------------------------
+
+ Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this:
+
+ : DOUBLE DUP + ;
+
+ and we have to turn this into:
+
+ pointer to previous word
+ ^
+ |
+ +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
+ ^ len pad codeword |
+ | V
+ LATEST points here points to codeword of DUP
+
+ There are several problems to solve. Where to put the new word? How do we read words? How
+ do we define : (COLON) and ; (SEMICOLON)?
+
+ FORTH solves this rather elegantly and as you might expect in a very low-level way which
+ allows you to change how the compiler works in your own code.
+
+ FORTH has an interpreter function (a true interpreter this time, not DOCOL) which runs in a
+ loop, reading words (using WORD), looking them up (using FIND), turning them into codeword
+ points (using >CFA) and deciding what to do with them. What it does depends on the mode
+ of the interpreter (in variable STATE). When STATE is zero, the interpreter just calls
+ each word as it looks them up. (Known as immediate mode).
+
+ The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the
+ interpreter just appends the codeword pointers to user memory (the HERE variable points to
+ the next free byte of user memory).
+
+ So you may be able to see how we could define : (COLON). The general plan is:
+
+ (1) Use WORD to read the name of the function being defined.
+
+ (2) Construct the dictionary entry header in user memory:
+
+ pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where
+ ^ | the interpreter will start appending
+ | V codewords.
+ +--|------+---+---+---+---+---+---+---+---+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
+ +---------+---+---+---+---+---+---+---+---+------------+
+ len pad codeword
+
+ (3) Set LATEST to point to the newly defined word and most importantly leave HERE pointing
+ just after the new codeword. This is where the interpreter will append codewords.
+
+ (4) Set STATE to 1. Go into compile mode so the interpreter starts appending codewords.
+
+ After : has run, our input is here:
+
+ : DOUBLE DUP + ;
+ ^
+ |
+ Next byte returned by KEY
+
+ so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads DUP,
+ gets its codeword pointer, and appends it:
+
+ +-- HERE updated to point here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+
+ len pad codeword
+
+ Next we read +, get the codeword pointer, and append it:
+
+ +-- HERE updated to point here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
+ len pad codeword
+
+
+
+*/
defcode ":",1,,COLON
xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
ret
+ defcode "CHAR",4,,CHAR
+ call _WORD // Returns %ecx = length, %edi = pointer to word.
+ xor %eax,%eax
+ movb (%edi),%al // Get the first character of the word.
+ push %eax // Push it onto the stack.
+ NEXT
+
/* This definiton of ' (TICK) is strictly cheating - it also only works in compiled code. */
defcode "'",1,,TICK
lodsl // Get the address of the next word and skip it.
andl $~3,%esi
NEXT
+ defcode "EMITSTRING",10,,EMITSTRING
+ mov $1,%ebx // 1st param: stdout
+ pop %ecx // 2nd param: address of string
+ pop %edx // 3rd param: length of string
+
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+
+ NEXT
+
// COLD must not return (ie. must not call EXIT).
defword "COLD",4,,COLD
.int INTERPRETER // call the interpreter loop (never returns)
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