Difference between revisions of "Forth parser"
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(→Implementation as a state machine) |
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state = scan | state = scan | ||
do | do | ||
| − | skip | + | word = "" |
| − | read word up to white space | + | skip until not white space or eof |
| − | + | if word starts with " | |
| + | then read word up to " or eof | ||
| + | if word does not end with " then exit loop | ||
| + | else read word up to white space or eof | ||
| + | if word ="" then exit loop | ||
match state with | match state with | ||
| scan -> match word with | | scan -> match word with | ||
| ":" -> state = func | | ":" -> state = func | ||
| − | | func -> | + | | ";" -> failwith "Found end of function before definition" |
| − | + | | "(" -> push state | |
| + | state = rem | ||
| + | | x -> failwith "Found illegal word %A outside definition" x ( this assumes that words can't exist outside functions ) | ||
| + | | rem -> match word with | ||
| + | | ")" -> pop state | ||
| + | | func -> match word with | ||
| + | | "(" -> push state | ||
| + | state = rem | ||
| + | | name -> print "Name of function is %A" name | ||
| + | state = def | ||
| + | | def -> match word with | ||
| + | | "(" -> push state | ||
| + | state = rem | ||
| + | | ";" -> print "End of function" | ||
| + | state = scan | ||
| + | | _ -> print "Assembling word %A" word | ||
| + | |||
// Check if compilation was successful | // Check if compilation was successful | ||
match state with | match state with | ||
| scan when word = "" -> print "Success" | | scan when word = "" -> print "Success" | ||
| − | | _ -> print "Error, exit in state | + | | _ -> print "Error, exit in state %A" state |
== Comment == | == Comment == | ||
Latest revision as of 06:19, 27 April 2018
Contents
Implementation as a state machine
state = scan
do
word = ""
skip until not white space or eof
if word starts with "
then read word up to " or eof
if word does not end with " then exit loop
else read word up to white space or eof
if word ="" then exit loop
match state with
| scan -> match word with
| ":" -> state = func
| ";" -> failwith "Found end of function before definition"
| "(" -> push state
state = rem
| x -> failwith "Found illegal word %A outside definition" x ( this assumes that words can't exist outside functions )
| rem -> match word with
| ")" -> pop state
| func -> match word with
| "(" -> push state
state = rem
| name -> print "Name of function is %A" name
state = def
| def -> match word with
| "(" -> push state
state = rem
| ";" -> print "End of function"
state = scan
| _ -> print "Assembling word %A" word
// Check if compilation was successful
match state with
| scan when word = "" -> print "Success"
| _ -> print "Error, exit in state %A" state
Comment
Anything between ( and ) that is not a part of a string or an immediate character should be ignored. So after detecting ( ignore all " until ) After an opening " ignore all ( and ) until closing "
String
When you reach ", start copying characters into a string dictionary until the matching " Emit LDC ... LEA instructions to point to where the string is located in the final binary The length of the string is stored in front of it.
Variable inference
Variables are stored locally on the return stack.
Any word that starts with ">" is a write to a variable.
For each word in function
If word starts with ">"
Add the word to an ordered list of unique words that starts with ">", remove the leading ">"
If there are more than 16 variables emit error
Emit STL #<index in the ordered list>
If word is in the list but does not start with ">"
Emit LDL #<index in the ordered list>
If there are more than 0 variables emit DIM #<largest index> as the first instruction of the function.
This can be skipped if the function calls no other functions and do not use the stack for any or purpose.
TODO: Check if every variable is read and eliminate the ones that are not in use and emit a warning.
This can be done with a simple bitmask to make sure a variable is written and then read.
How to encode constants
if number = 0 then
emit LDC #0
else
find the leftmost hex digit n that is not 0
emit LDC #n
zero out that digit
for each remaining digit n, zero or not
emit LDE #n
#0x1230 can be encoded as:
LDC #0x1
LDE #0x2
LDE #0x3
LDE #0x0
More efficient code can sometimes be achieved by using LDN LSL INC DEC DUP OVER and other opcodes.
Example 1
: lsl for dup + next ; ( Simple complete function that implements left shift by repeated addition ) ":" this tells the parser to define a new function "lsl" is the name of the function Adr Value 0 0xF0 ( for ) 1 0xE0 ( dup ) 2 0xC0 ( add ) 3 0xF1 ( next ) ";" tells the parser that it has reached the end of the definition The parser should create an "object" in memory like: "lsl",0xF0,0xE0,0xC0,0xF1 ( the return opcode is implied )
Example 2
: xorpattern
0 >yval
1080 for
0 >xval
1920 for
xval dup 1 + >xval yval
eor
xval yval plotpixel
next
yval 1 + >yval
next ;
Unique variable names being written to
0 "yval"
1 "xval"
DIM #1 ( Largest index of a variable is 1 )
LDC #0 ( 0 )
STL #0 ( >yval )
LDC #0x4 ( 1080 = 0x483 )
LDE #0x8
LDE #0x3
FOR ( for )
LDC #0 ( 0 )
STL #1 ( >xval )
LDC #0x7 ( 1920 = 0x780 )
LDE #0x8
LDE #0x0
FOR ( for )
LDC #1 ( xval )
LDC #0 ( yval )
LDC #<off> ( Load offset to the function plotpixel )
...
CALL
NEXT
LDC #0 ( yval )
INC
STC #0 ( yval )
NEXT
RETURN
Default opcode equivalency for forth words
Forth string - opcode + - add - - sub * - mul / - div
