Difference between revisions of "Forth parser"

Revision as of 17:35, 24 April 2018

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.

How to encode numbers

   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

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

Default opcode equivalency for forth words

Forth string - opcode 
+            - add
-            - sub
*            - mul
/            - div

Questions