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<head>
<meta charset="UTF-8">
<title>µEforth Internals</title>
<link rel="stylesheet" href="static/eforth.css">
</head>
<body>
<h1>µEforth Internals</h1>
{{MENU}}
<p>
µEforth (micro-Eforth) simplifies EForth even futher, by building just enough
of the core of the system in C to allow the rest to be be built in proper Forth.
</p>
<p>
A handful of "tricky" words that involve internal loops or many steps are built in their own
functions:
</p>
<pre>
FIND ( a n -- xt | 0 )
PARSE ( ch -- a n )
S&gt;NUMBER? ( a n -- n f | 0 )
CREATE ( "name" -- )
EVALUATE1 ( -- )
</pre>
<p>
This includes <code>EVALUATE1</code> which parses a single word and
interprets or compiles it (reusing <code>PARSE</code>,
<code>FIND</code>, and <code>S&gt;NUMBER?</code>).
</p>
<p>
See <a href="https://github.com/flagxor/eforth/blob/main/common/core.h">core.h</a>.
</p>
<p>
A few global variables connect parsing and compilation state between
C and Forth (by way of a memory region accessed via <code>'SYS</code>):
</p>
<pre>
'TIB --- Pointer to the Translation Input Buffer
#TIB --- Length of the Translation Input Buffer
&gt;IN --- Number of characters consumed from TIB
BASE --- Numeric base for printing and parsing
STATE --- State of compiling, -1 for compiling, 0 for interpreting
CURRENT --- Pointer to pointer to last word of current vocabulary
CONTEXT --- Pointer to pointer to last word of context vocabulary
'NOTFOUND --- Execution token of a handler to call on word not found
</pre>
<p>
Error handling is routed via a deferred callback in <code>'NOTFOUND</code>
used when a word is absent from the dictionary.
This is eventually directed to an error routing that prints
a proper error, once I/O and exceptions are available.
</p>
<p>
<a href="https://en.wikipedia.org/wiki/X_Macro">X-Macros</a>
are then used to build up a small set of core opcodes defined in 1-3 lines each:
</p>
<pre>
0= 0&lt; + U/MOD */MOD AND OR XOR
LSHIFT RSHIFT DUP SWAP OVER DROP
@ SL@ SW@ C@ ! L! W! C! SP@ SP! RP@ RP!
&gt;R R&gt; R@ : ; EXIT
EXECUTE BRANCH 0BRANCH DONEXT DOLIT
ALITERAL CELL DOES&gt; IMMEDIATE 'SYS
</pre>
<p><b>NOTE: Later to reduce the use of the RAM heap and improve performance,
additional non-essential extra opcodes were added in place of high-level
words.</b></p>
<p>
See <a href="https://github.com/flagxor/eforth/blob/main/common/opcodes.h">opcodes.h</a>.
</p>
<p>
I/O and access to systems outside Forth are connected via a few per platform words.
Typically this set of words should be minimal, while still allowing relevant parts
of the host system to be available to Forth.
</p>