110 lines
3.1 KiB
HTML
110 lines
3.1 KiB
HTML
<!DOCTYPE html>
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<!--
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Copyright 2021 Bradley D. Nelson
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<head>
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<meta charset="UTF-8">
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<title>µEforth Internals</title>
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<link rel="stylesheet" href="static/eforth.css">
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</head>
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<body>
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<h1>µEforth Internals</h1>
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{{MENU}}
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<p>
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µEforth (micro-Eforth) simplifies EForth even futher, by building just enough
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of the core of the system in C to allow the rest to be be built in proper Forth.
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</p>
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<p>
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A handful of "tricky" words that involve internal loops or many steps are built in their own
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functions:
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</p>
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<pre>
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FIND ( a n -- xt | 0 )
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PARSE ( ch -- a n )
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S>NUMBER? ( a n -- n f | 0 )
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CREATE ( "name" -- )
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EVALUATE1 ( -- )
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</pre>
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<p>
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This includes <code>EVALUATE1</code> which parses a single word and
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interprets or compiles it (reusing <code>PARSE</code>,
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<code>FIND</code>, and <code>S>NUMBER?</code>).
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</p>
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<p>
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See <a href="https://github.com/flagxor/eforth/blob/main/common/core.h">core.h</a>.
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</p>
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<p>
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A few global variables connect parsing and compilation state between
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C and Forth (by way of a memory region accessed via <code>'SYS</code>):
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</p>
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<pre>
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'TIB --- Pointer to the Translation Input Buffer
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#TIB --- Length of the Translation Input Buffer
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>IN --- Number of characters consumed from TIB
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BASE --- Numeric base for printing and parsing
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STATE --- State of compiling, -1 for compiling, 0 for interpreting
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CURRENT --- Pointer to pointer to last word of current vocabulary
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CONTEXT --- Pointer to pointer to last word of context vocabulary
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'NOTFOUND --- Execution token of a handler to call on word not found
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</pre>
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<p>
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Error handling is routed via a deferred callback in <code>'NOTFOUND</code>
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used when a word is absent from the dictionary.
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This is eventually directed to an error routing that prints
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a proper error, once I/O and exceptions are available.
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</p>
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<p>
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<a href="https://en.wikipedia.org/wiki/X_Macro">X-Macros</a>
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are then used to build up a small set of core opcodes defined in 1-3 lines each:
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</p>
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<pre>
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0= 0< + U/MOD */MOD AND OR XOR
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LSHIFT RSHIFT DUP SWAP OVER DROP
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@ SL@ SW@ C@ ! L! W! C! SP@ SP! RP@ RP!
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>R R> R@ : ; EXIT
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EXECUTE BRANCH 0BRANCH DONEXT DOLIT
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ALITERAL CELL DOES> IMMEDIATE 'SYS
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</pre>
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<p><b>NOTE: Later to reduce the use of the RAM heap and improve performance,
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additional non-essential extra opcodes were added in place of high-level
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words.</b></p>
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<p>
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See <a href="https://github.com/flagxor/eforth/blob/main/common/opcodes.h">opcodes.h</a>.
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</p>
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<p>
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I/O and access to systems outside Forth are connected via a few per platform words.
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Typically this set of words should be minimal, while still allowing relevant parts
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of the host system to be available to Forth.
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</p>
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