75 lines
3.3 KiB
HTML
75 lines
3.3 KiB
HTML
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<html lang="en">
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<head>
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<title>Memory Layout - SBCL Internals</title>
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<meta name="description" content="SBCL Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Character-and-String-Types.html#Character-and-String-Types" title="Character and String Types">
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<link rel="next" href="Reader-and-Printer.html#Reader-and-Printer" title="Reader and Printer">
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This manual is part of the SBCL software system. See the `README'
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file for more information.
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This manual is in the public domain and is provided with
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absolutely no warranty. See the `COPYING' and `CREDITS' files for
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more information.
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<body>
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<div class="node">
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<p>
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<a name="Memory-Layout"></a>
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Next: <a rel="next" accesskey="n" href="Reader-and-Printer.html#Reader-and-Printer">Reader and Printer</a>,
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Up: <a rel="up" accesskey="u" href="Character-and-String-Types.html#Character-and-String-Types">Character and String Types</a>
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<hr>
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</div>
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<!-- node-name, next, previous, up -->
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<h3 class="section">9.1 Memory Layout</h3>
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<p>Characters are immediate objects (that is, they require no heap
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allocation) in all permutations of build-time options. Even on a 32-bit
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platform with <code>:SB-UNICODE</code>, there are three bits to spare after
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allocating 8 bits for the character widetag and 21 for the character
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code. There is only one such layout, and consequently only one widetag
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is needed: the difference between <code>base-char</code> and <code>character</code>
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is purely on the magnitude of the <code>char-code</code>.
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<p>Objects of type <code>(simple-array nil (*))</code> are represented in memory
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as two words: the first is the object header, with the appropriate
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widetag, and the second is the length field. No memory is needed for
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elements of these objects, as they can have none.
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<p>Objects of type <code>simple-base-string</code> have the header word
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with widetag, then a word for the length, and after that a sequence of
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8-bit <code>char-code</code> bytes. The system arranges for there to be a
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null byte after the sequence of lisp character codes.
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<p>Objects of type <code>(simple-array character (*))</code>, where this is a
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distinct type from <code>simple-base-string</code>, have the header word with
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widetag, length, and then a sequence of 32-bit <code>char-code</code> bytes.
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Again, the system arranges for there to be a null word after the
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sequence of character codes.
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<p>Non-simple character arrays, and simple character arrays of non-unit
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dimensionality, have an array header with a reference to an underlying
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data array of the appropriate form from the above representations.
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</body></html>
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