📄 machexamp.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:45 2005 by faqproc version 2.7 --><!-- from source file null.sgml dated Wed Dec 21 13:03:25 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/null/machexamp.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 5.17</title><link href="confusion4.html" rev=precedes><link href="runtime0.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="confusion4.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="runtime0.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 5.17</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Seriously, have any actual machines really used nonzero nullpointers,or different representations for pointers to different types?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The Prime50 seriesused segment 07777, offset 0 for the null pointer,at least for PL/I.Later models used segment 0, offset 0 for null pointers in C,necessitating new instructions such as TCNP (Test C Null Pointer),evidentlyas a sop to<a href="sop.html" rel=subdocument>[footnote]</a>all the extant poorly-written C code which madeincorrect assumptions.Older, word-addressed Primemachineswere also notorious for requiring largerbyte pointers (<TT>char *</TT>'s)than word pointers (<TT>int *</TT>'s).</p><p>TheEclipse MV seriesfrom Data General has three architecturallysupported pointer formats (word, byte, and bit pointers),two of which are used by C compilers:byte pointers for <TT>char *</TT> and <TT>void *</TT>,and word pointers for everything else.For historical reasons during the evolutionof the 32-bit MV line from the 16-bit Nova line,word pointers and byte pointershad the offset, indirection, and ring protection bitsin different places in the word.Passing a mismatched pointer format to a functionresulted in protection faults.Eventually, the MV C compiler added many compatibility optionsto try to deal with code that had pointer type mismatch errors.</p><p>Some <TT>Honeywell-Bull</TT> mainframes usethe bit pattern 06000 for (internal) null pointers.</p><p>The CDC Cyber 180 Series has 48-bit pointersconsisting of a ring, segment, and offset.Most users (in ring 11) have null pointers of 0xB00000000000.It was common on old CDC ones-complement machinesto use an all-one-bits word as a special flag for all kinds of data,including invalid addresses.</p><p>The old HP 3000seriesuses a different addressing scheme for byte addressesthan for word addresses;like several of the machines aboveit therefore uses different representationsfor <TT>char *</TT> and <TT>void *</TT> pointersthan for other pointers.</p><p>The Symbolics Lisp Machine, a tagged architecture,does not even have conventional numeric pointers;it uses the pair <TT><NIL, 0></TT>(basically a nonexistent <TT><</TT>object, offset<TT>></TT> handle)as a C null pointer.</p><p>Depending on the``memory model'' in use,8086-familyprocessors (PC compatibles) may use16-bit data pointersand32-bit function pointers,or vice versa.</p><p>Some 64-bit Cray machines represent <TT>int *</TT>in the lower 48 bits of a word;<TT>char *</TT> additionally usessome ofthe upper 16 bits to indicatea byte address within a word.</p><p>Additional links:A<a href="wierdptrs.ct.html">message from Chris Torek</a>with more details about some of these machines.</p><p>References:K&R1 Sec. A14.4 p. 211<br></p><!-- aend --><p><hr><a href="confusion4.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="runtime0.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/null/machexamp.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -