c++ - Can we assume the last 2 bits of a memory address are 00 and reuse those bits? A windows 7 page fault blue screen -

-

my friend programming in c++ on 64-bit windows 7 pc , came crazy idea save little bit of memory: observed last 2 bits of memory addresses seemed 00, figured use bits other things , when memory address needed, use bit-mask set last 2 bits 0, either when writing or reading memory. reason why he's using last 2 bits needs work on 32-bit systems too. anyway, on windows 7 64-bit system got following blue screen error when running program:

page_fault_in_non_paged_area

could crazy memory savings idea causing this? i.e., can happen last 2 bits of memory address not 00, , he's accessing memory that's partly on 1 of memory pages, partly off page? in event, needs work on popular operating systems. question applies other operating systems well.

if (in windows 7 64 bit, @ least) scheme guaranteed work (if coded properly), else causing unusual blue screen crash?

your friend taking advantage of feature known tagged pointers. on windows, raymond chen has warning regarding on blog:

there no /8tb flag on 64-bit windows

a customer reported 64-bit application crashing on windows 8.1. traced problem fact user-mode address space 64-bit applications on windows 8.1 128tb, whereas 8tb on earlier versions of windows x64. ...

...

as how ended having dependency on address space being @ 8tb, didn't say, have guess: using unused bits tagging.

if going use tagged pointers, you need put tag bits in least significant bits, since bits control. example, if align objects on 16-byte boundaries, have 4 available bits tagging. if you're going use upper bits tagging, @ least verify upper bits available.

something more important watch out - memory pointer allocated os might aligned in way allows tagging, if intermediate memory manager sits between user's code , os (which case), manager allocates os memory internally , divides app use, pointers manager gives out app might not aligned in way allows tagging. cannot tag arbitrary memory pointer without knowing came or how aligned.


Comments

Post a Comment

Popular posts from this blog

c - Bitwise operation with (signed) enum value -

-
i using enumerator values flags: typedef enum { = 0x00, b = 0x01u, // u has no influence, expected c = 0x02u, // u has no influence, expected ... } enum_name; volatile unsigned char* reg = someaddress; *reg |= b; according misra-c:2004 bitwise operations shall not done signed type. unfortunately, compiler iar use signed int (or short or char) underlying type of enums, , option can find relates size, not signedness ("--enum-is-int"). according iar c/c++ development guide arm , pages 169 , 211, can define type of enum s if enable iar language extensions ( -e command-line option, or project > options > c/c++ compiler > language > allow iar extensions in ide). in particular, should define "sentinel" value, make sure compiler chooses correct type. prefers signed types, , uses smallest possible integer type, sentinel should largest positive integer corresponding unsigned integer type can describe. example, typedef enum
Read more

xslt - Unnest parent nodes by child node -

-
in xml there items 0-n attributes , item should copied each attribute new item 1 attribute. given xml this: <?xml version="1.0" encoding="utf-8" ?> <items> <item> <name>a</name> <attributes> <attribute> <key>attribute1</key> <value>1</value> </attribute> </attributes> </item> <item> <name>b</name> </item> <item> <name>c</name> <attributes> <attribute> <key>attribute1</key> <value>5</value> </attribute> <attribute> <key>attribute2</key> <value>2</value> </attribute> <attribute> <key>attri
Read more

YouTubePlayerFragment cannot be cast to android.support.v4.app.Fragment -

-
im stuck on 3 days , have tried have come across after lots of searching ,this post last resort, im trying open youtubeplayerfragment using navigation drawer youtubeplayerfragment cannot cast android.support.v4.app.fragment problem edit question : have updated code have used youtubeplayersupportfragment instead of youtubeplayerfragment ,i error in displayview method (case 4: fragment = new youtube(); break;) in main activity saying cannot convert youtube fragment , cannot logcat since cant run it edit question 2: code updated again have runtime problem ,heres logcat 07-22 22:15:09.782: e/androidruntime(20603): fatal exception: main 07-22 22:15:09.782: e/androidruntime(20603): process: net.frankandwalters, pid: 20603 07-22 22:15:09.782: e/androidruntime(20603): java.lang.nullpointerexception: attempt invoke virtual method 'void com.google.android.youtube.player.youtubeplayerview.a()' on null object reference 07-22 22:15:09.782: e/androidruntime(20603): @ com.goog
Read more