32bit Linux vs 64 bit Linux Post: 302318958

Sponsored Content

Operating Systems Linux Red Hat 32bit Linux vs 64 bit Linux Post 302318958 by robsonde on Saturday 23rd of May 2009 12:51:23 AM

05-23-2009

Registered User

Any 32 bit OS will only see / use 3.5GB of RAM.

Remember that in the absence of the 64 bit support, the OS memory manager is limited to a 4GB physical address space. Most of that address space is filled with RAM, but not all of it. Memory-mapped devices (such as your video card) will use some of that physical address space, as will the BIOS ROMs. After all the non-memory devices have had their say, there will be less than 4GB of address space available for RAM below the 4GB physical address boundary.

the motherboard assigned the ROMs and the hardware devices to the physical address space between 3.5GB and 4GB (occupying about 0.5GB of address space). When you start plugging in your memory chips, then, they are assigned physical addresses starting at the bottom, and then skip over the address space that has already been assigned to the hardware and ROM, then resume.

On this imaginary system, then, the 0.5GB of address space used for hardware and ROMs causes that much memory to get shoved upwards, and it ends up above the 4GB boundary. Without 64 bit support, the processor is capable only of addressing memory below the 4GB boundary, which means that the memory above that boundary is inaccessible. It's consuming electricity but isn't doing anything.

The solution is to go to 64-bit OS so that the processor can access the physical address space above the 4GB boundary.

So why don't we just map the ROMs and the hardware devices to space above 4GB??
then the CPU can't access the IO devices so you have system with 4GB of RAM and no video card......

any questions??

robsonde

View Public Profile for robsonde

Find all posts by robsonde

8 More Discussions You Might Find Interesting

1. Linux

Linux 32bit or 64bit

Hi, I want to know what is command to know which will tell wheather linux is 32 or 64 bit

2. Linux

Linux version v.s. 32bit/64bit

Where can I get a list that maps the each Linux version to corresponding 32/64 bits model? e.g. OS -> Model (ILP32, LP64, ...) RHLE3 -> ? RHLE4 -> ? RHLE5 -> ? ... It would be better if there is such a list that contains most of current UNIX OS versions. ...

3. Gentoo

compiling 32 bit application on 64 bit linux(x86_64)

hi all, i have a 64 bit linux machine. $uname -a Linux SVRDELLD30 2.6.9-42.ELsmp #1 SMP Tue Aug 15 10:35:26 BST 2006 x86_64 x86_64 x86_64 GNU/Linux here by default gcc creates 64 bit executable. but for some reason i want to create 32bit executable. first i want to create 32 bit object...

4. Red Hat

boot the 32 bit kernel on a 64 bit PPC Linux machine?

Hi all, I'm looking to cover a corner case for an upcoming test cycle. Is there a way to boot a RedHat Advanced Server 4 (update 3) installed on a Power PC machine to use a 32 bit kernel? This would be similar to what is done here -> https://www.unix.com/aix/26204-aix-platform.html I've done...

5. Solaris

32bit or 64 bit

Not really a Unix question as such :o, but what advantages or disadvantages are there between using 32bit or 64bit applications on a T5220 running Solaris 10? What about mixing them e.g. 64 bit app using 32 bit libraries or vice versa?

6. UNIX for Advanced & Expert Users

migrating unix mp-ras 32 bit to linux suse 64 bit

Hi. I need to migrate the whole unix environment from a Unix mp-ras 32 bit to a Linux Suse 64 bit. 1) can i use cpio to copy the data? 2) can i just copy the users from unix to linux or do i have to create them by hand 3) are there any other concerns i should worry about? thanx

7. Linux

Need assistance to enable more that 4GB RAM on Linux 32Bit OS.

How to enable more than 4GB RAM support on Linux 32bit OS? OS: CentOS release 5.4 (Final) Kernel version: 2.6.18-53.el5 Arch: 32Bit I got solution at Innovationframes.com • View topic - How to enable more than 4GB RAM support on Linux 32bit OS? but my question is the steps given...

8. High Performance Computing

AVX for Linux (32-bit)

For Intel processors we've a lot of new instruction sets (AVX and AVX2 already exists, AVX512 is announced). Therefore an application has to check during run time which instruction sets are available. I've written for that purpopse some procedures, which are callable from C or C++. A strange...

LEARN ABOUT SUNOS

kmem

mem(7D) 							      Devices								   mem(7D)

NAME

       mem, kmem, allkmem - physical or virtual memory access

SYNOPSIS

       /dev/mem

       /dev/kmem

       /dev/allkmem

DESCRIPTION

       The file /dev/mem is a special file that provides access to the physical memory of the computer.

       The  file  /dev/kmem  is  a special file that provides access to the virtual address space of the operating system kernel, excluding memory
       that is associated with an I/O device.

       The file /dev/allkmem is a special file that provides access to the virtual address space of the operating system kernel, including  memory
       that is associated with an I/O device.  You can use any of these devices to examine and modify the system.

       Byte  addresses	in  /dev/mem are interpreted as physical memory addresses. Byte addresses in /dev/kmem and /dev/allkmem are interpreted as
       kernel virtual memory addresses.  A reference to a non-existent location returns an error. See ERRORS for more information.

       The file /dev/mem accesses physical memory; the size of the file is equal to the amount of physical memory in the computer.  This size  may
       be  larger  than 4GB on a system running the 32-bit operating environment. In this case, you can access memory beyond 4GB using a series of
       read(2) and write(2) calls, a pread64() or pwrite64() call, or a combination of llseek(2) and read(2) or write(2).

ERRORS

       EFAULT	       Occurs when trying to  write(2) a read-only location (allkmem),	read(2) a write-only location  (allkmem),  or  read(2)	or
		       write(2) a non-existent or unimplemented location (mem, kmem, allkmem).

       EIO	       Occurs  when trying to read(2) or write(2) a memory location that is associated with an I/O device using the /dev/kmem spe-
		       cial file.

       ENXIO	       Results from attempting to mmap(2) a non-existent physical (mem) or virtual (kmem, allkmem) memory address.

FILES

       /dev/mem        Provides access to the computer's physical memory.

       /dev/kmem       Provides access to the virtual address space of the operating system kernel, excluding memory that is  associated  with	an
		       I/O device.

       /dev/allkmem    Provides  access  to  the virtual address space of the operating system kernel, including memory that is associated with an
		       I/O device.

SEE ALSO

       llseek(2), mmap(2), read(2), write(2)

WARNINGS

       Using these devices to modify (that is, write to) the address space of a live running operating system or to modify  the  state	of	 a
       hardware  device  is  extremely	dangerous  and	may  result  in a system panic if kernel data structures are damaged or if device state is
       changed.

SunOS 5.10							    18 Feb 2002 							   mem(7D)