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Top Forums UNIX for Dummies Questions & Answers translating physical/virtual addresses Post 302400522 by agaurav on Wednesday 3rd of March 2010 11:39:50 AM
Old 03-03-2010
translating physical/virtual addresses

Hi all,

I am new to Linux kernel/user space programming having been an assembly programmer in my previous life. I am now using 2.6.x kernel on an embedded CPU that has a few dedicated hardware blocks (including more CPU running just C-code, i.e., no operating system).

There is a single DRAM connected to this chip with one Linux CPU + multiple h/w blocks. No swapping since this is being done for an embedded device (SoC ASIC chip with a single 32-bit DDR2 interface, no hard-drive).

Question(s):

1. The Linux CPU needs to talk to hardware blocks that obviously physical DRAM addresses while Linux processes/threads use virtual addresses.

2. How do I translate these addresses back-n-forth? For example, a Linux process may want to allocate memory and then hand it off to a hardware block to write into it. Then after a while the process will read it.

3. Sometimes, the hardware block may write a physical address into the shared memory. The Linux CPU will read the shared memory and then convert the physical address to virtual memory and go read that location.

How does one achieve all of this? If this is being extremely stupid, then please let me know. Hopefully, you can give me some pointers. A website, book, code, man page, high-level thoughts, anything would be appreciated.

Thanks a lot,
Guraaf
 

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PHYS(2) 							System Calls Manual							   PHYS(2)

NAME
phys - allow a process to access physical addresses SYNOPSIS
phys(segreg, size, physadr) DESCRIPTION
The argument segreg specifies a process virtual (data-space) address range of 8K bytes starting at virtual address segregx8K bytes. This address range is mapped into physical address physadrx64 bytes. Only the first sizex64 bytes of this mapping is addressable. If size is zero, any previous mapping of this virtual address range is nullified. For example, the call phys(6, 1, 0177775); will map virtual addresses 0160000-0160077 into physical addresses 017777500-017777577. In particular, virtual address 0160060 is the PDP-11 console located at physical address 017777560. This call may only be executed by the super-user. SEE ALSO
PDP-11 segmentation hardware DIAGNOSTICS
The function value zero is returned if the physical mapping is in effect. The value -1 is returned if not super-user, if segreg is not in the range 0-7, if size is not in the range 0-127, or if the specified segreg is already used for other than a previous call to phys. BUGS
This system call is obviously very machine dependent and very dangerous. This system call is not considered a permanent part of the sys- tem. ASSEMBLER
(phys = 52.) sys phys; segreg; size; physadr PDP11 PHYS(2)
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