Here's[1] and example I use with my students. --Mark
[1] http://elinux.org/EBC_Exercise_11b_gpio_via_mmap On Monday, March 3, 2014 9:29:14 PM UTC-5, Charles Steinkuehler wrote: > > You are running on a system with virtual memory, so you can't just > access the physical memory location and expect it to work. There's a > re-mapping system of memory pages between your process address space and > the physical memory bus. > > The way you do this is to mmap the /dev/mem file, with appropriate > offset and length values. This gets you a memory pointer in your > program you can use to access the physical memory address you want. > > I'd provide full details, but I live in Topeka and am a KU fan! :) > > Actually, I'm typing on a netbook in a hotel room, or I'd dig something > up for you...it should be pretty easy to find on-line with a bit of a > search. If you're still stuck tomorrow, I can dig up some code when I'm > at the office. > > ...here's one example, not the best, but maybe it will get you started: > > > https://github.com/cdsteinkuehler/linuxcnc/blob/MachineKit-ubc/src/hal/drivers/hal_bb_gpio.c#L73 > > > On 03/03/14 14:48, dep...@phys.ksu.edu <javascript:> wrote: > > I've got a BeagleBone Black running Angstrom, and I want to directly > access > > the ARM335 registers from my C/C++ programs. A trivial example would be > to > > "read" the Device_ID at memory location 0x44E10600. To do this I tried: > > > > #include <iostream> > > using namespace std; > > > > unsigned long int *p; > > unsigned long int DevID = 0x44E10600; > > > > p1 = DevID; // <= this gives a compiler error: "invalid conversion from > > 'long unsigned int' to 'long unsigned int*' > > cout << *p1 << endl; // <= if I "cheat" around the compiler error, I > get a > > Segmentation fault from this statement > > > > // etc > > > > Note 1: just the p1 = DevI statement causes a fatal compiler error (as > > indicated in the comment. > > Note 2: I can get around this by using the -fpermissive compiler option, > > but then the cout statement (or any other way of extracting the contents > of > > that memory location) gives me the segmentation fault. > > > > I *think* this is a permissions thing -- which means I ought to be able > to > > fix it with some command. Is this correct? In any case, can someone > tell > > me how to read/write to a particular memory location using c/c++? Much > > obliged! > > > > BDD > > > > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
