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I can use the dd command to read a memory address (ex: 0x400):

dd if=/dev/mem count=4 bs=1 skip=$(( 0x400 ))

But how would I write a value to physical address (ex: 0x400)? I tried this:

function T_WRITE2 {
  printf $1 | dd of=/dev/mem count=4 bs=1 skip=$(( 0x400 ))
}

But I get the error:

dd: 'standard input': cannot skip to specified offset

(How do I get input to not skip any but the output to skip to the address to write?)

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  • Remember that promiscuous /dev/mem has been widely turned off for about 16 years, now. – JdeBP Feb 3 '20 at 15:44
  • @JdeBP 0x400–0x4FF remains accessible however, using /dev/mem (as root), as do various other areas of memory. – Stephen Kitt Feb 3 '20 at 16:54
  • Not necessarily. It all depends, from factors not stated in this question and not givens here. unix.stackexchange.com/q/532381/5132 – JdeBP Feb 3 '20 at 18:17
  • @JdeBP, true, I fell into the x86 pit of doom. – Stephen Kitt Feb 3 '20 at 18:33
1

skip skips bytes in the input; to write to a specific offset in the output, you need to “seek”, using the seek option:

dd of=/dev/mem count=4 bs=1 seek=$((0x400))
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  • I don't think that devices can be truncated. – mosvy Feb 3 '20 at 14:23
  • @mosvy they can’t, but dd might complain (then again, given that one of its main use-cases is writing to devices, it probably won’t). – Stephen Kitt Feb 3 '20 at 14:28
  • dd.c – mosvy Feb 3 '20 at 17:43

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