Accessing odd bytes

From: Robert Schwebel (robert@schwebel.de)
Date: Fri Jun 14 2002 - 07:31:40 EST

I have a strange effect on an embedded system (AMD Elan SC410,
Linux-2.4.18) while accessing a static RAM. The RAM is mapped to the bus
at 0x0200'0000. If I map it to user space this way:

  pSRAM = (unsigned short *)mmap(0, 0x00040000, PROT_READ + PROT_WRITE, MAP_SHARED, FD, 0x2000000);

and fill it like this:

  pByte=(char*)pSRAM;
  for (i=0; i<10; i++) {
    *pByte++=(char)i;
  }

  pByte=(char*)pSRAM;
  for (i=0; i<10; i++) {
    printf("i: %02i -> %03i\n", i, *pByte++);
  }

I see a mirroring effect:

  i: 00 -> 001
  i: 01 -> 001
  i: 02 -> 003
  i: 03 -> 003
  i: 04 -> 005
  i: 05 -> 005
  i: 06 -> 007
  i: 07 -> 007
  i: 08 -> 009
  i: 09 -> 009

Now I'm wondering how the kernel/processor handles odd byte access
exceptions. Can anybody give me a pointer where I could search or what my
problem could be?

Robert 

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 | Dipl.-Ing. Robert Schwebel | http://www.pengutronix.de |
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This archive was generated by hypermail 2b29 : Sat Jun 15 2002 - 22:00:31 EST