In subsection "Probing for ISA Memory" of section Using I/O Memory ( http://www.xml.com/ldd/chapter/book/ch08.html#t4 ) of Device Drivers Book, by Rubini, there is a code that reads a byte from memory, change this byte by a know pattern (it toggles every bit of the byte), write this changed byte to memory and read, ever in the same memory position. Then it unchanged the new-read byte to see if it is equal to old-read byte. If it is equal, then the book concludes that the memory region is RAM. Else book concludes the memory region is "empty". What does mean an "empty" region of memory? Why and how can a non-existing memory region be mapped? (below is the code in rubini's book): oldval = readb (base + add); /* Read a byte */ writeb (oldval^0xff, base + add); mb(); newval = readb (base + add); writeb (oldval, base + add); restore_flags(flags); if ((oldval^newval) == 0xff) { /* we reread our change: it's RAM */ printk(KERN_INFO "%lx: RAM\n", add); continue; } if ((oldval^newval) != 0) { /* random bits changed: it's empty */ printk(KERN_INFO "%lx: empty\n", add); continue; } Regards, Ribamar -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
