Programming Embedded Systems in C and C++

70 Chapter 6: Memory

baseAddress[offset] = pattern;

}

/*

* Check for address bits stuck high.

*/

testOffset = 0;

baseAddress[testOffset] = antipattern;

for (offset = sizeof(datum); (offset & addressMask) != 0; offset <<= 1)

{

if (baseAddress[offset] != pattern)

{

return ((datum *) &baseAddress[offset]);

}

}

baseAddress[testOffset] = pattern;

/*

* Check for address bits stuck low or shorted.

*/

for (testOffset = sizeof(datum); (testOffset & addressMask) != 0;

testOffset <<= 1)

{

baseAddress[testOffset] = antipattern;

for (offset = sizeof(datum); (offset & addressMask) != 0;

offset <<= 1)

{

if ((baseAddress[offset] != pattern) && (offset != testOffset))

{

return ((datum *) &baseAddress[testOffset]);

}

}

baseAddress[testOffset] = pattern;

}

return (NULL);

} /* memTestAddressBus() */

Device test

Once you know that the address and data bus wiring are correct, it is necessary to

test the integrity of the memory device itself. The thing to test is that every bit in

the device is capable of holding both 0 and 1. This is a fairly straightforward test

to implement, but it takes significantly longer to execute than the previous two.

For a complete device test, you must visit (write and verify) every memory loca-

tion twice. You are free to choose any data value for the first pass, so long as you

invert that value during the second. And because there is a possibility of missing

Memory Testing 71