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• The exam solution is released on Canvas.
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  • Your official exam grade will be released on Canvas next Monday (03/31).
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• Lab7 is released and will be due in two weeks (04/07/2025, 23:59).
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• Lab5 is released and will be due in two weeks (03/10/2025, 23:59).

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• Lab4 is released and will be due in two weeks (02/24/2025, 23:59).

• The coming office hour originally scheduled for Monday (02/03) is rescheduled to Tuesday (02/04) at 2PM.

• Lab3 is released and will be due in two weeks (02/10/2025, 23:59).

Context

In Week4 lecture, we failed to print the mtime value (8B) properly in C, with the helper function mtime_get(). We got either 0 or some random numbers.

unsigned long long mtime_get() {
    unsigned int low  = *(unsigned int*)(CLINT0_MTIME);
    unsigned int high = *(unsigned int*)(CLINT0_MTIME + 4);
    return (((unsigned long long)high) << 32) | (unsigned long long)low;
}

What happened?

This issue is due to using an incorrect format specifier for printf (or in the egos kernel, CRITICAL). The return value of mtime_get() is of type unsigned long long (8B), but we used %d (printing int, 4B) and %lx (printing long in hex, 4B). When printf meets a mismatch between format and type, its behavior is undefined, meaning the code can do literally anything.

Reproducing the problem

Want to reproduce this problem? Try the following code in grass.c:main():

unsigned long long a = mtime_get();      // use the above helper function

CRITICAL("%d (undefined behavior)", a);  // we did this in Week4 lecture
CRITICAL("%lx (undefined behavior)", a); // we did this in Week4 lecture
CRITICAL("%lx", (unsigned long)a);       // cast 8B to 4B
CRITICAL("%llx", a);                     // correct version

// print what has been stored on each byte
char *c = (char*)&a;
for (int i=0; i<8; i++) {
    CRITICAL("[%d]: %x", i, c[i]);
}

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