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Microsoft shares slumped after the software giant booked a large write-off to its Surface RT business after it slashed prices on the tablets to stimulate demand this week. Fourth-quarter earnings results also showed that its Windows 8 operating system has been so poorly received that it contributed to a revenue drop in its operating system software unit. The results raise new questions of whether Microsoft could transform itself into a ‘‘devices and services’’ company that is less reliant on providing PC software.