LOS ANGELES (AP) — Less than a year after Microsoft entered the tablet computer market with the Surface, the cracks are starting to show. The software giant on Thursday booked a large write-off to its Surface RT business after it slashed prices on the tablets to stimulate demand this week. Its quarterly earnings results also showed that Windows 8, an operating system designed to bridge the divide between PCs and tablets, has been so poorly received that it contributed to a revenue drop in its operating system software unit.