To compete with Apple’s iPad, Microsoft Corp. unveiled a tablet called the Surface. The device, comparably sized to the iPad, will run the forthcoming Windows 8 operating system, and a variant OS called Windows RT. Microsoft has provided software to tablet-style computing for three decades, but this is its first homegrown tablet. Executives have said this decision fits with its history to develop hardware when it is needed to bolster its software. However, this could cut into sales of Windows-powered tablets from the software giant’s allies. Microsoft’s move to make its own tablet “comes with consequences, which is complicating choices for consumers and complicating relations with third-party manufacturers,” said Sarah Rotman Epps, an analyst with Forrester Research Inc. The Surface’s rollout later this year will coincide with the arrival of Windows 8.