We present a new multithreaded implementation for the computationally demanding direct volume rendering (DVR) of volumetric datasets on desktop multicore processors using ray casting. The new implementation achieves interactive rendering of very large volumes, even on high resolution screens. Our implementation is based on a new algorithm that combines an object order traversal of the volumetric data followed by a focused ray casting. Using a very compact data structure, our method starts with a quick association of data subcubes with fine-grain screen tiles appearing along the viewing direction in front to back order. The next stage uses very limited ray casting on the generated sets of subcubes while skipping empty or transparent space and applying early ray termination in an effective way. Our multithreaded implementation makes use of new dynamic techniques to ensure effective memory management and load balancing. Our software enables a user to interactively explore large data sets through direct volume rendering while arbitrarily specifying a 2D transfer function. We test our system on a wide variety of well-known volumetric datasets on a two-processor Clovertown platform, each consisting of a Quad-Core 1.86 GHz Intel Xeon Processor. Our experimental tests demonstrate direct volume rendering at interactive rates for the largest datasets that can fit in the main memory on our platform. These tests also indicate a high degree of scalability, excellent load balancing, and efficient memory management across the datasets used.