Image Number 5 for United States Patent #6639603.
A display subsystem supports both normal mode and portrait mode displays. In normal mode, the scan starts at the upper left comer of the display. In portrait mode, the scan starts at the lower left comer of the display. The display subsystem includes a dual mapped display memory having a normal mode display area and a portrait mode display area. The portrait mode display area is defined by X-ofst.sub.(Virtual) and Y-ofst. X-ofst.sub.(Virtual) is a power of two that is greater than the real X-ofst supported by the display in portrait mode. Address requests from the CPU or software use high order bits to specify whether the address is in the normal or portrait mode display area. In addition, address requests to the portrait mode display area use the address space defined by X-ofst.sub.(Virtual) and Y-ofst. When the address request specifies the portrait mode display area, the address of the request is translated to account for the different mode of the display. Since X-ofst.sub.(Virtual) is a power of two, the X-coordinate of the display location specified by the address can be determined without a division operation, thereby allowing for fast address translation. Logic within the display subsystem rapidly translates the address and supports a range of Y-ofst values.