Direct memory access (DMA) controller with programmable bus release period for timing DMA transfers

In a system in which an image memory storing image data, an image processor, and a central processing unit for controlling the image memory and the image processor are connected to each other through a bus, an image data transfer controller for performing direct memory access transfer of the image data between said image processor and said image memory, comprising:

• setting means provided in said central processing unit and setting the number of data to be continuously transferred and a bus release period;
• direct memory access transfer processing means for repeatedly performing transfer processing provided on the side of said image processor for acquiring a right to control a bus and performing direct memory access transfer of image data corresponding to the set number of data to be continuously transferred between said image processor and said image memory and bus release processing for releasing the acquired right to control a bus for said central processing unit during the set bus release period until a predetermined number of image data are transferred;
• and
• set value changing means provided in said central processing unit and monitoring the state of a request to use said bus and changing the set value of said number of data to be continuously transferred and the set value of said bus release period on the basis of the state of the request to use the bus.