A display control device according to the present invention includes a common memory having two functions, one as a refresh memory for outputting a character code and the other as a character generator for generating a character font, an output character code from which is stored in a line buffer. In this case, a display address generator circuit generates a character address for generating the character code and a raster address for designating the character font, which are in turn switched by an address selector and outputted to said common memory which then outputs a video signal to a video control circuit based upon said character font. Additionally, there is provided a line buffer control circuit for outputting a read/write access control signal to said line buffer based upon the raster address.

A controller for a single port (typically FIFO) memory for a computer system allocates the flow of data from a single port dynamic RAM to several data requesters such as a graphics engine, a CPU, and a display screen. One parameter is assigned to the display screen and a second parameter is assigned to the other data requester. Then each memory cycle is assigned in duty cycle fashion to the data requesters associated with one or the other of the parameters. Thus, typically the screen display which requires large amounts of data will have a relatively large parameter value associated with it such as six, while the single parameter value associated with the graphic engine and CPU will have a lower value such as two. The screen display will in response access the memory for six memory cycles consecutively, while the graphics engine and CPU will access the memory for only two memory cycles. The FIFO associated with the screen display is thus kept relatively full so that the screen is refreshed at all times and no FIFO empty detection is thus needed. The memory controller includes a loadable counter for counting down the loaded parameter, and a state machine for loading in the values, decrementing the values held in the counter, and switching between the two parameters.

An improved display controller provides commands for changing a display. The controller includes a component for determining whether or not the command provided thereby includes an attribute changing command therein. If a NOCHANGE mode has been set, and the controller component determines that an implicit attribute changing command is included in the display command, the attribute changing command is inhibited. If a direct, explicit, attribute changing command is detected, the attribute change is performed. If the NOCHANGE mode has not been set, all attribute changing commands are executed, whether implicitly or explicitly provided.

An interface unit connected between a first bus and a second bus, the first bus having connected thereto a memory system and the second bus having connected thereto a plurality of devices which perform read operations directed at the memory system, the interface unit including a buffer memory having an input that is electrically coupled to the first bus for receiving data that is sent from the memory system over the first bus and an output that is electrically coupled to the second bus for delivering data to the second bus for transfer to a requesting device, the requesting device being one of the plurality of devices; a plurality of prefetch size registers equal in number to the plurality of devices, wherein each of the plurality of prefetch size registers stores a number for a different corresponding one of the plurality of devices, wherein the number stored in each of the plurality of prefetch size registers specifies an amount of data that is prefetched from memory in response to a data read request from the corresponding device; and a control unit which in response to a data read request from any one of the plurality of devices prefetches into the buffer a predetermined amount of data from the system memory, the predetermined amount being determined by the number stored in prefetch size register corresponding to the requesting device.