Graphic processing apparatus with clipping circuit

Claim 1

A graphic apparatus comprising: means for displaying a graphic pattern; bit map memory means for storing graphic data indicative of said graphic pattern at addresses corresponding to a coordinate plane of said display means; clip register means for storing coordinates which define a predetermined clip area in said coordinate plane within which a different pattern is desired to be displayed; a comparator for comparing said coordinates in said clip register means with said addresses to determine if said addresses fall within said predetermined clip area; memory control means for selectively controlling a write operation of said graphic data into said bit map memory means based on a comparison signal from said comparator; select means for receiving: (1) output data from said bit map memory means as first input data, and (2) data corresponding to said different graphic pattern as second input data, and for selecting one of said first and second input data and supplying said selected data to said bit map memory means; and select control means for controlling said select means to produce one of: (a) all bits of said first input data, when said addresses of said bit map memory means are outside said clip area, or (b) predetermined bits of said first input data corresponding to said graphic pattern and predetermined bits of said second input data corresponding to said different graphic pattern, said predetermined bits of said second input data replacing bits in said clip area other than said predetermined bits of said first input data when said addresses of said bit map means are within said clip area, in accordance with the coordinates stored in said clip register means in response to said comparison signal from said comparator.