Method and apparatus for improved block transfers in computer graphics

Abstract

A method and apparatus for transferring a block of pixel data from a source multi-line frame buffer area to a destination multi-line frame buffer area in a raster-type computer graphics display system. The method steps are performed by a state machine embedded in a macro function unit within a computer graphics subsystem: It is first determined whether the first raster line corresponding to the destination multi-line frame buffer area is at least a minimum number of raster lines ahead of the current raster line. If so, the state machine follows a fast path, during which the pixel data are transferred one line at a time from the source multi-line frame buffer area to the destination multi-line frame buffer area until the last line of pixel data in the block has been transferred. Then, the transfer operation stops. If not, then the state machine follows a slow path. In the slow path, the state machine times the line-by-line transfers of pixel data so as intentionally to remain behind the raster throughout the transfer operation: The state machine checks before each line is transferred to make certain that the destination line is behind the raster. But after each line is written, the state machine also checks to see whether the next destination line is ahead of the raster; this may occur if a vertical retrace has just begun. If so, the slow path procedure is interrupted, and the state machine switches to the fast path for the remainder of the transfer operation.