Method and apparatus for compressing and decompressing print data in the

Abstract

An improved printer is provided which includes a separate hardware compression module and hardware decompression module contained within the ASIC of the print engine. A 'Request Counter' register is decremented each time memory is requested to store a block of bitmap image data. When the Request Counter decreases to a predetermined value, the current size of the remaining 'free memory' space is evaluated. If this free memory is less than another predetermined value (a 'low water mark'), then blocks of data that have already been stored in the printer's memory are sent to the compression module to be compressed before the printer literally runs out of free memory, and the printer can continue storing and processing more incoming print job data. By thus preserving some of the free memory, the printer can continue to process or rasterize more print data while the compression hardware simultaneously operates independently to compress one or more blocks of data. Since the compressor hardware operate independently of the printer's microprocessor, the printer can continue to process further data during the actual compression operations, thereby preventing the printer's microprocessor from becoming idle, which otherwise would become a very inefficient utilization of the printer's processing power. The present invention is especially useful in processing color data, especially in printers containing relatively small quantities of RAM, since color data typically requires four (4) planes of bitmap image data per page to be printed.