HP LaserJet 4 Service Manual - Page 126

Memory Management The printer uses several different types of memory. This memory can be divided into two different categories, Read Only Memory (ROM) and Random Access Memory (RAM). ROM memory consists of the memory located on the Formatter PCA and any font cartridges that may be installed in the printer. RAM memory consists of RAM memory resident on the Formatter PCA, SIMM memory installed in the printer, and the Non-Volatile Memory (NVRAM) located on the Formatter PCA. In general, the ROM is responsible for storing the instructions for the operation of the processor and font information. The RAM memory is used to store temporary information that will be used by the printer for a particular print job or a series of print jobs. This could consist of soft fonts, macros or graphics patterns. RAM memory is also used by the printer to store data that is being processed into the raster image that will control the laser and create the printed page. The NVRAM is used to store printer configuration information that can be input from the printer front panel. This information stored in NVRAM will remain resident in the printer even if the power to the printer is turned off. Data Processing The formatter CPU is responsible for the overall control of all functions related to the formatter. One of the CPU's main functions is to maintain communication with the print engine. This is done by the print engine interface and the DC Controller PCA. The DC Controller PCA monitors all print engine functions such as fuser temperature and paper jam status. The print engine status is communicated by the DC Controller to the formatter CPU by the print engine interface. The CPU also controls the process that converts ASCII data received from the host into the raster data. This raster data is sent to the laser scanner assembly via the DC Controller. Finally the CPU comunicates printer status to the user through the control panel via the front panel interface. The front panel interface also transmits front panel inputs to the CPU. 5-14 Functional Overview