Printing Data Flow

The data flow between your application, the printer driver, the spooler, and the kernel device driver is shown in the following figure. In addition to the data flow for the printer data stream, the data flow for screen output is shown. From an application's point of view, creating output for a printer is conceptually the same as creating screen output.

Overview of the Application Interface and Data Flow

There are three routes that the application's printer data can follow:

• Base Printing

  Base printing is provided primarily for non-PM programs writing complete printer data streams, including all printer control codes, directly to a printer port. It also provides compatibility for applications that run under DOS or Microsoft** Windows**.

• Queued PM Printing

  Queued printing is recommended for all PM programs. It provides the most flexibility, both for the application and the user.

  Print jobs created by PM applications are queued on a spooler queue. The spool files are processed and finally sent to the printer asynchronously from the application.

  Note: If the spooler is disabled, queued print jobs perform as though they were submitted for PM direct printing.

• Direct PM Printing

  Conceptually, direct printing is the same as queued printing for the application interface, but the spooler is bypassed. Therefore, the data is sent directly to the printer, and the application has to wait until the printing is finished. Following are some of the reasons for PM applications to avoid direct printing:

  • The print job can interfere with other users whose jobs are destined for this printer. Print output could be mixed with the other print jobs

  • The spooler allows only one job at a time to print to a particular port to avoid mixing of print job output. If your application tries to print directly to the same port while another job is printing, your user must wait for the current job to complete before the user's job can begin.

  • The printing application loses some of its multitasking advantages because one job must complete before a second job is submitted.

  • The printer device driver does not protect a print job from job property and printer property mismatches.

  • Pictures printed directly can be different from those printed through a spooler. The default setting for direct printing is to print pictures their actual size, whereas the default for queue printing is scaled to fit for the output area.
  Direct printing is recommended only for specialized applications that use dedicated hard copy devices. Print jobs that have a certain degree of security associated with them, such as a corporate payroll or confidential documents, might best be handled with direct printing because files in the spooler can be copied.

  Jobs that are very large, that is, over 10MB, that you do not want copied in the spool file, also might best be printed directly.