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01 - Overview and Concepts

This document explains the ideas behind SINTRAN III printing and spooling. Once you understand the peripheral-file model and the spooling-file / queue model here, the configuration, operator, and user commands in the other documents follow naturally.

Sources: ../../Reference-Manuals/ND-60.050.06 SINTRAN III Users Guide.md section 3.8 "The Spooling System"; ../../Reference-Manuals/ND-60.128.5 EN SINTRAN III Reference Manual.md.

1. What is a "printer" to SINTRAN?

SINTRAN III does not print to hardware directly from user programs. Instead, every peripheral - including every printer - is represented by a peripheral file: an ordinary file-system name that is tied to a logical device number. When a program opens the peripheral file and writes to it, SINTRAN's device driver turns those writes into I/O on the corresponding hardware controller.

Standard peripheral file names for print devices (from the SINTRAN III Users Guide / Commands Reference, Appendix D "Standard Peripheral File Names"):

Peripheral file name Device Typical hardware device number (octal)
LINE-PRINTER Line printer 430, 431, ...
PRINTER Matrix printer (and terminals used as printers) 414, 415, 417
LINE-PRINTER-1, LINE-PRINTER-2, ... Versatec printer/plotter (when it is the only line printer) 603-606
VERSATEC-1, VERSATEC-2, ... Versatec printer/plotter (when another line printer exists) 603-606
TERMINAL A terminal - can also act as PRINTER 300 and up

The physical side of these devices is covered in 02-HARDWARE-DEVICES.md.

Key point: a printer is just a file name bound to a device number. This is why the same commands and the same file-system model apply to printers, disks, tapes, and terminals.

2. Two ways to print: direct vs spooled

2.1 Direct (no spooling)

You can COPY-FILE (or have a program write) straight to the peripheral file. While that copy runs, the physical printer is reserved to that user, and the user program runs at the speed of the printer. If the printer is a 200 line per minute line printer, the program is stuck for as long as printing takes, and nobody else can use that printer. For a single user with a fast printer this is fine; on a multi-user timesharing system it is not.

2.2 Spooled (the normal case)

Spooling (Simultaneous Peripheral Operation On-Line) decouples the program from the printer:

  1. The user's output is copied quickly to a disk file (a spooling file).
  2. That file is placed on a spooling queue for the target printer.
  3. A background spooling program later reads the queue and empties each file onto the physical printer, one at a time.

The user program finishes as soon as the fast disk copy is done and continues with other work; the slow printing happens in the background. This is the whole reason spooling exists.

Users Guide 3.8: "When the file is closed, the file is linked to a spooling queue for the peripheral and eventually emptied on the peripheral."

3. Peripheral files vs spooling files: the "versions" mechanism

This is the mechanism that makes SINTRAN spooling work, and it is unusual, so it is worth stating precisely.

A peripheral file can be created in more versions than there are physical devices. The rule (Users Guide 3.8):

  • A version that is connected to a device number is a peripheral file (it maps to real hardware).
  • A version that is not connected to a device number is a spooling file (it is just disk space that looks like the printer).

Example from the Users Guide: LINE-PRINTER is created with ten versions. Version 1 is the real peripheral file (device number 5, a line printer). The other nine versions are spooling files - disk buffers.

When spooling is running and a user opens the peripheral (LINE-PRINTER), SINTRAN does not give them the hardware. It gives them the first free spooling file of that printer. The user writes into that disk file. When they close it, SINTRAN links it onto the spooling queue, and the spooling program eventually empties it to the physical device.

Because there are multiple spooling-file versions, several users can "open the printer" at the same time (or one user can open it several times) - each gets a different spooling file. This is how many users share one slow printer without blocking each other.

4. The spooling queue

The spooling queue is the ordered list of files waiting to be printed on a given peripheral. Entries reach the queue in two ways:

  1. Automatically, when a user closes a spooling file they wrote into (see above).
  2. Explicitly, with @APPEND-SPOOLING-FILE, which lets a user queue any file (not just a spooling file) and ask for a number of copies.

You inspect the queue with @LIST-SPOOLING-QUEUE, reorder it with @MOVE-SPOOLING-QUEUE-ENTRY, and remove entries with @REMOVE-FROM-SPOOLING-QUEUE or @DELETE-SPOOLING-FILE. These are covered in 05-USER-COMMANDS.md.

Note: the file currently being printed is not shown in the queue - printing of it has already started (Users Guide 3.8 example).

5. The spooling page pool

Spooling files consume disk space, and that space belongs to user SYSTEM. To stop spooling from eating all of SYSTEM's disk, SINTRAN caps the number of pages the spooling files may use. The default limit is 500 pages.

  • @SPOOLING-PAGES-LEFT reports how many pages remain.
  • @GIVE-SPOOLING-PAGES <n> raises the limit by n.
  • @TAKE-SPOOLING-PAGES <n> lowers it by n.

If the pool is exhausted (limit reached, or user SYSTEM has no free pages), any user program currently writing to a spooling file is put into a wait state. The spooling program then prints one of the queued spooling files, returns its pages to the free pool, and wakes the waiting programs (Users Guide 3.8). So the system self-throttles rather than failing outright.

(1 page = 1024 words = 1KW = 2KB, per the project memory convention.)

6. Life of a print job (end to end)

Putting it together, here is what happens when spooling is running and a user prints a file to LINE-PRINTER:

   User program / COPY-FILE
        |
        |  opens "LINE-PRINTER"
        v
   +--------------------------+
   | First FREE spooling file |   (a disk file, one of the LINE-PRINTER versions)
   +--------------------------+
        |
        |  user writes data (fast, disk speed) then CLOSES the file
        v
   +--------------------------+
   |     Spooling queue       |   (ordered list for this printer)
   +--------------------------+
        |
        |  background spooling program picks the next entry
        v
   +--------------------------+
   |  Physical printer (dev.  |   printed at printer speed;
   |  no. via the driver)     |   pages returned to the pool afterwards
   +--------------------------+

Meanwhile the user program has long since moved on. Multiple users feed the queue concurrently through different spooling-file versions.

7. Where to go next