Oversimplified Picture of a printf() operation

+---------------------------------------------------------------------------------------+
| User Mode Programs                                                                    |
|                                                                                       |
|   your program                                                                        |
|  +------------+                                                                       |
|  |main() {    |                                                                       |
|  |   ...      |                                                                       |
|  | printf();  |-------+                                                               |
|  |   ...      |<--+   |                                                               |
|  +------------+   |   |   printf() function                                           |
|                   |   |   +--------------+                                            |
|                   |   +-->|printf() {    |                                            |
|                   |       | create string|                                            |
|                   |       |    from args |                                            |
|                   |       | write(string)|-------+                                    |
|                   +-------| return;      |<--+   |                                    |
|                           +--------------+   |   |   fs/read_write.c                  |
|                                              |   |   +---------------+                |
|                                              |   +-->|write() {      |                |
|                                              |       |   ...         |                |
|                                              |       | movl 4,%eax   |                |
|                                              |       | int  $0x80    |                |
|                                              |       | ...           |                |
|                                              +-------| return;       |                |
|                                                      +---------------+                |
|                                                          |       ^                    |
|----------------------------------------------------------|-------|---------------------
| Kernel Mode Programs                                     |       |                    |
|                                                          v       |                    |
|                                              int 80  +---------------+                |
|                                              handler |syscall(0, ...)|                |
|                                                      |      ...      |                |
|                                                      |syscall(4, ...)|                |
| We have ignored the software                         | set up printer|                |
| for the various busses (e.g. USB)                    | with HP driver|                |
|                                                      +---------------+                |
|                                                          |       ^                    |
|                  <-----------+---------------------------+       |                    |
|                              |                           |       |                    |
|                  ------------|-------+-------------------|------>|                    |
|                              v       |                   v       |                    |
|                   Screen +---------------+        HP +---------------+                |
|                   Device | send commands |    Device | send commands |                |
|                   Driver | and string to |    Driver | and string to |                |
|                          | device, wait  |           | device, wait  |                |
|                          | for interrupt |           | for interrupt |                |
|                          +---------------+           +---------------+                |
|                              |       ^                   |       ^                    |
+------------------------------|-------|-------------------|-------|--------------------|
| Hardware                     V       |                   v       |                    |
|                  Display +---------------+        HP +---------------+                |
|                 Hardware |               |   Printer | output string |                |
|                          |               |           | generate      |                |
|                          |               |           | interrupt     |                |
|                          |               |           | when done     |                |
|                          +---------------+           +---------------+                |
|                                                                                       |
+---------------------------------------------------------------------------------------|