Digi BL1800 user manual Advantages of Cooperative Multitasking

used to collect some operations that are helpful to do once on every pass through the loop. Place the cursor on this function name BigLoopTop() and hit <Ctrl-H>to learn more.

The statement at (3) waits for a time delay, in this case 200 ms. The costatement is being executed on each pass through the big loop. When a waitfor condition is encountered the first time, the current value of MS_TIMER is saved and then on each subsequent pass the saved value is compared to the current value. If a waitfor condition is not encoun- tered, then a jump is made to the end of the costatement (4), and on the next pass of the loop, when the execution thread reaches the beginning of the costatement, execution passes directly to the waitfor statement. Once 200 ms has passed, the statement after the waitfor is executed. The costatement has the property that it can wait for long periods of time, but not use a lot of execution time. Each costatement is a little program with its own statement pointer that advances in response to conditions. On each pass through the big loop, as little as one statement in the costatement is executed, starting at the current posi- tion of the costatement’s statement pointer. Consult the Dynamic C User’s Manual for more details.

The second costatement in the program debounces the switch and maintains the variable vswitch. Debouncing is performed by making sure that the switch is either on or off for a long enough period of time to ensure that high-frequency electrical hash generated when the switch contacts open or close does not affect the state of the switch. The abort state- ment is illustrated at (5). If executed, the internal statement pointer is set back to the first statement within the costatement, and a jump to the closing brace of the costatement is made.

At (6) a use for a shadow register is illustrated. A shadow register is used to keep track of the contents of an I/O port that is write only - it can’t be read back. If every time a write is made to the port the same bits are set in the shadow register, then the shadow register has the same data as the port register. In this case a test is made to see the state of the LED and make it agree with the state of vswitch. This test is not strictly necessary, the output regis- ter could be set every time to agree with vswitch, but it is placed here to illustrate the concept of a shadow register.

To illustrate the use of snooping, use the watch window to observe vswitch while the program is running. Add the variable vswitch to the list of watch expressions. Then tog- gle vswitch and the LED. Then type <Ctrl-U>to observe vswitch again.

4.3.1 Advantages of Cooperative Multitasking

Cooperative multitasking, as implemented with language extensions, has the advantage of being intuitive. Unlike preemptive multitasking, variables can be shared between different tasks without having to take elaborate precautions. Sharing variables between tasks is the greatest cause of bugs in programs that use preemptive multitasking. It might seem that the biggest problem would be response time because of the big loop time becoming long as the program grows. Our solution for that is a device caused slicing that is further described in the Dynamic C User’s Manual.

50 Jackrabbit (BL1800)