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| author | gdisirio <gdisirio@35acf78f-673a-0410-8e92-d51de3d6d3f4> | 2009-01-19 13:31:37 +0000 |
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| committer | gdisirio <gdisirio@35acf78f-673a-0410-8e92-d51de3d6d3f4> | 2009-01-19 13:31:37 +0000 |
| commit | 0810f1daac3c33d194d573d82ec436021e3e7255 (patch) | |
| tree | 41909345afc1c1e949a18119563243b9adfc8648 /docs/src/saveram.dox | |
| parent | f8b4fca89a8dd31989a3d21e4c6fb4175aba4110 (diff) | |
| download | ChibiOS-0810f1daac3c33d194d573d82ec436021e3e7255.tar.gz ChibiOS-0810f1daac3c33d194d573d82ec436021e3e7255.tar.bz2 ChibiOS-0810f1daac3c33d194d573d82ec436021e3e7255.zip | |
git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@643 35acf78f-673a-0410-8e92-d51de3d6d3f4
Diffstat (limited to 'docs/src/saveram.dox')
| -rw-r--r-- | docs/src/saveram.dox | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/docs/src/saveram.dox b/docs/src/saveram.dox new file mode 100644 index 000000000..fec810c81 --- /dev/null +++ b/docs/src/saveram.dox @@ -0,0 +1,66 @@ +/**
+ * @page article_saveram Saving RAM by declaring thread functions "noreturn"
+ * @{
+ * One of the problems, when writing embedded multi-threaded applications,
+ * is that the thread functions do save the registers in the function
+ * entry code even if the system does not require it, exiting such
+ * a function would terminate the thread so there is no need to preserve
+ * the register values. This can waste tens of bytes for each thread.<br>
+ * Consider the following code:
+ * @code
+#include <ch.h>
+
+static WORKING_AREA(waMyThread, 64);
+
+static t_msg MyThread(void *arg) {
+
+ while (!chThdShoudTerminate()) {
+ /* Do thread inner work */
+ }
+ return 1;
+}
+
+main() {
+ chSysInit();
+ ...
+ chThdCreate(NORMALPRIO, 0, waMyThread, sizeof(waMyThread), MyThread, NULL);
+ ...
+ chSysPause();
+}
+ * @endcode
+ * The resulting ASM code for the thread function would be something like this:
+ * @code
+MyThread:
+ stmfd sp!, {r4, r5, r6, lr}
+ ...
+ ldmfd sp!, {r4, r5, r6, pc}
+ * @endcode
+ * Being that function a thread there is no need to save those registers, in
+ * embedded applications often the RAM is a scarce resource. That space can be
+ * saved by modifying the code as follow, using some advanced GCC extensions:
+ * @code
+#include <ch.h>
+
+static BYTE8 waMyThread[UserStackSize(64)];
+
+__attribute__((noreturn)) void MyThread(void *arg) {
+
+ while (!chThdShoudTerminate()) {
+ /* Do thread inner work */
+ }
+ chThdExit(1);
+}
+
+main() {
+ chSysInit();
+ ...
+ chThdCreate(NORMALPRIO, 0, waMyThread, sizeof(waMyThread), (t_tfunc)MyThread, NULL);
+ ...
+ chSysPause();
+}
+ * @endcode
+ * This will make GCC believe that the function cannot return and there is no
+ * need to save registers. The code will be a bit less readable and less
+ * portable on other compilers however.
+ */
+/** @} */
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