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+title = "xpost: Cooperative-multitasking studies (matcha-threads)"
+
+[taxonomies]
+tags = ["threading", "linux", "x86", "arm", "riscv"]
++++
+
+This is a cross post to a **cooperative-multitasking implementation** that I
+did in the past and hosted on github [>> matcha-threads <<][matcha].
+
+Cooperative-multitasking allows to perform the thread scheduling in user space.
+Executing threads need to give the control back to the `scheduler` such that
+other threads can run. Since control is returned explicitly to the scheduler,
+threads need to **"cooperate"**.
+
+The following code snippet shows an example of two such threads:
+```cpp
+#include "lib/executor.h"
+#include "lib/thread.h"
+#include <cstdio>
+
+void like_tea(nMatcha::Yielder& y) {
+    std::puts("like");
+    y.yield();
+    std::puts("tea");
+}
+
+int main() {
+    nMatcha::Executor e;
+    e.spawn(nMatcha::FnThread::make(like_tea));
+    e.spawn(nMatcha::FnThread::make([](nMatcha::Yielder& y) {
+        std::puts("I");
+        y.yield();
+        std::puts("green");
+    }));
+    e.run();
+    return 0;
+}
+```
+
+Which gives the following output when being run:
+```text
+I
+like
+green
+tea
+```
+
+The main focus of that project was to understand the fundamental mechanism
+underlying cooperative-multitasking and implement such a `yield()` function as
+shown in the example above.
+
+Looking at the final implementation, the yield function does the following:
+```text
+yield:
+  1. function prologue
+  2. push callee-saved regs to current stack
+  3. swap stack pointers (current - new)
+  4. pop callee-saved regs from new stack
+  5. function epilogue & return
+```
+
+Implementations for different ISAs are available here:
+- [x86_64][yield-x86]
+- [arm64][yield-arm64]
+- [armv7a][yield-arm]
+- [riscv64][yield-rv64]
+
+[matcha]: https://github.com/johannst/matcha-threads
+[yield-x86]: https://github.com/johannst/matcha-threads/blob/master/lib/arch/x86_64/yield.s
+[yield-arm]: https://github.com/johannst/matcha-threads/blob/master/lib/arch/arm/yield.s
+[yield-arm64]: https://github.com/johannst/matcha-threads/blob/master/lib/arch/arm64/yield.s
+[yield-rv64]: https://github.com/johannst/matcha-threads/blob/master/lib/arch/riscv64/yield.s