Added stack tutorial

0x0001a_stack/0x0001a_stack.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include "pico/stdlib.h"
+
+
+int main()
+{
+    stdio_init_all();
+
+    while (true) {
+        __asm volatile(
+            // Save a low register and the link register.
+            "push {r4, lr}\n"
+            // Intentionally unordered: the encoded list is still r2, r3, r6.
+            "push {r3, r2, r6}\n"
+            // Thumb PUSH cannot encode high registers r8-r12.
+            "stmdb sp!, {r9, r10}\n"
+            // Restore each group in reverse order to return SP to its start.
+            "ldmia sp!, {r9, r10}\n"
+            "pop {r2, r3, r6}\n"
+            "pop {r4, lr}\n"
+            ::: "memory");
+    }
+}