diff --git a/arch/arm/core/exc_exit.S b/arch/arm/core/exc_exit.S
index 051f8ea506c..f1af77e6256 100644
--- a/arch/arm/core/exc_exit.S
+++ b/arch/arm/core/exc_exit.S
@@ -72,6 +72,7 @@ SECTION_SUBSEC_FUNC(TEXT, _HandlerModeExit, z_arm_int_exit)
 
 SECTION_SUBSEC_FUNC(TEXT, _HandlerModeExit, z_arm_exc_exit)
 #if defined(CONFIG_CPU_CORTEX_R)
+    /* r0 contains the caller mode */
     push {r0, lr}
 #endif
 
@@ -116,11 +117,17 @@ _EXIT_EXC:
 #if defined(CONFIG_CPU_CORTEX_M)
     bx lr
 #elif defined(CONFIG_CPU_CORTEX_R)
-    /*
-     * r0-r3 are either the values from the thread before it was switched out
-     * or they are the args to _new_thread for a new thread
-     */
+    /* Restore the caller mode to r0 */
     pop {r0, lr}
+
+    /*
+     * Restore r0-r3, r12 and lr stored into the process stack by the mode
+     * entry function. These registers are saved by _isr_wrapper for IRQ mode
+     * and z_arm_svc for SVC mode.
+     *
+     * r0-r3 are either the values from the thread before it was switched out
+     * or they are the args to _new_thread for a new thread.
+     */
     push {r4, r5}
 
     cmp r0, #RET_FROM_SVC
diff --git a/arch/arm/core/swap_helper.S b/arch/arm/core/swap_helper.S
index 4fa0a329080..c6bd30038a8 100644
--- a/arch/arm/core/swap_helper.S
+++ b/arch/arm/core/swap_helper.S
@@ -39,6 +39,10 @@ GDATA(_kernel)
  * When PendSV is pended, the decision that a context switch must happen has
  * already been taken. In other words, when z_arm_pendsv() runs, we *know* we
  * have to swap *something*.
+ *
+ * For Cortex-R, PendSV exception is not supported by the architecture and this
+ * function is directly called either by _IntExit in case of preemption, or
+ * z_arm_svc in case of cooperative switching.
  */
 
 SECTION_FUNC(TEXT, z_arm_pendsv)
@@ -328,7 +332,10 @@ _thread_irq_disabled:
 #endif
 #endif /* CONFIG_TRACING */
 
-    /* exc return */
+    /*
+     * Cortex-M: return from PendSV exception
+     * Cortex-R: return to the caller (_IntExit or z_arm_svc)
+     */
     bx lr
 
 #if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
@@ -514,13 +521,25 @@ valid_syscall_id:
 
     /* return from SVC to the modified LR - z_arm_do_syscall */
     bx lr
-#endif
+#endif /* CONFIG_USERSPACE */
 
 #elif defined(CONFIG_ARMV7_R)
+
+/**
+ *
+ * @brief Service call handler
+ *
+ * The service call (svc) is used in the following occasions:
+ * - Cooperative context switching
+ * - IRQ offloading
+ * - Kernel run-time exceptions
+ *
+ * @return N/A
+ */
 SECTION_FUNC(TEXT, z_arm_svc)
     /*
      * Switch to system mode to store r0-r3 to the process stack pointer.
-     * Save r12 and the lr as we will be swapping in another process and
+     * Save r12 and the lr as we could be swapping in another process and
      * returning to a different location.
      */
     push {r4, r5}