fixed issues with flickering on frame sync

crates/prometeu-kernel/src/prometeu_os.rs

@@ -68,6 +68,10 @@ pub struct PrometeuOS {
     /// Debugging flag to execute exactly one instruction or frame regardless of budget.
     pub debug_step_request: bool,
 
+    /// When true, the next logical frame must rearm the entrypoint call before running
+    /// to avoid resuming at a pending RET after a FRAME_SYNC safe point.
+    needs_prepare_entry_call: bool,
+
     /// Wall-clock time of system startup.
     boot_time: Instant,
 }
@@ -109,6 +113,7 @@ impl PrometeuOS {
             certifier: Certifier::new(cap_config.unwrap_or_default()),
             paused: false,
             debug_step_request: false,
+            needs_prepare_entry_call: false,
             boot_time,
         };
 
@@ -218,8 +223,11 @@ impl PrometeuOS {
 
             // If the VM is not currently executing a function (e.g. at the start of the app
             // or after the entrypoint function returned), we prepare a new call to the entrypoint.
-            if vm.call_stack.is_empty() {
+            // Additionally, if the previous slice ended with FRAME_SYNC, we must force a rearm
+            // so we don't resume execution at a pending RET on the next tick.
+            if self.needs_prepare_entry_call || vm.call_stack.is_empty() {
                 vm.prepare_call(&self.current_entrypoint);
+                self.needs_prepare_entry_call = false;
             }
 
             // Reset telemetry for the new logical frame
@@ -285,6 +293,12 @@ impl PrometeuOS {
                         self.logical_frame_active = false;
                         self.logical_frame_remaining_cycles = 0;
 
+                        // If the slice ended in FRAME_SYNC, ensure the next tick starts a fresh
+                        // call to the entrypoint instead of resuming at the RET that follows.
+                        if run.reason == LogicalFrameEndingReason::FrameSync {
+                            self.needs_prepare_entry_call = true;
+                        }
+
                         // If we were doing a "step frame" debug command, pause now that the frame is done.
                         if self.debug_step_request {
                             self.paused = true;

test-cartridges/test01/run.sh

@@ -0,0 +1,7 @@
+#!/bin/zsh
+
+set -e
+
+sdk/prometeu build .
+cp build/program.pbc cartridge/program.pbc
+sdk/prometeu run cartridge