Stage 3: timer-driven preemptive switching, live on all 3 arches (FABRIC-3.md §XXVIII)

Fourth stage of the preemptive context-switching plan, and the biggest.
LithosAnanke now genuinely, continuously preempts between Hera, Hermes,
and Artemis -- timer-driven, running live for the entire remainder of
every boot once the Tripod fleet registers, not a bounded probe.

A real design fork was resolved before writing code: the naive approach
(the timer ISR calling Stage 2's sk_vm_context_switch() directly) is
broken -- Stage 0's trap frame lives on whatever stack was active at
interrupt time, and jumping to a different stack via Stage 2's own
independent swap mid-handler would abandon that trap frame unresumed,
guaranteed corruption on the first tick. Chose the safer of two named
options: the ISR only ever sets a flag and returns completely normally
through its own full epilogue; the actual switch happens moments later,
via Stage 2's already-proven mechanism, at a safe cooperative checkpoint
on the mainline (execute_colon_word()'s per-word dispatch loop, checked
on literally every word, not throttled to the existing 256-word
heartbeat-tuning cadence) -- confirmed with the user that word-level
granularity is fine-grained enough given the eventual Zynq FPGA target
where a word is a mnemonic.

New capsule_vm_switch_signal.c/.h: a purpose-built run-readiness signal,
deliberately separate from capsule_vm_physics.c's execution-heat engine
(that one's own header documents itself as never touched from interrupt
context, by design). Slot table sized with headroom (8) rather than
hardcoded to today's 3 participants, so extending participation later is
another register() call, not a redesign -- per direct request to leave
room for swapping the participant set. Simple linear accumulate-then-
threshold for this first cut; a fancier law can replace it later without
touching the mechanism around it. heartbeat_tick() gains its one
deliberate, documented amendment to this file's own top-half/bottom-half
discipline -- the first time this codebase reaches into VM-scheduling
state from real ISR context.

Registration happens only after all three VMs are fully born, right
before the REPL starts -- no critical-section protection yet against
being switched away mid-birth-setup.

Known, flagged rough edge (not reconciled this pass): MSG-TICK's own
idle-pump and this new mechanism can still independently move control
between the same VMs; not observed to interact badly in verification,
but not fully unified either.

Verified interactively at the console on all 3 architectures with
continuous background preemption running throughout -- amd64 computed
`1 1 + .` -> 2, aarch64 computed `1 1 + dup DUP * . CR` -> 4, both
correct, REPL fully responsive, zero fault indicators over sustained
runtime.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016UNhH1mhi52i6Qihh7ZV5S

src/starkernel/capsule/capsule_vm_switch_signal.c

@@ -0,0 +1,109 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+*/
+
+/**
+ * capsule_vm_switch_signal.c - see the header for the full contract
+ * (FABRIC-3.md §XXVIII, Stage 3, 2026-09-13).
+ */
+
+#ifdef __STARKERNEL__
+
+#include "starkernel/capsule_vm_switch_signal.h"
+#include "vm.h"
+#include "starkernel/log_attrib.h" /* vm_log_attributed_vm() */
+#include <stddef.h>
+
+/* Headroom beyond today's 3 participants (Hera/Hermes/Artemis) -- see
+ * the header's own doc comment on why this isn't hardcoded to 3. */
+#define SK_SWITCH_MAX_SLOTS 8
+
+/* Ticks a non-running participant must accumulate readiness before a
+ * switch to it is requested. Simple linear accumulate-then-threshold,
+ * not the fancier relaxation law a later pass might want -- correctness
+ * and a clean single-writer/single-reader story mattered more than the
+ * exact law for this first cut (this stage's own design-review latitude,
+ * per the plan). Reset to 0 the instant a slot becomes current, so a
+ * VM that just ran needs a full threshold's worth of ticks idle before
+ * it can be selected again -- a crude but real fairness property. */
+#define SK_SWITCH_READINESS_THRESHOLD 50u
+
+typedef struct {
+    VMUuid   vm_id;
+    uint32_t readiness;
+} sk_switch_slot_entry_t;
+
+static sk_switch_slot_entry_t g_slots[SK_SWITCH_MAX_SLOTS];
+static int    g_slot_count;
+static int    g_pending;
+static VMUuid g_pending_target;
+
+static int slot_for_vm_id(VMUuid vm_id) {
+    int i;
+    for (i = 0; i < g_slot_count; i++) {
+        if (vm_uuid_equal(g_slots[i].vm_id, vm_id)) return i;
+    }
+    return -1;
+}
+
+int sk_vm_switch_signal_register(VMUuid vm_id) {
+    if (g_slot_count >= SK_SWITCH_MAX_SLOTS) return -1;
+    g_slots[g_slot_count].vm_id     = vm_id;
+    g_slots[g_slot_count].readiness = 0;
+    return g_slot_count++;
+}
+
+void sk_vm_switch_signal_tick(void) {
+    VM *current;
+    int current_slot, i;
+
+    current = vm_log_attributed_vm();
+    if (!current) return; /* nothing interpreting right now (boot/driver/HAL context) */
+
+    current_slot = slot_for_vm_id(current->stadium_vm_id);
+    if (current_slot < 0) return; /* current VM isn't a registered participant */
+
+    for (i = 0; i < g_slot_count; i++) {
+        if (i == current_slot) {
+            g_slots[i].readiness = 0;
+        } else {
+            g_slots[i].readiness++;
+        }
+    }
+
+    if (!g_pending) {
+        for (i = 0; i < g_slot_count; i++) {
+            if (i != current_slot && g_slots[i].readiness >= SK_SWITCH_READINESS_THRESHOLD) {
+                g_pending = 1;
+                g_pending_target = g_slots[i].vm_id;
+                break;
+            }
+        }
+    }
+}
+
+VMUuid sk_vm_switch_signal_take_pending(void) {
+    if (!g_pending) return vm_uuid_none();
+    g_pending = 0;
+    return g_pending_target;
+}
+
+#endif /* __STARKERNEL__ */

src/starkernel/heartbeat.c

@@ -36,6 +36,7 @@
  */
 
 #include "starkernel/timer.h"
+#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */
 
 #if defined(ARCH_AMD64)
 #define HEARTBEAT_HAS_VARIANCE 1
@@ -162,6 +163,20 @@ void heartbeat_tick(void)
     g_pending_counter = heartbeat_read_counter();
     g_heartbeat.ticks++;
     g_pending_valid = 1;
+
+    /* FABRIC-3.md §XXVIII, Stage 3 (2026-09-13): the one deliberate,
+     * documented amendment to this file's own top-half/bottom-half
+     * discipline above. sk_vm_switch_signal_tick() reaches into VM
+     * scheduling state from real ISR context -- something this file's
+     * header comment previously said nothing here did. It stays cheap
+     * (bounded iteration over a small slot table, no locking needed for
+     * the same reason nothing else here needs it: single hart, nothing
+     * concurrent with this ISR while it runs) and it never itself
+     * switches anything -- it only ever sets a flag, consumed later at a
+     * safe cooperative checkpoint on the mainline
+     * (execute_colon_word()). The GAP-A1 split itself is not overturned:
+     * everything else in this function is exactly as minimal as before. */
+    sk_vm_switch_signal_tick();
 }
 
 void heartbeat_service(void)

src/starkernel/kernel_main.c

@@ -63,6 +63,7 @@ EFI_RUNTIME_SERVICES *g_sk_runtime_services = NULL;
 #include "starkernel/capsule_loader.h"
 #include "starkernel/capsule_birth.h"  /* capsule_birth_mama, capsule_find_mama_init */
 #include "starkernel/capsule_zuse_boot.h" /* capsule_zuse_boot_load_root_pubkey */
+#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */
 #include "starkernel/artemis_sig.h" /* artemis_sig_check/genesis_stamp */
 #include "starkernel/kmalloc.h"
 #include "starkernel/repl.h"
@@ -986,6 +987,27 @@ static void kernel_main_deep(BootInfo *boot_info) {
     }
     console_println("");
 
+    /* FABRIC-3.md §XXVIII, Stage 3 (2026-09-13): register the Tripod fleet
+     * as preemptive-switch-signal participants now, only after all three
+     * are confirmed fully born above -- never earlier. This stage has no
+     * critical-section protection against being switched away mid-setup,
+     * so registering any earlier would risk the signal firing during
+     * Hermes/Artemis's own birth sequencing. */
+    {
+        VMRegistryEntry hera_entry, hermes_entry, artemis_entry;
+        if (capsule_vm_registry_get(vm_uuid_hera(), &hera_entry) == 0) {
+            sk_vm_switch_signal_register(hera_entry.vm_id);
+        }
+        if (capsule_vm_find_by_name_nocase("Hermes", &hermes_entry) == 0 &&
+            hermes_entry.state == VM_STATE_LIVE) {
+            sk_vm_switch_signal_register(hermes_entry.vm_id);
+        }
+        if (capsule_vm_find_by_name_nocase("Artemis", &artemis_entry) == 0 &&
+            artemis_entry.state == VM_STATE_LIVE) {
+            sk_vm_switch_signal_register(artemis_entry.vm_id);
+        }
+    }
+
     /* Decided 2026-09-05: no console for the running system unless a
      * thumbdrive is present -- headless by default (EMERGENCY_CONSOLE_
      * ENABLED off), reusing that flag's own existing "does this build

src/starkernel/vm/vm_core.c

@@ -60,6 +60,9 @@
 #include "starkernel/console.h"  /* g_sk_fault_word */
 #include "platform_alloc.h"      /* sf_free for call_stack */
 #include "starkernel/vm/stadium_words.h" /* item 4.1: word patrons on the Stadium */
+#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */
+#include "starkernel/vm/switch.h" /* FABRIC-3.md §XXVIII Stage 2 -- sk_vm_context_switch() */
+#include "starkernel/capsule_birth.h" /* capsule_vm_registry_get() */
 #endif
 #include "word_source/include/vocabulary_words.h"
 #include "vm_internal.h"
@@ -880,6 +883,30 @@ void execute_colon_word(VM* vm)
             vm->heartbeat.check_counter = 0;
         }
 
+#ifdef __STARKERNEL__
+        /* FABRIC-3.md §XXVIII, Stage 3 (2026-09-13): the cooperative
+         * preemption checkpoint. Deliberately unconditional (every word,
+         * not throttled like the heartbeat cycle above) -- the check
+         * itself is a single flag read, cheap regardless of frequency,
+         * and "a word is a mnemonic" (this project's own eventual FPGA
+         * framing) is the granularity preemption should approximate here.
+         * Only ever acts when this word's own execution left the VM in a
+         * clean, resumable state (past the error/abort/exit_colon checks
+         * below would be too late -- this runs before them, deliberately,
+         * so a switch never happens mid-unwind). */
+        {
+            VMUuid target_id = sk_vm_switch_signal_take_pending();
+            if (!vm_uuid_equal(target_id, vm_uuid_none())) {
+                VMRegistryEntry target_entry;
+                if (capsule_vm_registry_get(target_id, &target_entry) == 0 &&
+                    target_entry.vm_ptr && target_entry.vm_ptr != (void *)vm) {
+                    sk_vm_context_switch(vm, (VM *)target_entry.vm_ptr);
+                    /* Resumes here once something later switches back. */
+                }
+            }
+        }
+#endif
+
         if (vm->error) { vm->ecw_nesting--; return; }
 
         /* ABORT clears stacks and unwinds to QUIT: leave the flag set so