Launcher: Fix crash after 40 launches

A long running test (tests/ramcard.lua) that restarted multiple times
including launching DeskTop.system and rebooting via $C700 would cause
the stack to "leak", eventually tickling a CFFA2 firmware issue where
the return address was calculated improperly and we'd crash to the
monitor. I'd assumed (incorrectly) that ProDOS would set the stack
pointer on startup.

Reset the stack pointer in the launcher, because no-one else does.

Thanks to @peterferrie for diagnosing the issue.

Author: inexorabletash

RELEASE-NOTES.md

@@ -14,6 +14,7 @@ Project Page: https://github.com/a2stuff/a2d
 
 ### Launcher
 
+* Fix crash after 40 restarts.
 
 ### DeskTop
 

src/desktop.system/desktop.system.s

@@ -99,6 +99,10 @@ start:
         ;; Do this for good measure.
         cli
 
+        ;; Clear stack, because ProDOS doesn't.
+        ldx     #$FF
+        txs
+
         jsr     Check128K       ; QUITs if check fails
         jsr     ClearScreenEnable80Cols
         jsr     CheckRAMEmpty   ; QUITs if user cancels

tests/lib/apple2.lua

@@ -874,4 +874,16 @@ end
 
 --------------------------------------------------
 
+function apple2.IsCrashedToMonitor()
+  local cpu = manager.machine.devices[":maincpu"]
+  local sp = cpu.state.SP.value
+  if sp == 0x1FE and apple2.ReadMemory(sp) == 0x4E and apple2.ReadMemory(sp+1) == 0xEB then
+    return true
+  else
+    return false
+  end
+end
+
+--------------------------------------------------
+
 return apple2

tests/stack_exhaustion.lua

@@ -0,0 +1,26 @@
+--[[ BEGINCONFIG ========================================
+
+MODELARGS="-sl1 ramfactor -sl2 mouse -sl6 '' -sl7 cffa2 -aux ext80"
+DISKARGS="-hard1 $HARDIMG"
+
+======================================== ENDCONFIG ]]--
+
+-- A CFFA2 is specified the repro tickles a firmware issue where low
+-- stack causes improper address calculation and we crash to the
+-- monitor. Low stack being a problem is not specific to the CFFA,
+-- however.
+
+test.Step(
+  "Stack exhaustion (takes about 122 seconds to run)",
+  function()
+    local cpu = manager.machine.devices[":maincpu"]
+
+    -- Prior to fix, crashes around iteration 40
+    for i = 1,50 do
+      --print(string.format("i=%d SP=%04X", i, cpu.state.SP.value))
+      a2d.CloseAllWindows()
+      a2d.OpenPath("/A2.DESKTOP/DESKTOP.SYSTEM")
+      a2d.WaitForRestart()
+      test.Expect(not apple2.IsCrashedToMonitor(), "should not have crashed to monitor")
+    end
+end)