headless: $ED opcode = clean power-off (exit + battery save)

The unused SM83 opcode $ED now sets gb->poweroff, which breaks the headless
loop so the normal cart_free()->cart_save() path runs. Lets a guest OS shut the
emulator down cleanly (and flush its .sav) instead of relying on an external
timeout/SIGTERM, which could SIGKILL before the save. run_frame() also stops on
poweroff so exit is immediate.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/cpu.c b/src/cpu.c
index 9e7f685..5b73724 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -407,6 +407,8 @@ int cpu_step(GB *gb) {
 
     case 0xCB: do_cb(gb); break;
 
+    case 0xED: gb->poweroff = true; break;  // (illegal on HW) clean-exit signal
+
     default:
         // 0x40-0xBF block: LD r,r' and ALU A,r
         if (op >= 0x40 && op < 0x80) {