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.

3 files changed, 5 insertions, 2 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) {
diff --git a/src/gb.h b/src/gb.h
index 7218648..7959e60 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -116,6 +116,7 @@ struct GB {
     bool serial_log;   // print serial output to stderr (blargg tests)
     int  serial_out_fd; // if >=0, transmitted serial bytes written here (stdout)
     int  serial_in_fd;  // if >=0, received serial bytes read here (stdin), else 0xFF
+    bool poweroff;      // set by the $ED opcode: request a clean emulator exit
 };
 
 void gb_init(GB *gb);
diff --git a/src/main.c b/src/main.c
index 648ded2..935c9e7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -24,7 +24,7 @@ static void run_frame(GB *gb) {
     gb->ppu.frame_ready = false;
     // safety bound in case LCD is off (no frame_ready)
     u64 budget = gb->cycles + 70224 * 2;
-    while (!gb->ppu.frame_ready && gb->cycles < budget)
+    while (!gb->ppu.frame_ready && gb->cycles < budget && !gb->poweroff)
         cpu_step(gb);
 }
 
@@ -120,7 +120,7 @@ int main(int argc, char **argv) {
         double start_t = now_sec();
         u64 start_cyc = gb->cycles;
         const double HZ = 4194304.0;
-        while (running) {
+        while (running && !gb->poweroff) {
             run_frame(gb);              // advance ~a frame's worth of cycles
             if (!uncapped) {
                 double target = start_t + (gb->cycles - start_cyc) / HZ;