Fix HALT IME=1 interrupt servicing

If we check interrupts before decoding instructions, HALT IME=1 case
works properly.

Fixes #15, #19 and #20.

src/acceptance_tests/mod.rs

@@ -23,6 +23,9 @@ fn div_timing() { run_acceptance_test("div_timing") }
 #[test]
 fn ei_timing() { run_acceptance_test("ei_timing") }
 
+#[test]
+fn halt_ime1() { run_acceptance_test("halt_ime1") }
+
 #[test]
 fn if_ie_registers() { run_acceptance_test("if_ie_registers") }
 

src/cpu/mod.rs

@@ -280,8 +280,6 @@ impl<H> Cpu<H> where H: Bus {
         self.hardware.emulate(self.time);
       }
     } else {
-      self.decode();
-
       match self.ime_change {
         ImeChange::None => (),
         ImeChange::Soon(value) => {
@@ -308,10 +306,11 @@ impl<H> Cpu<H> where H: Bus {
             let pc = self.regs.pc;
             self.push_u16(pc);
             self.regs.pc = interrupt.get_addr();
-            return;
           }
         }
       }
+
+      self.decode();
     }
   }
 

tests/Makefile

@@ -1,6 +1,6 @@
 TESTS := \
 	add_sp_e_timing boot_div boot_div2 call_timing call_timing2 call_cc_timing\
-	call_cc_timing2 div_timing ei_timing if_ie_registers intr_timing\
+	call_cc_timing2 div_timing ei_timing halt_ime1 if_ie_registers intr_timing\
 	intr_timing2 jp_timing jp_cc_timing ld_hl_sp_e_timing oam_dma_restart\
 	oam_dma_timing pop_timing push_timing rapid_di_ei reti_intr_timing rst_timing
 

tests/halt_ime1/Makefile

@@ -0,0 +1 @@
+include ../common/template.mk

tests/halt_ime1/test.s

@@ -0,0 +1,39 @@
+; If IME=1, HALT is expected to immediately service an interrupt.
+; So, if the interrupt service routine doesn't return,
+; the instruction after HALT should never get executed
+
+.incdir "../common"
+.include "common.i"
+
+  ei
+
+  ; B = 0
+  xor a
+  ld b, a
+
+  ; Enable timer interrupt
+  ld a, $04
+  ld_ff_a IE, a
+
+  ; TIMA = $F0
+  ld a, $F0
+  ld_ff_a TIMA, a
+
+  ; Start timer at 262144 Hz
+  ld a, $05
+  ld_ff_a TAC, a
+
+  halt
+  ; This should never get executed
+  inc b
+
+  test_failure
+
+test_finish:
+  ; GBP MGB-001 / GBC CGB-001 / GBASP AGS-101 (probably DMG/GBA as well)
+  save_results
+  assert_b $00
+  jp print_results
+
+.org $50
+  jp test_finish