gba: add bios interrupt flags (#5445)

* gba: add bios interrupt flags

Adds a new GBA register for interrupt flags, the register is equivalent
to the IF register, but is intended for BIOS functions.

Acknowledging the interrupts with this register allows us to use BIOS
halt functions such as VBlankIntrWait and IntrWait, which we can use to
get rid of busy loops in the GBA code.

* fix formatting

---------

Co-authored-by: zoey <git@zoey.si>

src/device/gba/gba.go

@@ -85,6 +85,9 @@ var (
 	SIO = (*SIO_Type)(unsafe.Add(unsafe.Pointer(REG_BASE), uintptr(0x0134)))
 
 	INTERRUPT = (*INTERRUPT_Type)(unsafe.Add(unsafe.Pointer(REG_BASE), uintptr(0x0200)))
+
+	// BIOS interrupt flags
+	BIOS_IF = (*volatile.Register16)(unsafe.Pointer(uintptr(0x03007FF8)))
 )
 
 // Main memory sections

src/runtime/interrupt/interrupt_gameboyadvance.go

@@ -23,6 +23,7 @@ func handleInterrupt() {
 	for i := 0; i < 14; i++ {
 		if flags&(1<<uint(i)) != 0 {
 			gba.INTERRUPT.IF.Set(1 << uint(i)) // acknowledge interrupt
+			gba.BIOS_IF.Set(1 << uint(i))      // acknowledge interrupt for BIOS
 			callInterruptHandler(i)
 		}
 	}