targets/gba: implement interrupt handler

Thanks to Kyle Lemons for the inspiration and original design. The
implementation in this commit is very different however, building on top
of the software vectoring needed in RISC-V. The result is a flexible
interrupt handler that does not take up any RAM for configuration.

src/machine/machine_gameboyadvance.go

@@ -8,6 +8,25 @@ import (
 	"unsafe"
 )
 
+// Interrupt numbers as used on the GameBoy Advance. Register them with
+// runtime/interrupt.New.
+const (
+	IRQ_VBLANK  = 0
+	IRQ_HBLANK  = 1
+	IRQ_VCOUNT  = 2
+	IRQ_TIMER0  = 3
+	IRQ_TIMER1  = 4
+	IRQ_TIMER2  = 5
+	IRQ_TIMER3  = 6
+	IRQ_COM     = 7
+	IRQ_DMA0    = 8
+	IRQ_DMA1    = 9
+	IRQ_DMA2    = 10
+	IRQ_DMA3    = 11
+	IRQ_KEYPAD  = 12
+	IRQ_GAMEPAK = 13
+)
+
 // Make it easier to directly write to I/O RAM.
 var ioram = (*[0x400]volatile.Register8)(unsafe.Pointer(uintptr(0x04000000)))
 

src/runtime/interrupt/interrupt_gameboyadvance.go

@@ -0,0 +1,36 @@
+// +build gameboyadvance
+
+package interrupt
+
+import (
+	"runtime/volatile"
+	"unsafe"
+)
+
+var (
+	regInterruptEnable       = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000200)))
+	regInterruptRequestFlags = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000202)))
+	regInterruptMasterEnable = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000208)))
+)
+
+// Enable enables this interrupt. Right after calling this function, the
+// interrupt may be invoked if it was already pending.
+func (irq Interrupt) Enable() {
+	regInterruptEnable.SetBits(1 << uint(irq.num))
+}
+
+//export handleInterrupt
+func handleInterrupt() {
+	flags := regInterruptRequestFlags.Get()
+	for i := 0; i < 14; i++ {
+		if flags&(1<<uint(i)) != 0 {
+			regInterruptRequestFlags.Set(1 << uint(i)) // acknowledge interrupt
+			callInterruptHandler(i)
+		}
+	}
+}
+
+// callInterruptHandler is a compiler-generated function that calls the
+// appropriate interrupt handler for the given interrupt ID.
+//go:linkname callInterruptHandler runtime.callInterruptHandler
+func callInterruptHandler(id int)

src/runtime/runtime_arm7tdmi.go

@@ -3,6 +3,7 @@
 package runtime
 
 import (
+	_ "runtime/interrupt" // make sure the interrupt handler is defined
 	"unsafe"
 )
 

targets/gameboy-advance.ld

@@ -1,10 +1,13 @@
 OUTPUT_ARCH(arm)
 ENTRY(_start)
 
+/* Note: iwram is reduced by 96 bytes because the last part of that RAM
+ * (starting at 0x03007FA0) is used for interrupt handling.
+ */
 MEMORY {
-    ewram   : ORIGIN = 0x02000000, LENGTH = 256K /* on-board work RAM (2 wait states) */
-    iwram   : ORIGIN = 0x03000000, LENGTH = 32K  /* in-chip work RAM (faster) */
-    rom     : ORIGIN = 0x08000000, LENGTH = 32M  /* flash ROM */
+    ewram   : ORIGIN = 0x02000000, LENGTH = 256K   /* on-board work RAM (2 wait states) */
+    iwram   : ORIGIN = 0x03000000, LENGTH = 32K-96 /* in-chip work RAM (faster) */
+    rom     : ORIGIN = 0x08000000, LENGTH = 32M    /* flash ROM */
 }
 
 __stack_size_irq = 1K;

targets/gameboy-advance.s

@@ -17,9 +17,7 @@ _start:
     .byte   0x00,0x00              // Checksum (80000BEh)
 
 start_vector:
-    mov     r0, #0x4000000                 // REG_BASE
-    str     r0, [r0, #0x208]
-
+    // Configure stacks
     mov     r0, #0x12                      // Switch to IRQ Mode
     msr     cpsr, r0
     ldr     sp, =_stack_top_irq            // Set IRQ stack
@@ -27,7 +25,21 @@ start_vector:
     msr     cpsr, r0
     ldr     sp, =_stack_top                // Set user stack
 
+    // Configure interrupt handler
+    mov     r0, #0x4000000                 // REG_BASE
+    ldr     r1, =handleInterruptARM
+    str     r1, [r0, #-4]                  // actually storing to 0x03007FFC due to mirroring
+
+    // Enable interrupts
+    mov     r1, #1
+    str     r1, [r0, #0x208]               // 0x04000208 Interrupt Master Enable
+
     // Jump to user code (switching to Thumb mode)
     ldr     r3, =main
     bx      r3
 
+// Small interrupt handler that immediately jumps to a function defined in the
+// program (in Thumb) for further processing.
+handleInterruptARM:
+    ldr     r0, =handleInterrupt
+    bx      r0