diff --git a/src/runtime/gc_blocks.go b/src/runtime/gc_blocks.go
index 71e88e4a..462097f6 100644
--- a/src/runtime/gc_blocks.go
+++ b/src/runtime/gc_blocks.go
@@ -277,6 +277,10 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		size += align(unsafe.Sizeof(layout))
 	}
 
+	if interrupt.In() {
+		runtimePanic("alloc in interrupt")
+	}
+
 	gcTotalAlloc += uint64(size)
 	gcMallocs++
 
diff --git a/src/runtime/interrupt/interrupt_avr.go b/src/runtime/interrupt/interrupt_avr.go
index 25c8caa0..0af71a89 100644
--- a/src/runtime/interrupt/interrupt_avr.go
+++ b/src/runtime/interrupt/interrupt_avr.go
@@ -34,3 +34,12 @@ func Restore(state State) {
 		"state": state,
 	})
 }
+
+// In returns whether the system is currently in an interrupt.
+//
+// Warning: this always returns false on AVR, as there does not appear to be a
+// reliable way to determine whether we're currently running inside an interrupt
+// handler.
+func In() bool {
+	return false
+}
diff --git a/src/runtime/interrupt/interrupt_cortexm.go b/src/runtime/interrupt/interrupt_cortexm.go
index a7127c3e..a34dff78 100644
--- a/src/runtime/interrupt/interrupt_cortexm.go
+++ b/src/runtime/interrupt/interrupt_cortexm.go
@@ -50,3 +50,13 @@ func Disable() (state State) {
 func Restore(state State) {
 	arm.EnableInterrupts(uintptr(state))
 }
+
+// In returns whether the system is currently in an interrupt.
+func In() bool {
+	// The VECTACTIVE field gives the instruction vector that is currently
+	// active (in handler mode), or 0 if not in an interrupt.
+	// Documentation:
+	// https://developer.arm.com/documentation/dui0497/a/cortex-m0-peripherals/system-control-block/interrupt-control-and-state-register
+	vectactive := uint8(arm.SCB.ICSR.Get())
+	return vectactive != 0
+}
diff --git a/src/runtime/interrupt/interrupt_gameboyadvance.go b/src/runtime/interrupt/interrupt_gameboyadvance.go
index 72f2a819..b14f8059 100644
--- a/src/runtime/interrupt/interrupt_gameboyadvance.go
+++ b/src/runtime/interrupt/interrupt_gameboyadvance.go
@@ -2,6 +2,8 @@
 
 package interrupt
 
+// This is good documentation of the GBA: https://www.akkit.org/info/gbatek.htm
+
 import (
 	"runtime/volatile"
 	"unsafe"
@@ -36,8 +38,11 @@ func (irq Interrupt) Enable() {
 	regInterruptEnable.SetBits(1 << uint(irq.num))
 }
 
+var inInterrupt bool
+
 //export handleInterrupt
 func handleInterrupt() {
+	inInterrupt = true
 	flags := regInterruptRequestFlags.Get()
 	for i := 0; i < 14; i++ {
 		if flags&(1<<uint(i)) != 0 {
@@ -45,6 +50,7 @@ func handleInterrupt() {
 			callInterruptHandler(i)
 		}
 	}
+	inInterrupt = false
 }
 
 // Pseudo function call that is replaced by the compiler with the actual
@@ -115,3 +121,8 @@ func Restore(state State) {
 	// Restore interrupts to the previous state.
 	regGlobalInterruptEnable.Set(uint16(state))
 }
+
+// In returns whether the system is currently in an interrupt.
+func In() bool {
+	return inInterrupt
+}
diff --git a/src/runtime/interrupt/interrupt_none.go b/src/runtime/interrupt/interrupt_none.go
index caafc285..255fca9e 100644
--- a/src/runtime/interrupt/interrupt_none.go
+++ b/src/runtime/interrupt/interrupt_none.go
@@ -23,3 +23,9 @@ func Disable() (state State) {
 // calling Disable, this will not re-enable interrupts, allowing for nested
 // cricital sections.
 func Restore(state State) {}
+
+// In returns whether the system is currently in an interrupt.
+func In() bool {
+	// There are no interrupts, so it can't be in one.
+	return false
+}
diff --git a/src/runtime/interrupt/interrupt_tinygoriscv.go b/src/runtime/interrupt/interrupt_tinygoriscv.go
index 14d9a588..2b97a2f1 100644
--- a/src/runtime/interrupt/interrupt_tinygoriscv.go
+++ b/src/runtime/interrupt/interrupt_tinygoriscv.go
@@ -27,3 +27,12 @@ func Disable() (state State) {
 func Restore(state State) {
 	riscv.EnableInterrupts(uintptr(state))
 }
+
+// In returns whether the system is currently in an interrupt.
+func In() bool {
+	// There is one exception that has the value 0 (instruction address
+	// misaligned), but it's not very likely and even if it happens, it's not
+	// really something that can be recovered from. Therefore I think it's safe
+	// to ignore it. It's handled specially (in handleException).
+	return riscv.MCAUSE.Get() != 0
+}
diff --git a/src/runtime/interrupt/interrupt_xtensa.go b/src/runtime/interrupt/interrupt_xtensa.go
index be8d4dc7..fe962f04 100644
--- a/src/runtime/interrupt/interrupt_xtensa.go
+++ b/src/runtime/interrupt/interrupt_xtensa.go
@@ -29,3 +29,11 @@ func Restore(state State) {
 		"state": state,
 	})
 }
+
+// In returns whether the system is currently in an interrupt.
+//
+// Warning: interrupts have not been implemented for Xtensa yet so this always
+// returns false.
+func In() bool {
+	return false
+}
diff --git a/src/runtime/runtime_fe310.go b/src/runtime/runtime_fe310.go
index 22afca40..01cc3ba1 100644
--- a/src/runtime/runtime_fe310.go
+++ b/src/runtime/runtime_fe310.go
@@ -25,6 +25,12 @@ func main() {
 	// Zero the threshold value to allow all priorities of interrupts.
 	sifive.PLIC.THRESHOLD.Set(0)
 
+	// Zero MCAUSE, which is set to the reset reason on reset. It must be zeroed
+	// to make interrupt.In() work.
+	// This would also be a good time to save the reset reason, but that hasn't
+	// been implemented yet.
+	riscv.MCAUSE.Set(0)
+
 	// Set the interrupt address.
 	// Note that this address must be aligned specially, otherwise the MODE bits
 	// of MTVEC won't be zero.
@@ -73,6 +79,10 @@ func handleInterrupt() {
 		// misaligned loads). However, for now we'll just print a fatal error.
 		handleException(code)
 	}
+
+	// Zero MCAUSE so that it can later be used to see whether we're in an
+	// interrupt or not.
+	riscv.MCAUSE.Set(0)
 }
 
 // initPeripherals configures periperhals the way the runtime expects them.
diff --git a/src/runtime/runtime_k210.go b/src/runtime/runtime_k210.go
index 298a420f..5998de69 100644
--- a/src/runtime/runtime_k210.go
+++ b/src/runtime/runtime_k210.go
@@ -31,6 +31,12 @@ func main() {
 		kendryte.PLIC.PRIORITY[i].Set(0)
 	}
 
+	// Zero MCAUSE, which is set to the reset reason on reset. It must be zeroed
+	// to make interrupt.In() work.
+	// This would also be a good time to save the reset reason, but that hasn't
+	// been implemented yet.
+	riscv.MCAUSE.Set(0)
+
 	// Set the interrupt address.
 	// Note that this address must be aligned specially, otherwise the MODE bits
 	// of MTVEC won't be zero.
@@ -93,6 +99,10 @@ func handleInterrupt() {
 		// misaligned loads). However, for now we'll just print a fatal error.
 		handleException(code)
 	}
+
+	// Zero MCAUSE so that it can later be used to see whether we're in an
+	// interrupt or not.
+	riscv.MCAUSE.Set(0)
 }
 
 // initPeripherals configures periperhals the way the runtime expects them.