1. A computing device comprising: a processor; a controller; a first memory storing system boot code, wherein the first memory is accessible by both the processor and the controller over a bus; a second memory electrically isolated from the bus and storing a system boot code copy, wherein the second memory is accessible by the controller but inaccessible by the processor, wherein the controller is to, during a period in which the processor is not accessing the first memory, replace the system boot code in the first memory with the system boot code copy from the second memory.