I’ve recently started getting into low level stuff and looking into bootloaders and operating systems etc…
As I understand it, for ARM processors at least, peripherals are initialized by the bootloader and then they are mapped into the physical memory space. From here, code can access the peripherals by simply writing values to the memory space mapped to the peripherals registers. Later if the chip has a MMU, it can be used to further remap into virtual memory spaces. Am I right?
What I don’t understand are (assuming what I have said above is correct):
- How does the bootloader initialize the peripherals if they haven’t been mapped to an address space yet?
- With virtual memory mapping, there are tables that tell the MMU where to map what. But what determines where peripherals are mapped in physical memory?
When a device boots, the MMU is turned off and you will be typically running in supervisor mode. This means that any addresses provide are physical addresses.
Each ARM SOC (system on Chip) will have a memory map. The correspondece of addresses to devices is determined by which physical data and address line are connect to which parts of the processor. All this information can be found in a Technical reference manual. For OMAP4 chips this can be found here.
There are several ways to connect off-chip device. One is using the GPMC. Here you will need to sepcify the address in the GPMC that you want to use on the chip.
When the MMU is then turned on, these addresses may change depending on how the MMU is programmed. Typically direct access to hardware will also only be available in kernel mode.