Confused over memory mapping

I've r开发者_开发知识库ecently 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.

Though this is an old question, thought of answering this as it might help some others like me trying to get sufficient answers from stackoverflow.

you explanation is almost correct but want to give little explanation on this one: peripherals are initialized by the bootloader and then they are mapped into the physical memory space

Onchip peripherals already have a predefined physical address space. For other external IO mapped peripherals (like PCIe), we need to config a physical addr space, but their physical address space range is still predefined. They cannot be configured at random address space.

Now to your questions, here are my answers.. How does the bootloader initialize the peripherals if they haven't been mapped to an address space yet?

As I mentioned above, all (on-chip)peripherals have physical address space predefined (usually will be listed in Memory map chapter of processor RM). So, boot loaders (assuming MMU is off) can directly access them.

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?

With VMM, there are page tables (created and stored in physical DRAM by kernel) that tells MMU to map virtual addr to physical addr. In linux kernel with 1G kernel virt space (say kernel virtual addrs from 0xc0000000-0xffffffff), on-chip peripherals will need to have a VM space from within the above kernel VM space (so that kernel & only kernel can access it); and page tables will be setup to map that peripheral virt addr to its actual physical addr (the ones defined in RM)

You can't remap peripherals in ARM processor, all peripheral devices correspond to fixed positions in memory map. Even registers are mapped to internal RAM memory that has permanent fixed positions. The only things you can remap are memory devices like SRAM, FLASH, etc. via FSMC or similar core feature. You can however remap a memory mapped add-on custom peripheral that is not part of the core itself, lets say a hard disk controller for instance, but what is inside ARM core its fixed.

A good start is to take a look at processor datasheets at company sites like Philips and ST, or ARM architecture itself at www.arm.com.