circle64 | A 64-bit C bare metal environment for Raspberry Pi

You are looking for "memory mapped input-output".

The most common way for CPU to talk to external hardware is through memory bus - the same bus that is used to access normal memory.

First, keep in mind that CPUs interaction with memory involves not just reads and writes, but also things like bus error handling (invalid access), arbitration (multiple devices accessing the same memory) and routing (CPU might want to access multiple memory devices). To handles this, a bus protocol is used.

To write or read external memory CPU has to initiate a transaction. The exact sequence of this is defined by used bus protocol, but it usually involves steps like:

• Sending transcation address, type, length etc.
• Receiving response - allowed or declined (bus error).
• Transmitting the actual data if transaction was allowed.

The device that initiates transcation is called master or initiator, while device that is responsible for handling the transaction is called slave or target.

Device that decides what slave handles the transaction is called decoder or router. So transaction will usually go from master to decoder and then to slave device.

Bus protocol essentialy provides a way to transfer data to or from a device. This device can be a memory device or anything else. In case of memory device, its controller handles transcations by writing or reading data to or from memory cell array.

If you are familiar with Object Oriented Programming, you can think of CPU being connected to external devices via interface which allows reading and writing at specified address. Implementation of this interface can do anything. And this how memory mapped I/O is done - CPU gets connected to a bunch of devices, each receiving transactions at specific address range. Write data to one address and this data will be received by memory device which will store it to memory cell array, write data to another address and it will be received by, lets say, SD controller and will be interpreted as "send SEND_STATUS command to SD card".

If you are also familiar with modern operating systems, you can think of "everything is a file" abstraction. Some files are just plain files e.g. they act as memory. Other files are different. Just like reading /proc/cpuinfo on Linux gives you information about your CPUs, reading at some address can provide you with information on what IRQs are currently pending or tell you how many incoming messages mailbox has at the moment.

Examples of bus protocols are AXI and AHB. AHB is simpler, AXI is more complex and faster protocol. In case of Raspberry PI it is most likely AXI protocol that is used to connect CPU to hardware.

So regarding your question, those two functions are used to access registers of external devices via memory mapped I/O. Everything else you got right:

• volatile is used to prevent compiler from removing, reordering or in any way changing those memory accesses - without this hardware will not do what we want it to do.
• u32 is used because many device will not even support anything but 32-bit accesses to keep hardware simple.