Physical RAM is completely different from the address space. For a simple example, the control register for the UART serial port peripheral allows you to 'flip

Physical RAM is completely different from the address space. For a simple example, the control register for the UART serial port peripheral allows you to 'flip an off switch' by writing a 0 to the whole register, or to turn the UART transmit and receive functions by writing a one to the ninth and tenth bits, and to turn on the FIFO buffer by writing a 1 to the first bits. In this code, that control register has an address defined: #define UART0_CR ((volatile unsigned int*)(0x3F000000+0x00201030)) And the code can interact with the peripheral as described by writing `UART0_CR = 0;` or `UART0_CR = 0x301;`. There is no RAM storage at 4,236,263,616 bytes from the start of memory, but this peripheral exists at and handles writes to that address space. Take a look at https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-..., page 5. It's part of the address space, but not part of RAM. Relatedly, one useful feature of some ARM processors is the bitwise section of the address space. You want to set the FIFO bit above, but you don't know if the Tx, Rx, or both is on? You need to do the equivalent of: temp = UART0_CR; // Either 0x100, 0x200 0x300, or 0x000 // UART0_CR = 0x001; <- Won't work, you've deleted the Tx and Rx temp = temp | 0x001; // Bitwise OR sets last bit only UART0_CR = temp; which takes 3 cycles and a scratch register. Instead, on at least some Cortex M microcontrollers, there's a copy of UART0_CR (and all the other peripheral registers) expanded in a part of the address space where each word is just one bit of the nominal register, so you can write in a single cycle without a scratch register.