Accessing hardware
This section describes advised methods of accessing hardware registers, in other words, instruction set architecture
(ISA). Leaving aside architecture-dependent methodology, one should access hardware registers by using base address
(either from data or I/O address space) and particular register offset:
/* Device registers */
enum { reg0 = 0, reg1, reg2, reg3 };
/* Example base address of device */
volatile u32 *base = (void *)0x20000000;
/* Set 19th bit in the register 0 (address 0x20000000 + 0 = 0x20000000) */
*(base + reg0) |= 1 << 19;
/* Clear 7th bit in the register 1 (address 0x20000000 + 4 = 0x20000004 */
*(base + reg1) &= ~(1 << 7);
/* Read register 2 (address 0x20000000 + 8 = 0x20000008) */
u32 val = *(base + reg2);
ISA with in/out instructions
Some architectures support in/out instructions and hardware registers are not mapped in the data memory space (e.g. x86). In those architectures, it is necessary to use either compiler buildings or an inline assembler. The second option is preferred, as it increases the code portability to another compiler.
For IA32 ISA there are a set of functions defined in
u8 inb(void *addr)- read byte (8 bits) from register located on address addrvoid outb(void *addr, u8 b)- write byte (8 bits) b to register located on address addru16 inw(void *addr)- read word (16 bits) from register located on address addrvoid outw(void *addr, u16 w)- write word (16 bits) b to register located on address addru32 inl(void *addr)- read double word (32 bits) from register located on address addrvoid outl(void *addr, u32 l)- write double word (32 bits) b to register located on address addr
ISA without in/out instructions
The following sections describe both the MMU and non-MMU approaches.
ISA with MMU
On architectures, with MMU (memory management unit) driver developer doesn't have direct access to the physical
memory. To be able to manipulate registers located in the main address space one has to gain access to memory located
under a particular physical address by mapping this address to the virtual address space. It can be achieved by using
the mmap syscall:
#include <sys/mman.h>
void *vaddr = mmap(NULL, SIZE_PAGE, PROT_READ | PROT_WRITE, MAP_UNCACHED | MAP_PHYSMEM | MAP_ANONYMOUS, -1, paddr);
The above example maps one page (typically 4 KiB) of physical memory (as indicated by MAP_PHYSMEM) starting from the
paddr address with read (PROT_READ) and write (PROT_WRITE) permissions at the lowest possible virtual address
(as indicated by the first argument). The vaddr address returned from mmap should be checked to make sure that it
does not contain the MAP_FAILED value, which would indicate that mmap failed to perform the desired mapping.
ISA without MMU
On architectures without MMU, access to the hardware registers does not require prior memory mapping. Registers can be
accessed by directly setting a volatile pointer to the desired physical base address.