The legacy method was present in the original PCI, and it is called Configuration Access Mechanism (CAM). It allows for 256 bytes of a device's address space to be reached indirectly via two 32-bit registers called PCI CONFIG_ADDRESS and PCI CONFIG_DATA. These registers are at addresses and in the x86 I/O address space. For example, a software driver (firmware, OS kernel or kernel driver) can use these registers to configure a PCI device by writing the address of the device's register into CONFIG_ADDRESS, and by putting the data that is supposed to be written to the device into CONFIG_DATA. Since this process requires a write to a register in order to write the device's register, it is referred to as "indirection".