CHAPTER 8. Registers
Registers are the interface that hardware provides to firmware; this is the primary means of communication between the two. Firmware reads from and writes to those registers. Hardware communicates information to firmware by making it available in registers for firmware to read. Firmware configures hardware and launches tasks in hardware by writing to registers. More specifically, each hardware block in the chip has a set of registers. Each block typically has a device driver within firmware that accesses that set of registers.
The term, register, has two slight variations in meaning that overlap each other. One is as an addressable portal through which firmware accesses hardware. The other is as a bank of flip-flops. Figure ...

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