Programming Embedded Systems in C and C++

86 Chapter 7: Peripherals

it is the data pointed to that is subject to change without notice. This is a very sub-

tle point, and it is easy to confuse yourself by thinking about it too much. Just

remember that the location of a register is fixed, though its contents might not be.

And if you use the volatile keyword, the compiler will assume the same.

The primary disadvantage of the other type of device registers, I/O-mapped regis-

ters, is that there is no standard way to access them from C or C++. Such registers

are accessible only with the help of special machine-language instructions. And

these processor-specific instructions are not supported by the C or C++ language

standards. So it is necessary to use special library routines or inline assembly (as

we did in Chapter 2) to read and write the registers of an I/O–mapped device.

The Device Driver Philosophy

When it comes to designing device drivers, you should always focus on one eas-

ily stated goal: hide the hardware completely. When you’re finished, you want the

device driver module to be the only piece of software in the entire system that

reads or writes that particular device’s control and status registers directly. In addi-

tion, if the device generates any interrupts, the interrupt service routine that

responds to them should be an integral part of the device driver. In this section,

I’ll explain why I recommend this philosophy and how ...