Variable defined by the register keyword

Register I = rep3_dma_con;
for (I = 0; I <10; I ++);

/* Register is a storage type of variable, called register.
If a variable is defined as this type, it will be stored in general-purpose registers, so the storage speed is fast.
the benefit of this type is that if you need to use this variable frequently, you can avoid storing it from memory multiple times.

This improves the running efficiency.
it is just a prompt for the compiler. Whether it is register or not depends on how the compiler handles it. Not a mandatory method
modern Compiler optimization technology is good for storing variables into registers, so this keyword for modern compilers

it is useless, but it is still very useful in early compilers
*/