Page "Register window" Paragraph 3

While registers are almost a universal solution to performance, they do have a drawback.

Different parts of a computer program all use their own temporary values, and therefore compete for the use of the registers.

Since a good understanding of the nature of program flow at runtime is very difficult, there is no easy way for the developer to know in advance how many registers they should use, and how many to leave aside for other parts of the program.

In general these sorts of considerations are ignored, and the developers, and more likely, the compilers they use, attempt to use all the registers visible to them.

In the case of processors with very few registers to begin with, this is also the only reasonable course of action.