Processor registers

Each programmable device, and Intel processors are not an exception, has a set of general purpose registers--memory cells located physically on the die, thus providing low latency access. They are used for temporary storage of data that a processor operates on or data that is frequently accessed (if the amount of general purpose registers allows this). The amount and bit size of registers on an Intel CPU vary in accordance with the current mode of operation. An Intel CPU has at least two modes:

• Real mode: This is the good old DOS mode. When the processor is powered up, it starts in the real mode, which has certain limitations, such as the size of the address bus, which is only 20 bits, and the segmented memory space.
• Protected mode: This was first introduced in 80286. This mode provides access to larger amount of memory, as it uses different memory segmentation mechanisms. Paging, introduced in 80386, allows even easier memory addressing virtualization.

Since about 2003, we also have the so-called long mode--64-bit registers/addressing (although, not all 64 bits are used for addressing yet), flat memory model, and RIP-based addressing (addressing relative to the instruction pointer register). In this book, we will work with 32-bit protected (there is such a thing as the 16-bit protected mode, but that is out of scope) and Long, which is a 64-bit mode of operation. The long mode may be considered a 64-bit extension of the protected mode, which evolved from 16-bit to 32-bit. It is important to know that registers accessible in the earlier mode are also accessible in the newer mode, meaning that the registers that were accessible in the real mode are also accessible in the protected mode, and that registers accessible in the protected mode would also be accessible in the long mode (if long mode is supported by the processor). There are a few exceptions regarding the bit width of certain registers and we will look at this soon in this chapter. However, since 16-bit modes (real and 16-bit protected modes) are no longer used by application developers (with minor possible exceptions), in this book, we will work on protected and long modes only.