Exploring General-Purpose Registers in 64-bit Mode

Exploring General-Purpose Registers in 64-bit Mode

Table of Contents

1. Introduction
2. General-Purpose Registers in 64-bit Mode
3. Differences in Operand Sizes
4. Available Registers in 64-bit Mode
5. Accessing Registers at Different Operand Levels
6. Transitioning from 64-bit Mode to Compatibility Mode
7. Understanding Real Address Mode
8. The Memory Model in Real Address Mode
9. Limitations on Accessing Byte Registers
10. Operand Size Determines Valid Bits in Destination Register

General-Purpose Registers in 64-bit Mode

In this article, we will delve into the details of general-purpose registers in the 64-bit mode of the Intel processor. Just like in the 32-bit mode, there are 16 general-purpose registers available. The default operand size remains at 32 bits, so there are a few similarities between the two modes. However, there are also some differences to note.

Differences in Operand Sizes

One major difference in the 64-bit mode is that general-purpose registers can work with either 32-bit or 64-bit operands. This means that even though a register like EAX is 32 bits in size, it can still be used in a 64-bit program. The suffix "D" after a register number indicates a 32-bit operand size. Therefore, if a 32-bit operand size is specified, all the registers, including the E registers and our 8 through our 15, are available for use.

Available Registers in 64-bit Mode

In 64-bit mode, all the E registers, as well as our 8 through our 15, can be accessed. These eight registers represent the new general-purpose registers introduced in this mode. Additionally, the XMM registers are preserved across transitions from 64-bit mode to compatibility mode. This means that these registers are not only available in 64-bit mode but also when working with 32-bit programs on a 64-bit machine.

Accessing Registers at Different Operand Levels

When working with registers, you can use different levels of operand sizes, such as byte, WORD, double word, and quad word. Using the registers without the "REX" prefix allows you to reference them at their default sizes. However, when using the "REX" prefix, you can make reference to additional registers, including the 64-bit registers.

Transitioning from 64-bit Mode to Compatibility Mode

While transitioning from 64-bit mode to compatibility mode, the values of our 8 through our 15 and the XMM registers become undefined. This happens when switching from 64-bit mode through compatibility mode to legacy or real code mode. However, these registers are preserved across transitions, which means they retain their values as you switch back and forth between the modes.

Understanding Real Address Mode

Real address mode is a memory model used for the Intel 8086 processor, which was released in 1978. It is still supported when you're in compatibility mode. In this mode, you work with 16-bit operand sizes. It is important to note that real address mode is different from real code mode, which is related to ancient programs.

The Memory Model in Real Address Mode

In real address mode, the processor supports the real address memory model. This Paragraph provides background information on the memory model used by the Intel 8086 processor in real address mode. It is Mentioned here to clarify the difference between real address mode and real code mode.

Limitations on Accessing Byte Registers

In 64-bit mode, there are limitations on accessing byte registers. Instructions cannot reference the high bytes, as indicated by the examples given. However, instructions can reference legacy low bytes and new byte registers simultaneously. The architecture enforces this limitation by changing high-byte references to low-byte references. The low 8 bits on the low registers (RB and P) are used for this purpose.

Operand Size Determines Valid Bits in Destination Register

Depending on the operand size, the number of valid bits in the destination general-purpose register is determined. When operating with 64-bit operands, the result will be 64 bits. Similarly, 32-bit operands generate 32-bit results. However, 8-bit and 16-bit operands only generate 8-bit or 16-bit results, respectively. The upper 56 bits or 48 bits of the destination register remain unmodified by the operation. If an 8-bit or 16-bit result is intended for 64-bit address calculation, explicit sign extension is required.

In conclusion, while working in 64-bit mode, it is important to understand the nuances of general-purpose registers. Knowing the available registers and their operand limitations can help optimize your code for performance and compatibility.