One of the most fundamental distinctions between x64 and x86 is the size of their registers. In x64 architecture, registers are 64 bits wide, enabling them to handle 64-bit data at once. In contrast, x86 registers are 32 bits wide, limiting them to 32-bit data manipulation. This increased register size in x64 is pivotal because it facilitates larger memory addressing and more extensive data processing capabilities.

Link For more info: https://phoenixnap.com/kb/x64-vs-x86

The x64 architecture provides a distinct advantage by introducing more general-purpose registers compared to x86. While x86 has EAX, EBX, ECX, and EDX as its primary registers, x64 extends this set to include R8-R15. These extra registers offer valuable storage and manipulation options, contributing to improved performance and versatility in handling complex operations.

Link For more info: https://www.tutorialspoint.com/assembly_programming/assembly_registers.htm

Another crucial difference lies in how registers are used in these architectures. In x86, ESP (stack pointer) and EBP (base pointer) registers are often employed for managing the stack. However, in x64, RSP serves as the stack pointer, and RBP is commonly used as a frame pointer. These changes reflect variations in calling conventions and stack management between the two architectures.

Link For more info: https://www.cs.virginia.edu/~evans/cs216/guides/x86.html