x64 is a generic name for the 64-bit extensions to Intel’s and AMD’s 32-bit x86 instruction set architecture (ISA). There are a number of useful resources on the web:
- Introduction to x64 Assembly
- Intel® 64 and IA-32 Architectures Software Developer Manuals
- AMD64 Architecture Programmer’s Manual, Volumes 1-5
- System V Application Binary Interface AMD64 Architecture Processor Supplement
There are many other unofficial documents available on the Web: