Celeron is a discontinued series of low-end IA-32 and x86-64 computer microprocessor models targeted at low-cost personal computers, manufactured by Intel. The first Celeron-branded CPU was introduced on April 15, 1998, and was based on the Pentium II.
Intel Celeron Covington
Intel Celeron Mendocino 300 MHz in SEPP package
Top of a Mendocino-core Socket 370 Celeron (PPGA package)
Underside of a Mendocino-core Socket 370 Celeron, 333 MHz
x86-64 is a 64-bit version of the x86 instruction set, first announced in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mode.
AMD Opteron, the first CPU to introduce the x86-64 extensions in April 2003
The five-volume set of the x86-64 Architecture Programmer's Manual, as published and distributed by AMD in 2002