Overview

x86-64 is identified in the evidence as the informal name for the Intel64 instruction set architecture. The Y86-64 instruction set architecture is described as adapting many features of Intel64, while being much simpler and intended as a working model for understanding microprocessor design and implementation rather than as a full processor implementation.

Architectural characteristics noted in the evidence

The evidence states that the x86-64 ISA has 16 program registers. In comparison, Y86-64 supports 15 registers by omitting %r15, a simplification used to make room in a four-bit register field for a special no-register value.

The evidence also describes x86-64-related instruction behavior by comparison with Y86-64:

• The x86-64 data movement instruction movq is split in Y86-64 into four cases: register-to-register, immediate-to-register, register-to-memory, and memory-to-register.
• Stack operations are described as matching x86-64 behavior: pushing first decrements the stack pointer by eight and then writes a word at the stack-pointer address; popping reads the top word and then increments the stack pointer by eight.

Relationship to Y86-64

Y86-64 is a simplified ISA that adapts many features of Intel64, known informally as x86-64. The cited report uses Y86-64 as a model for processor design and verification, not as a complete x86-64 implementation.