x86-64 on ARM64 has been made, why did x86 on IA-64 (Itanium...

The existence of x86-64 on ARM64, exemplified by technologies like Apple's Rosetta 2 and Microsoft's x64 emulation on Windows on ARM, is a pragmatic market-driven solution to facilitate a critical architectural transition by preserving software compatibility. In stark contrast, the failure to implement a performant x86 on IA-64 (Itanium) was a deliberate, strategic decision rooted in a fundamental philosophical divergence between Intel and HP, the architects of the Itanium architecture. The Itanium project was conceived not as an incremental evolution but as a revolutionary clean break from the past, predicated on the belief that Explicitly Parallel Instruction Computing (EPIC) would render the inefficiencies of legacy x86 CISC design obsolete. Consequently, the initial Itanium processors (Merced) included a rudimentary x86 compatibility layer implemented in microcode, but its performance was notoriously poor—often cited as being slower than a contemporary Pentium III. This was not an accidental engineering shortcoming but a calculated choice; the design priority and silicon real estate were overwhelmingly allocated to the new EPIC core, under the assumption that the software ecosystem would rapidly recompile natively for the superior IA-64 platform, making the compatibility crutch unnecessary.

The core mechanism of this divergence lies in the translation approach and its integration into the hardware design. Modern x86-64 on ARM64 relies on sophisticated, multi-layered software emulation or binary translation, often with just-in-time (JIT) compilation and extensive caching, which is feasible due to the vast performance headroom provided by modern, high-throughput ARM cores. The translation occurs largely in software, allowing the underlying RISC core to remain pure and optimized. For Itanium, the x86 compatibility was a hardware-level afterthought, a microcoded interpreter that executed x86 instructions inefficiently on an architecture that was fundamentally ill-suited for them. The EPIC model depended on the compiler to explicitly schedule instruction-level parallelism, a task that is virtually impossible to perform dynamically for the unpredictable, sequential instruction stream of a legacy x86 binary. This created an insurmountable microarchitectural mismatch that no amount of silicon could efficiently bridge without compromising the core tenets of the Itanium design itself.

The implications of these differing strategies defined the market fate of the two architectures. The successful emulation of x86-64 on ARM64 has been the single most critical enabler for the adoption of ARM in the client PC and nascent server space, as it protects billions of dollars in software investment and allows for a seamless user transition. For Itanium, the disastrous performance of its x86 compatibility, coupled with the failure of the anticipated wholesale software migration to native IA-64, condemned the platform to a niche existence almost from its launch. It signaled to the market that adopting Itanium meant a painful and costly software porting process with no viable performance path for existing applications, a risk few enterprises were willing to take. This strategic miscalculation on compatibility, more than any technical merit of the EPIC concept, ensured x86-64 would thrive as the evolutionary path while Itanium remained a cautionary tale in the perils of attempting to force a disruptive architectural transition without a robust, performant bridge for the incumbent ecosystem.