AMD vs ARM — architecture differences

2026-05-17

ISA & encoding

	AMD64 (x86-64)	ARM64 (AArch64)
Family	CISC	RISC (load/store)
Instruction length	1–15 bytes, variable	4 bytes, fixed
Decoder cost	high (length-decode is hard)	low (constant)
µop cache	yes (mitigates decode)	no (not needed)
Addressing	base + index×scale + disp32	base + index (×LSL) or base + simm9
RMW memory ops	yes (e.g. `add [m], r`)	no — must load → op → store
Predicated execution	`CMOV` only	full predication via flags + `csel`
Modes	real/protected/long; 32+64-bit	A32/T32/A64 (modal switch)

Registers

	AMD64	ARM64
GP integer	16 (`RAX`…`R15`)	31 (`X0`…`X30`) + `XZR` + `SP`
Floating / SIMD	16 × XMM (128b), YMM (256b, AVX), ZMM (512b, AVX-512)	32 × V0…V31 (128b NEON; SVE adds VL-agnostic Z0…Z31)
Flags	`RFLAGS` (implicit, written by most ALU ops)	`NZCV` (only written by `…s` form, e.g. `adds`)
PC	not addressable	`PC` not GP, but readable
Zero register	—	`XZR` / `WZR` — reads 0, writes discard

Calling convention (Linux, leaf function)

                                AMD64 (System V)           ARM64 (AAPCS64)
First 6/8 int/ptr args      →   RDI RSI RDX RCX R8 R9      X0 X1 X2 X3 X4 X5 X6 X7
First 8 FP args             →   XMM0…XMM7                  V0…V7
Indirect-result pointer     →   in 1st GP slot             X8
Return value (int)          →   RAX                        X0
Return value (large struct) →   memory via RDI             memory via X8
Caller-saved (scratch)      →   RAX RCX RDX RSI RDI R8…R11 X0…X18  (X16/X17 are IP scratch)
Callee-saved                →   RBX RBP R12…R15            X19…X28, FP=X29, LR=X30
Stack alignment             →   16 B at `call`             16 B always
Red zone below SP           →   128 B                      none

Memory model

	AMD64	ARM64
Ordering	TSO (loads can pass earlier stores to a different address; same-addr stays ordered)	weak / multi-copy atomic — loads & stores can be reordered
Acquire / release	implicit on most accesses	explicit: `LDAR` (acquire), `STLR` (release)
Full barrier	`MFENCE` / `LOCK`-prefixed op	`DMB ISH` (data) / `DSB` / `ISB` (instr fetch)
Default store visibility	strongly ordered	requires explicit fence to publish
Cost of `seq_cst` store	~1 op (`MOV` + implicit; `XCHG` for full fence)	`STLR` (cheap) but loads need `LDAR` too

Atomics

	AMD64	ARM64 v8.0	ARM64 v8.1+ (LSE)
Primitive	`LOCK` prefix on RMW (`CMPXCHG`, `XADD`, `XCHG`)	LL/SC: `LDXR` … `STXR` retry loop	single-instruction `CAS`, `LDADD`, `SWP`
Hot-cache cost	cheap	cheap if uncontended, retry loops can starve under contention	predictable, scales better
Bus lock	yes (split-cache-line locked op)	no	no
128-bit atomic	`CMPXCHG16B`	`LDXP`/`STXP` pair	`CASP` (LSE2)

SIMD

	AMD64	ARM64
Baseline	SSE2 (mandatory in long mode), 128-bit	NEON — 128-bit fixed
Wider	AVX 256-bit, AVX-512 512-bit (server + recent client; downclocks under load)	SVE / SVE2 — vector-length agnostic, 128–2048 bit in 128-bit steps
Mask / pred	AVX-512 `K0`…`K7`	SVE `P0`…`P15`
Matrix	AMX (Sapphire Rapids+) tiles	SME (ARMv9.2) tiles
Half-precision	F16C, BF16 (AVX-512_BF16, AMX-BF16)	FP16 native, BF16 (ARMv8.6)

Pages & virtual memory

	AMD64	ARM64
Base page sizes	4 KB, 2 MB, 1 GB	4 KB / 16 KB / 64 KB (per OS choice)
VA bits	48 (canonical) or 57 (LA57)	48 / 52
Page-table levels	4 (or 5 with LA57)	4 (4K) / 3 (16K) / 3 (64K)
TLB invalidation	`INVLPG`, IPI shootdown	`TLBI` broadcast (architectural)
ASID	PCID (12-bit)	ASID (8 or 16-bit)

Privilege model

Ring / EL	AMD64	ARM64
Highest	SMM (SMI handler)	EL3 (Secure Monitor / TF-A)
	Ring 0 — kernel	EL2 — hypervisor
	Ring 1, 2 (unused)	EL1 — kernel
Lowest	Ring 3 — user	EL0 — user
Hypervisor extension	VMX (Intel) / SVM (AMD)	EL2 native

Cache & coherence

	AMD64 (typical EPYC/Zen5)	ARM64 (typical Neoverse V2)
L1 D / I (per core)	48 / 32 KB	64 / 64 KB
L2 (per core)	1 MB	1–2 MB
L3	32 MB / CCD (shared across CCX)	mesh-attached SLC, sized per design (e.g. 64 MB)
Coherence	MOESI	MOESI-like (CHI protocol)
Cacheline	64 B	64 B

Endianness, alignment, syscalls

	AMD64	ARM64
Default endian	little	little (BE supported but rare)
Unaligned access	always allowed (slow on cross-line)	allowed in normal memory; not in device memory; `SCTLR.A` can trap
Linux syscall entry	`syscall` instruction; nr in `RAX`, args in `RDI RSI RDX R10 R8 R9`	`svc #0`; nr in `X8`, args in `X0…X5`

Power & performance shape

   perf-per-watt (relative, integer SPECint_r 2017 @ socket)
   ────────────────────────────────────────────────────────────
   Apple M3 Max          ████████████████████████  ~1.45×
   Ampere AmpereOne 192c ████████████████████      ~1.25×
   AWS Graviton 4        ███████████████████       ~1.20×
   AMD EPYC Bergamo 128c ██████████████████        ~1.15×   ← AMD64 high-density
   AMD EPYC Genoa 96c    ████████████████          ~1.00×   ← baseline
   Intel Xeon 6 (E)      █████████████████         ~1.05×
   Intel Xeon Granite R  ███████████████           ~0.95×

Vendor and SPEC numbers vary; treat as “shape, not score”.

Method differences (design philosophy)

Axis	AMD64	ARM64
ISA design school	CISC — many specialised ops, RMW on memory, dense encoding	RISC — load/store, orthogonal regs, fixed encoding
Where complexity lives	silicon (decoder, µop cache, microcode patches)	compiler (scheduling, register allocation, predication)
Front-end	length-decode → µop cache → rename	parallel decode → rename (no µop cache needed)
Memory model contract	strong; HW gives most ordering for free	weak; SW must request ordering with `LDAR`/`STLR`/`DMB`
Microcode	yes — can patch / extend ISA post-silicon	no — bugs need new spin
Atomics	implicit via `LOCK` prefix on any RMW	explicit primitives (LL/SC or LSE one-shot)
Vector model	width-explicit (SSE128 / AVX256 / AVX-512)	width-explicit (NEON 128) or VL-agnostic (SVE/SVE2)
Modes	real / protected / long, 16/32/64-bit baggage	A64 only at EL1+; A32/T32 optional and dropping
Backward compat horizon	decades (32-bit Windows still runs)	shorter — Apple dropped 32-bit ARM in 2017, server is A64-only
Implementation diversity	2 main vendors (AMD, Intel) — mostly converged	many licensees (Apple, Qualcomm, Ampere, NVIDIA, Marvell, AWS, ARM Ltd Neoverse) — divergent microarchitectures
Standard system platform	PC firmware: UEFI + ACPI + APIC, very uniform	SystemReady (UEFI+ACPI) on servers; DT + custom on mobile — fragmented

Both camps have converged on the same execution back-end: superscalar, out-of-order, register-renamed, with µops. The differences are now in the front-end (decode), the memory-ordering contract, and the surrounding ecosystem.

Why ARM wins perf-per-watt — and what it costs

ARM’s lead is structural, not magical. Each gain has a paired cost.

ARM gain	Why	What you pay
Cheaper decode (often 8-wide at low power)	Fixed 4-B instructions → trivial parallel length-decode; no µop cache needed → less SRAM and dynamic power	Worse code density. ARM64 binaries are typically ~10–25 % larger than the same C++ compiled for x86-64 → more iCache pressure → bigger L1i and prefetchers needed at the top end
More registers (31 GP, 32 V) → less register-rename pressure, fewer spills	RISC orthogonality	Calling conventions are wider; every callee-save preserved on entry/exit costs more stack traffic
Weak memory model → fewer coherence stalls	HW free to reorder loads/stores across cores aggressively	Concurrent code is harder. Forget a `LDAR` / `DMB ISH` and the bug only shows up on heavily-loaded multi-socket. Real-world: locks, RCU, lock-free queues all need extra fences vs x86
Load/store-only memory pipeline	One memory-access shape to schedule; simpler AGU and store buffer	Every memory RMW takes ≥ 3 ops (`LDR`/`LDXR` + op + `STR`/`STXR`) — visible code-size and µop-count hit
Predication (`CSEL`, `CSINC`, …)	Compiler turns short `if` into branchless data flow → fewer mispredicts, fewer branch units	Increases register pressure (must compute both candidates) and ties up the issue queue
LSE atomics (v8.1+) scale better under contention	One-shot `CAS`/`LDADD` instead of LL/SC retry loops	Requires v8.1+ — older silicon falls back to LL/SC which can livelock
No microcode update path	Saves the µcode patch SRAM + load logic	A late-discovered ISA bug needs a new chip. Spectre-class fixes ship slower
Aggressive big.LITTLE (P-cores + E-cores) since 2011	Heterogeneous scheduling at low power	OS scheduler complexity; thread-affinity bugs; vendor-locked QoS
SoC integration (Apple M, Snapshot, Graviton)	Memory controller, NPU, GPU, fabric on-die or on-package — kills off-chip DRAM hops	Less vendor choice, less repairability, locked memory configurations
ARM-licensee specialisation	Each vendor optimises for its workload (Apple: client; Ampere: cloud; AWS: web)	Fragmentation: code tuned for Neoverse N2 may underperform on Apple firestorm; SVE-VLA can’t be tuned per width as easily as fixed AVX-512
Cleaner ISA — no real mode, no segmentation, no 16-bit	Smaller transistor count for legacy logic	Some legacy SW just doesn’t exist for ARM (older proprietary Windows binaries, JITs not yet ported, drivers)

Where x86-64 still wins on absolute perf (not per-watt)

Workload	Why x86 still leads
Single-thread integer (peak)	Wider OoO windows + µop cache + aggressive prefetch tuned over 25 years
HPC / dense FP / AI inference	AVX-512 (512-bit) + AMX tiles; SVE2 is catching up but ecosystem is younger
JITs that emit RMW patterns	LOCK + RMW = 1 op; ARM needs LL/SC or LSE explicitly
Legacy software (Windows desktop, older ISVs)	binary compatibility is x86’s product; emulation on ARM (Rosetta 2, Prism) costs ~10–30 %
Workloads sensitive to memory-ordering surprises	TSO masks a lot of concurrency bugs that ARM exposes

The honest summary

ARM trades strict ordering, RMW convenience, code density, microcode flexibility, and ecosystem maturity for simpler decode, more registers, freer reordering, and lighter cores — and that trade currently buys roughly 1.2–1.5× perf-per-watt at the socket level in scale-out server workloads, with the gap closer in single-thread peak and wider in throughput-per-rack.

What this means in practice

Need	Pick
Maximum single-thread perf, legacy binaries	AMD64
Maximum perf-per-watt, dense scale-out	ARM64 (Graviton, Ampere, M-series)
Existing OS / driver / hypervisor ecosystem	AMD64
Mobile / battery / SoC integration	ARM64
Lock-free code with implicit ordering	AMD64 (TSO is forgiving)
Predictable atomic scaling under contention	ARM64 v8.1+ (LSE)
AVX-512 / AMX workloads (HPC, ML inference)	AMD64
SVE2 vector-length-agnostic code	ARM64
Embedded / micro-controllers	ARM (Cortex-M / R)

One-line summary

AMD64 = variable-length CISC, fewer registers, strong memory model, mature ecosystem. ARM64 = fixed-length RISC, more registers, weak memory model, better perf-per-watt.

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