Fossmark

A single-threaded CPU benchmark for ARM64 (AArch64), with the numeric kernels hand-written in assembly and a small portable C driver to run and score them.

What it measures

Nine workloads, each a tight assembly kernel:

#	Test	What it exercises
1	Integer Math	64-bit ALU: madd, umulh/smulh, udiv/sdiv, bit ops
2	Floating Point Math	scalar double: fmadd, fdiv, fsqrt
3	Prime Numbers	sieve of Eratosthenes to 2,000,000 (strided memory + ALU)
4	Extended Instructions	NEON/ASIMD: 128-bit integer, widening, table, float vectors
5	Compression	LZ77 match-finder over a 4 MiB corpus (branchy, cache probe)
6	Encryption	ChaCha20, 20 rounds, NEON, over 1 MiB
7	Physics	512-body direct-summation gravity, double precision
8	Sorting	in-place heapsort of 1M uint32 (branch + cache stress)
9	Single-Threaded	dependent-load pointer chase over 16 MiB (memory latency)

Each test auto-calibrates its iteration count until it runs long enough to be timed reliably, then reports the best of several runs (the run least disturbed by the OS scheduler). Every kernel returns a checksum that the driver verifies across runs, so a miscompiled or non-deterministic kernel is caught rather than silently mis-scored.

Scoring

Each test's raw rate is normalised against a reference machine into a unitless score, and the overall is a weighted geometric mean of those scores:

S_i     = TARGET * (rate_i / REF_i)                       (per-test score)
Overall = TARGET * exp( Σ w_i·ln(rate_i/REF_i) / Σ w_i )  (weighted geo. mean)

The reference rates are the tuning machine's own rates, and TARGET is 20000, so that machine scores ~20000 on every test and overall. Scaling is linear in performance: a machine half as fast scores ~10000, one 10× slower ~2000, and a future machine twice as fast ~40000 — so there is unbounded room both below and above the reference.

The weights reflect each test's influence on everyday, common-workload user experience — integer/general-purpose throughput and memory-latency-bound responsiveness matter most; specialised floating-point and physics matter least. This mirrors the weighted, integer-dominant approach of mainstream suites such as Geekbench 6 (which splits integer/FP roughly 65/35 and combines real-world workloads with a weighted mean).

Test	Weight
Integer Math	20%
Single-Threaded	16%
Compression	14%
Sorting	12%
Extended Instructions	11%
Floating Point	9%
Encryption	8%
Prime Numbers	6%
Physics	4%

Everything above is configurable via #defines at the top of src/main.c: FM_TARGET_SCORE, the nine FM_REF_* reference rates, and the nine FM_WEIGHT_* weights. Weights are relative — the code normalises by their sum, so you can change one without rebalancing the rest. To re-baseline for a different reference machine, set each FM_REF_* to that machine's measured rate.

Note: the pointer-chase (Single-Threaded) test measures raw memory latency and is the noisiest to sample, so the overall typically varies ~1–2% run to run.

"Runs on all operating systems"

The assembly is OS-independent: src/fossmark.S contains no system calls, no libc calls, and no external relocations. Every routine is a pure function of its arguments under the AAPCS64 calling convention, so the same source assembles and runs correctly on Linux (ELF), macOS (Mach-O), Windows (COFF) and the BSDs. It avoids x18 (reserved on Darwin/Windows) and the v8–v15 callee-saved vector bank.

A single binary that runs everywhere is not possible — Linux, macOS and Windows use incompatible executable formats and system-call ABIs. So the portable C driver (src/main.c) supplies the per-OS parts (timing, memory, I/O), and you build one binary per platform. The Linux build is named fossmark-linux-arm64.

Build

make            # builds dist/fossmark-<os>-<arch> for the host
make linux-arm64
make linux-amd64
make macos-arm64
make macos-amd64
make bench      # build and run the benchmark
make test       # build and run the kernel correctness tests

Both macOS targets can be built on either Apple Silicon or Intel Macs; Apple Clang selects the requested architecture with -arch. They produce dist/fossmark-macos-arm64 and dist/fossmark-macos-amd64, respectively.

Or by hand:

cc -O2 src/main.c src/fossmark.S -o dist/fossmark-linux-arm64 -lm

On macOS the same command produces a native binary (name it fossmark-macos-arm64); on Windows use clang from the LLVM/MSVC toolchain.

Testing

src/test_kernels.c is a standalone harness that validates each kernel against an independent reference or invariant — the sieve against a C reference sieve, the NEON ChaCha20 against a scalar reference anchored to the RFC 8439 known-answer vector, the sort against qsort, the N-body step against conservation of momentum, and so on. It exits non-zero if any check fails.

make test