# SUB-OS: Modular Monolithic Linux-Compatible Operating System
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[](https://github.com/subhobhai943/SUB-OS/releases/tag/v1.0.0)
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[](LICENSE)
**SUB-OS** is a multi-architecture, production-grade modular monolithic operating system kernel engineered in **C, C++17, Assembly, freestanding bare-metal Rust (`no_std`), and the custom SUB Language (`.sb`)**. Built for high performance, memory safety, modularity, and Unix/Linux compatibility, SUB-OS supports **64-bit x86 (`x86_64`)**, **64-bit ARM (`aarch64`)**, and **32-bit ARM (`armv8i` / AArch32)** targets.
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## ๐ Key Architecture & Capabilities
- ๐งถ **Weave Scheduler & Real Context Switching (`kernel/sched.c`, `arch/*/cpu/switch.*`)**:
- **Genuine multitasking, not a state relabel**: `sub_ctx_switch()` is per-architecture assembly (x86_64/aarch64/armv8i) that saves and restores the callee-saved register set plus flags on each thread's own kernel stack, with a first-run trampoline that launches a freshly forged thread frame.
- **Tiered rotation with behavioural weaving**: runnable threads thread through `WEAVE_NLANES` priority lanes; the dispatcher always pulls the lowest non-empty lane round-robin. A thread that yields voluntarily is *woven up* toward low latency, one that burns its whole quantum and is preempted is *woven down* toward throughput โ the lane floats on observed behaviour, not a static nice value.
- **Boot-time self-test**: before hardware interrupts come up, the Weave scheduler drives three cooperative kernel threads to completion on separate stacks (verified under QEMU: correct round-robin interleave, clean return to the boot thread) โ proof the switch is real.
- ๐งต **Kernel Concurrency Core (`kernel/wait.c`, `kernel/futex.c`, `kernel/rcu.c`)**:
- **Wait Queues & Completions**: Sleeper parking with timed waits, one-shot completion barriers, and a static entry pool so a blocking path never re-enters the allocator (`waitinfo`).
- **Futex Hash Table**: 32 buckets with value re-check on the slow path, requeue support, and an `fmutex_t` sleeping mutex built on the uncontended-fastpath protocol (`futexinfo`).
- **Tiny RCU**: Lock-free readers with nesting depth tracking, deferred `call_rcu`/`kfree_rcu` callbacks, grace-period accounting, and `rcu_assign_pointer`/`rcu_dereference` publish barriers (`rcuinfo`).
- ๐งฑ **Core Data Structures (`lib/rbtree.c`, `lib/kfifo.c`, `lib/hashtable.c`)**:
- **Red-Black Tree**: Linux-compatible parent-pointer rbtree with full insert/erase rebalancing, forward and reverse in-order iteration, and a black-height invariant validator.
- **kfifo Ring Buffer**: Power-of-two byte ring with free-running indices, so the wrap path costs one mask instead of a branch per byte.
- **Hash Table**: Separately-chained FNV-1a table with string or `u64` keys and rehash-on-growth at a 3/4 load factor.
- ๐พ **Block Page Cache (`mm/page_cache.c`)**:
- Write-back cache of 4 KB pages keyed by `(device, page index)`, indexed through the kernel hash table.
- CLOCK second-chance eviction with dirty write-back, pinning, per-device flush and invalidate, and live hit-rate telemetry (`pagecache`).
- ๐งช **In-Kernel Test Harness (`kernel/ktest.c`)**:
- KUnit-style suites with assertion macros, per-case pass/fail reporting and timing.
- **6 built-in suites, 21 cases, 735 assertions** covering rbtree, kfifo, hashtable, RCU, futex, wait queues, page cache and libcore โ runnable from the shell (`ktest`) or the desktop KTest Runner.
- ๐จ **Boot Logo & Splash Screen (`init/bootlogo.c`, `scripts/mklogo.py`)**:
- **Two presentations of one artwork**: an ASCII rendition on the text console and serial line from the first moments of boot, then the real bitmap as a graphical splash once the video driver is up at step [5/14].
- **4-bit palette-indexed asset**: 160x160 pixels in 12.5 KB rather than 100 KB, with a transparency mask so the mark composites onto the gradient instead of a flat rectangle.
- **Animated progress bar** tied to the real boot stages, held for a minimum duration (`splashtime=