Architecture Overview¶

High-Level Components¶

graph TD
    BL["Boot Loader (GRUB to kernel.asm)"]
    K["Kernel - Scheduler, Memory Mgmt, Interrupts"]
    VM["VmImpl / JVM - Classloader, JIT Compiler, VM Magic"]
    PS["Plugin System - PluginDescriptor, PluginManager, Extension Points"]
    CS["Core Services - Naming, Security, Logging"]
    DRV["Drivers - PCI, USB, IDE, Net, Video"]
    FS["Filesystems - Ext2, FAT, ISO9660, NFS, NTFS"]
    NS["Network Stack - IPv4, TCP, UDP, DNS"]
    SH["Shell - Commands, Aliases, Plugins"]
    GUI["GUI / AWT - Video, Input, Thinlet"]

    BL --> K
    K --> VM
    VM --> PS
    PS --> CS
    PS --> DRV
    PS --> FS
    CS --> NS
    CS --> SH
    CS --> GUI

Key Design Principles¶

1. Java-First¶

95%+ of code is Java. Only ~25 assembly files for: - kernel.asm — entry point, GDT/IDT setup, context switch - vm.asm — VM primitives, stack frame layout, safepoints - mm32.asm / mm64.asm — Memory management, page tables - apic.asm — Local APIC, IPI handling

2. Isolate-Based Architecture¶

JNode uses Isolates (similar to processes but lighter-weight): - Each isolate has its own classloader, heap, thread groups - Communication via channels (typed message passing) - Security via capabilities (unforgeable references) - See Isolate Implementation

3. Plugin System¶

• PluginDescriptor (XML) declares exports/imports
• PluginManager resolves dependencies at boot
• Extension points for: filesystems, drivers, shell commands, network layers
• See Plugin System

4. MMTk Integration¶

• Memory Management Toolkit bindings for garbage collection
• Available MMTk plans: org.jnode.vm.memmgr.mmtk.ms (MarkSweep), ...genrc (generational), ...nogc (no collection)
• Default memory manager is org.jnode.vm.memmgr.def; MMTk plans are experimental
• See MMTk Bindings

5. VMMagic Annotations¶

• @Uninterruptible, @Inline, @Offset, @Address
• Enable low-level operations in Java
• Processed by JNasm and BootImageBuilder
• See VMMagic Annotations

Boot Sequence¶

1. GRUB loads kernel.asm at 1 MB
2. kernel.asm: GDT, IDT, paging, enter protected mode
3. VmImpl.: Boot classloader, VM magic init
4. PluginManager: Load default-plugin-list.xml
5. Core services: Naming, Security, DeviceManager
6. Drivers: PCI enumeration, device matching
7. Filesystems: Mount root (ISO9660 → ramdisk)
8. Shell: Start init isolate → command prompt

See Boot Sequence for detailed trace.

Memory Layout (x86, 32-bit)¶

0xFFFFFFFF ┌─────────────────────┐
           │   Kernel Space      │  1 GB (0xC0000000–0xFFFFFFFF)
           │   (Identity mapped) │
0xC0000000 ├─────────────────────┤
           │   User / Isolate    │  3 GB (0x00000000–0xBFFFFFFF)
           │   Heaps, Stacks     │
0x00000000 └─────────────────────┘

• Kernel runs at high addresses (negative pointers)
• Isolates allocated in low 3 GB
• Direct memory via VmUnsafe for DMA, framebuffer

See Paging Implementation and Memory Management.

Threading & Scheduling¶

• Green threads mapped 1:1 to kernel threads
• Priority-based preemptive scheduler
• Yieldpoints at method calls, loop backedges, allocations
• IsolateThread = Java thread + kernel context
• See Core Thread Scheduling

JIT Compilers¶

• L1A/L1B: baseline bytecode compilers; jnode.compiler accepts L1A, L1B, or default (→ L1A)
• L2: optimizing compiler, used only in test harness (testCompilers)
• See JIT Compilers

Further Reading¶