wrote a basic bootloader

2025-02-25 01:14:52 +00:00
commit 012b0454ed
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+# Boot Process and System Layout
+
+## Disk Layout
+```
+FAT32 Partition
+┌─────────────────────────┐
+│ /                       │
+├─────────────────────────┤
+│ ├── EFI/               │
+│ │   └── BOOT/          │
+│ │       └── BOOTX64.EFI│ <- UEFI bootloader
+├─────────────────────────┤
+│ ├── boot/              │
+│ │   └── bootloader.bin │ <- BIOS bootloader
+├─────────────────────────┤
+│ └── kernel             │ <- Main kernel binary
+└─────────────────────────┘
+```
+
+## Kernel Binary Format
+```
+Kernel Header Structure
+┌─────────────────────────┐
+│ Magic Number (4 bytes)  │ 0x00
+├─────────────────────────┤
+│ Entry Point (8 bytes)   │ 0x04
+├─────────────────────────┤
+│ Stack Pointer (8 bytes) │ 0x0C
+├─────────────────────────┤
+│ Flags (4 bytes)         │ 0x14
+├─────────────────────────┤
+│ Text Offset (4 bytes)   │ 0x18
+├─────────────────────────┤
+│ Text Size (4 bytes)     │ 0x1C
+├─────────────────────────┤
+│ Data Offset (4 bytes)   │ 0x20
+├─────────────────────────┤
+│ Data Size (4 bytes)     │ 0x24
+└─────────────────────────┘
+```
+
+## Memory Layout (After Boot)
+```
+Virtual Memory Layout
+┌─────────────────────┐ 0xFFFFFFFF_FFFFFFFF
+│    Higher Half      │
+├─────────────────────┤ 0xFFFFFFFF_FF600000
+│     Recursive       │
+│    Page Mapping     │
+├─────────────────────┤ 0xFFFFFFFF_C0000000
+│   Kernel Stacks     │
+├─────────────────────┤ 
+│   Kernel Heap       │
+├─────────────────────┤ 0xFFFFFFFF_80000000
+│   Kernel Code       │
+├─────────────────────┤ 0x00007FFF_FFFFFFFF
+│       Guard         │
+├─────────────────────┤ 0x00007FFF_00000000
+│    User Space       │
+├─────────────────────┤ 0x0000000000400000
+│       Guard         │
+└─────────────────────┘ 0x0000000000000000
+
+Physical Memory Layout
+┌─────────────────────┐
+│    Available RAM    │
+├─────────────────────┤ 
+│   Kernel Binary     │ <- Loaded at 1MB (0x100000)
+├─────────────────────┤
+│   Reserved/BIOS     │
+└─────────────────────┘ 0x00000000
+```
+
+## Boot Process
+
+### BIOS Boot Flow
+1. BIOS loads MBR (stage1.bin)
+2. Stage 1 bootloader:
+   - Loads Stage 2 bootloader (stage2.bin) starting at sector 2048
+3. Stage 2 bootloader:
+   - Switches to protected mode
+   - Sets up initial page tables
+   - Finds and loads kernel from FAT32
+   - Enables long mode
+   - Jumps to kernel entry point
+
+### UEFI Boot Flow
+1. UEFI firmware loads BOOTX64.EFI
+2. UEFI bootloader:
+   - Gets memory map
+   - Finds and loads kernel
+   - Exits boot services
+   - Sets up page tables
+   - Enables long mode
+   - Jumps to kernel entry point
+
+### Kernel Entry Point
+```rust
+extern "C" {
+   fn kmain(magic: u64, boot_info: *const BootInfo) -> !;
+}
+```   
+
+## Common Boot Environment
+Both bootloaders must provide:
+
+### CPU State
+```
+- Long mode enabled
+- Paging enabled
+- Interrupts disabled
+- GDT set up for long mode
+- IDT not required (kernel will set up)
+```
+
+### Register State
+```
+RAX = Boot magic value (e.g., 0xCAFEBABE)
+RBX = Pointer to boot info structure
+RCX = 0
+RDX = 0
+RSI = 0
+RDI = 0
+RBP = 0
+RSP = Valid stack pointer (as specified in kernel header)
+```
+
+### Boot Info Structure
+```c
+struct BootInfo {
+    uint64_t magic;          // Boot info magic number
+    uint64_t mem_map_addr;   // Physical address of memory map
+    uint64_t mem_map_size;   // Size of memory map
+    uint64_t fb_addr;        // Framebuffer address (if available)
+    uint32_t fb_width;       // Framebuffer width
+    uint32_t fb_height;      // Framebuffer height
+    uint32_t fb_pitch;       // Framebuffer pitch
+    uint8_t  fb_bpp;         // Bits per pixel
+    uint8_t  boot_type;      // 0 = BIOS, 1 = UEFI
+    uint8_t  reserved[6];    // Padding to 64-bit align
+};
+```
+
+## Required Kernel Features
+1. Position-independent code (PIC)
+2. No assumptions about physical memory layout beyond boot info
+3. Own interrupt handling
+4. Own memory management after boot
+
+## Development Notes
+1. Kernel must be compiled with:
+   - No red zone
+   - No MMX/SSE initially
+   - Position-independent code
+   - No standard library dependencies
+
+2. Testing can be done with:
+   ```bash
+   # BIOS boot
+   qemu-system-x86_64 disk.img
+
+   # UEFI boot
+   qemu-system-x86_64 -bios /usr/share/OVMF/OVMF_CODE.fd disk.img
+   ```