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docs/Stevia Notes.md
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# **Bootloader Documentation**
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## **1. Calling Conventions**
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### __cdecl16near Calling Convention
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- **Purpose**: For calling near (within the same segment) functions in 16-bit code.
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- **Stack Management**: Caller cleans up the stack after the function call.
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- **Parameter Passing**: Parameters are pushed onto the stack from right to left.
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- **Return Address**: A near return address (16-bit) is pushed onto the stack.
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- **Return Value**: Placed in the AX register.
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Example:
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```assembly
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; Caller
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push param2
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push param1
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call near_func
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add sp, 4 ; Clean up the stack (2 parameters * 2 bytes each)
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; Callee (near_func)
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near_func:
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push bp
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mov bp, sp
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; Function body
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mov sp, bp
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pop bp
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ret
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```
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### __cdecl16far Calling Convention
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- **Purpose**: For calling far (across different segments) functions in 16-bit code.
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- **Stack Management**: Caller cleans up the stack after the function call.
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- **Parameter Passing**: Parameters are pushed onto the stack from right to left.
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- **Return Address**: A far return address (32-bit, consisting of a segment and an offset) is pushed onto the stack.
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- **Return Value**: Placed in the AX register.
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Example:
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```assembly
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; Caller
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push param2
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push param1
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call far_func
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add sp, 4 ; Clean up the stack (2 parameters * 2 bytes each)
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; Callee (far_func)
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far_func:
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push bp
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mov bp, sp
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; Function body
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mov sp, bp
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pop bp
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retf ; Far return
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```
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### **Key Differences**
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- **Return Address**: `__cdecl16near` uses a 16-bit return address; `__cdecl16far` uses a 32-bit return address (segment:offset).
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- **Function Scope**: `__cdecl16near` is for functions within the same segment; `__cdecl16far` is for functions that may be in different segments.
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- **Return Instruction**: `__cdecl16near` uses `ret`; `__cdecl16far` uses `retf` (far return).
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### **Register Usage**
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#### Caller-Saved (Volatile) Registers
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- **AX**: Accumulator, often used for return values.
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- **CX**: Counter register.
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- **DX**: Data register, used for I/O operations.
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- **SI/DI**: String operation indexes.
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#### Callee-Saved (Non-Volatile) Registers
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- **BP**: Base pointer, used for stack frame management.
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- **SP**: Stack pointer.
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- **BX**: Base register.
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Example:
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```assembly
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; Caller
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push param2
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push param1
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call near_func
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add sp, 4 ; Clean up the stack (2 parameters * 2 bytes each)
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; Callee (near_func)
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near_func:
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push bp
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mov bp, sp
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; Save callee-saved registers if used
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push bx
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push si
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push di
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; Function body
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; Use AX, CX, DX freely
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; Restore callee-saved registers
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pop di
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pop si
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pop bx
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mov sp, bp
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pop bp
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ret
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```
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## **2. E820 Memory Map Usage**
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### **Address Range Descriptor Structure**
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| Offset | Name | Description |
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|--------|---------------|---------------------------------------|
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| 0 | BaseAddrLow | Low 32 bits of base address |
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| 4 | BaseAddrHigh | High 32 bits of base address |
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| 8 | LengthLow | Low 32 bits of length in bytes |
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| 12 | LengthHigh | High 32 bits of length in bytes |
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| 16 | Type | Address type of this range |
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### **E820 Function Call**
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#### Input
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- **EAX**: Function code `E820h`.
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- **EBX**: Continuation value for physical memory retrieval (0 for the first call).
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- **ES:DI**: Buffer pointer to an Address Range Descriptor structure.
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- **ECX**: Buffer size (minimum size 20 bytes).
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- **EDX**: Signature ('SMAP').
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#### Output
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- **CF**: Carry flag (indicates success/failure).
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- **EAX**: Signature ('SMAP').
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- **ECX**: Buffer size (number of bytes returned).
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- **EBX**: Continuation value for subsequent E820 calls.
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### **Address Type Values**
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| Value | Pneumonic | Description |
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|-------|----------------------|-----------------------------------------------------------|
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| 1 | AddressRangeMemory | Available RAM usable by the operating system. |
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| 2 | AddressRangeReserved | Reserved by the system, unusable by the operating system. |
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## **3. Example Calculations**
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### **Partition Offset**
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- Partition 1 offset = LBA 0x800 = 0x100000
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- `bsSectorSize = 512`
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### **First FAT Sector**
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- `first_fat_sector = bsResSector = 32 => (32*512) = 0x4000`
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- `first_fat_sector = 0x100000 + 0x4000 = 0x104000`
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### **Total FAT Sectors**
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- `total_fat_sectors = fat_sectors * number_of_FATs = 2001 * 2 = 4002`
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- `total_fat_size = total_fat_sectors * bsSectorSize = 0x1F4400`
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### **First Data Sector**
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- `first_data_sector = FatStartSector + FatAreaSize = 0x104000 + 0x1F4400 = 0x2F8400`
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### **FAT Table Look Up**
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```c
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if the cluster we got from the table entry was cluster 354
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fat_sector = 354 / 128 = 2
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fat_entry = 354 mod 128 = 98
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so we load the 3rd (indexed from 0) fat table sector and read the 98th entry
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```
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Example:
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```c
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fat_table_offset = (first_fat_sector + 2) * 512
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fat_table = *(fat_table_offset)
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disk_read(fat_table[98])
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```
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## **4. Global Descriptor Table (GDT)**
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### **Segment Attributes**
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- **Pr**: Present bit (must be 1 for valid selectors).
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- **Privl**: Privilege level (0 = kernel, 3 = user).
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- **S**: Descriptor type (set for code/data segments, cleared for system segments).
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- **Ex**: Executable bit (set if segment contains code).
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- **DC**: Direction/Conforming bit (for data or code segments).
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- **RW**: Readable/Writable (depends on segment type).
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### **Granularity (Gr)**
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- **Gr**: Granularity bit (0 = byte granularity, 1 = 4 KiB blocks).
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- **Sz**: Size bit (0 = 16-bit mode, 1 = 32-bit mode).
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### **GDT Entry Construction**
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Each GDT entry is 8 bytes:
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- First DWORD: Limit (0:15), Base (0:15)
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- Second DWORD: Base (16:31), Attributes (8:12)
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## **5. Memory Layout Example**
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### **Low Memory (First MiB)**
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| Start | End | Size | Type | Description |
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|-------------|-------------|-----------------|-----------------------|----------------------------------|
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| 0x00000000 | 0x000003FF | 1 KiB | RAM (partially unusable) | Real Mode IVT (Interrupt Vector Table) |
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| 0x00000400 | 0x000004FF | 256 bytes | RAM (partially unusable) | BDA (BIOS data area) |
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| 0x00000500 | 0x00007BFF | almost 30 KiB | RAM - free for use | Conventional memory |
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| 0x00007C00 | 0x00007DFF | 512 bytes | RAM (partially unusable) | OS BootSector |
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| 0x00007E00 | 0x0007FFFF | 480.5 KiB | RAM - free for use | Conventional memory |
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| 0x00080000 | 0x0009FFFF | 128 KiB | RAM (partially unusable) | EBDA (Extended BIOS Data Area) |
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| 0x000A0000 | 0x000FFFFF | 384 KiB | various (unusable) | Video memory, ROM Area |
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### **Extended Memory (Above 1 MiB)**
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| Start | End | Size | Description |
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|-------------|-------------|-----------------|------------------------|
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| 0x00100000 | 0x00EFFFFF | 14 MiB | RAM - free for use |
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| 0x00F00000 | 0x00FFFFFF | 1 MiB | Possible memory-mapped hardware (ISA) |
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| 0x01000000 | ? | ? | More extended memory |
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| 0xC0000000 | 0xFFFFFFFF | 1 GiB | Memory mapped PCI devices, BIOS, etc. |
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| 0x0000000100000000 | ? | ? | RAM - usable in PAE/64-bit mode |
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