wrote a basic bootloader

2025-02-25 01:14:52 +00:00
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+# BIOS Interrupt Vector Table (IVT)
+
+## Overview
+The BIOS Interrupt Vector Table (IVT) is a crucial data structure in real mode that maps interrupt numbers to their handler routines. It is located at the very beginning of memory, starting at physical address 0x0000:0x0000.
+
+## Structure
+- The IVT contains 256 entries (0x00 to 0xFF)
+- Each entry is 4 bytes long:
+  - 2 bytes for the offset
+  - 2 bytes for the segment
+- Total size: 256 * 4 = 1024 bytes (0x400)
+
+```
+Memory Layout:
+0x0000:0x0000 - Int 0x00 vector (Divide by zero)
+0x0000:0x0004 - Int 0x01 vector (Single step)
+0x0000:0x0008 - Int 0x02 vector (NMI)
+...
+0x0000:0x0040 - Int 0x10 vector (Video services)
+...
+0x0000:0x03FC - Int 0xFF vector (Last vector)
+```
+
+## Common BIOS Interrupts
+- **Int 0x10**: Video Services
+  - AH=0x00: Set video mode
+  - AH=0x0E: Write character in TTY mode
+  - AH=0x13: Write string
+
+- **Int 0x13**: Disk Services
+  - AH=0x00: Reset disk system
+  - AH=0x02: Read sectors
+  - AH=0x03: Write sectors
+  - AH=0x41: Check extensions present
+  - AH=0x42: Extended read sectors
+  - AH=0x43: Extended write sectors
+
+- **Int 0x16**: Keyboard Services
+  - AH=0x00: Read keystroke
+  - AH=0x01: Check for keystroke
+
+## How It Works
+1. When an interrupt occurs (via hardware or `int` instruction):
+   - CPU pushes FLAGS, CS, and IP onto stack
+   - CPU disables further interrupts
+   - CPU looks up handler address in IVT
+   - CPU jumps to handler address
+
+2. Example of int 0x10 call:
+```nasm
+mov ah, 0x0E    ; TTY output function
+mov al, 'A'     ; Character to print
+int 0x10        ; Call BIOS video interrupt
+```
+
+3. The BIOS handler:
+   - Receives control
+   - Reads parameters from registers
+   - Performs requested operation
+   - Returns via IRET instruction
+
+## Important Notes
+1. BIOS interrupts are only available in real mode
+2. When transitioning to protected mode:
+   - BIOS interrupts become unavailable
+   - Must set up new Interrupt Descriptor Table (IDT)
+   - Must provide own interrupt handlers
+
+3. Some BIOS operations require interrupts to be enabled:
+   - Disk I/O (int 0x13)
+   - Keyboard input (int 0x16)
+   - Some video operations (int 0x10)
+
+4. Memory Map Considerations:
+   - IVT: 0x0000 - 0x03FF
+   - BIOS Data Area: 0x0400 - 0x04FF
+   - Your code should not overwrite these areas