Building an OS from Scratch (Day 2: Reading Files from Disk and Accepting User Input.)

5 min readJun 5, 2025

🔧 What We’re Doing Today

Welcome back! It’s been a while since our first post on February 10, 2025, where we printed our first “Hello World” from the bootloader.

Today, we’re taking a step forward by reading a file from disk, and accepting user input from the keyboard. That means we’re introducing:

Basic file table logic
Reading sectors from the disk
A mini text input prompt
Dynamic messages like Hello your username is:Adarsh,

⚙️ Tools Used

Same as before:

NASM for assembling code
QEMU for emulation
Text editor (Nano or VS Code)
WSL / Linux terminal

📁 Directory Structure

TWZ/
├── src/
│   └── main.asm
├── build/
│   ├── main.bin
│   └── floppy.img

🧠 Concept Overview

Today’s bootloader will:

Load itself to memory
Load a file table from sector 2
Check if the file “HELLO” exists
Load the content of that file into memory
Print the content using BIOS
Ask for the user’s name
Display a greeting with that name

📜 File Table Format

Stored at sector 2.
Each entry is:

filename (8 bytes) | size (1 byte) | start_sector (1 byte)

So to look for a file named “HELLO”, we just compare the first 8 bytes of each entry in memory with 'HELLO '.

🧾 Assembly Code

org 0x7C00
bits 16

start:
    jmp main

a; =============================
; Print string pointed by SI
; =============================
puts:
    push si
    push ax
.print:
    lodsb
    or al, al
    jz .done
    mov ah, 0x0E
    int 0x10
    jmp .print
.done:
    pop ax
    pop si
    ret

; =============================
; Get a key from keyboard (result in AL)
; =============================
getchar:
    mov ah, 0x00
    int 0x16
    ret

; =============================
; Read a line into ES:DI
; Max 32 chars, terminates on Enter
; =============================
read_line:
    xor cx, cx
.next_char:
    call getchar
    cmp al, 0x0D        ; Enter key?
    je .done

    ; Echo the character
    mov ah, 0x0E
    int 0x10
    stosb
    inc cx
    cmp cx, 32
    jne .next_char
.done:
    mov al, 0
    stosb
    ret

; =============================
; Read sector into ES:BX
; DL = sector number
; =============================
read_sector:
    push ax
    push dx
    mov ah, 0x02
    mov al, 1
    mov ch, 0
    mov cl, dl
    inc cl              ; Convert to 1-based
    mov dh, 0
    mov dl, 0x00
    int 0x13
    pop dx
    pop ax
    ret

main:
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00

    ; === Print boot message ===
    mov si, msg_boot
    call puts

    ; === Read file table from sector 2 ===
    mov bx, 0x8000
    mov dl, 2
    call read_sector

    ; === Search for file "HELLO" ===
    mov si, 0x8000
    mov di, filename
    mov cx, 8
    repe cmpsb
    jne notfound

    ; === Load the file ===
    mov al, [si]        ; size
    mov dl, [si+1]      ; start sector
    mov bx, 0x9000
    call read_sector

    ; === Print file content ===
    mov si, 0x9000
    call puts

    ; === Ask for user input ===
    mov si, newline
    call puts
    mov si, msg_prompt
    call puts

    mov di, 0x9100
    mov es, ax
    call read_line

    ; === Final greeting ===
    mov si, newline
    call puts

    mov si, msg_hello
    call puts

    mov ax, es
    mov ds, ax
    mov si, 0x9100
    call puts

    mov si, newline
    call puts

    mov si, msg_tail
    call puts

hang:
    jmp hang

notfound:
    mov si, msg_nf
    call puts
    jmp hang

; =============================
; Data
; =============================
filename: db 'HELLO   '
msg_boot:   db 'Booting OS...', 0x0D, 0x0A, 0
msg_nf:     db 'File Not Found!', 0x0D, 0x0A, 0
msg_prompt: db 'Enter your name: ', 0
msg_hello:  db 'Hello your username is: ', 0
msg_tail:   db 0x0D, 0x0A, 'welcome to the', 0x0D, 0x0A, 0
newline:    db 0x0D, 0x0A, 0

times 510 - ($ - $$) db 0
dw 0xAA55org 0x7C00
bits 16

start:
    jmp main

; Print string at SI
puts:
    push si
    push ax
.loop:
    lodsb
    or al, al
    jz .done
    mov ah, 0x0E
    int 0x10
    jmp .loop
.done:
    pop ax
    pop si
    ret

; Get a character from keyboard
getchar:
    mov ah, 0x00
    int 0x16
    ret

; Read up to 32 characters to ES:DI
read_line:
    xor cx, cx
.next:
    call getchar
    cmp al, 0x0D
    je .done
    mov ah, 0x0E
    int 0x10
    stosb
    inc cx
    cmp cx, 32
    jne .next
.done:
    mov al, 0
    stosb
    ret

; Read 1 sector from DL into ES:BX
read_sector:
    push ax
    push dx
    mov ah, 0x02
    mov al, 1
    mov ch, 0
    mov cl, dl
    inc cl
    mov dh, 0
    mov dl, 0x00
    int 0x13
    pop dx
    pop ax
    ret

main:
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00

    mov si, msg_boot
    call puts

    ; Load file table from sector 2
    mov bx, 0x8000
    mov dl, 2
    call read_sector

    ; Search for 'HELLO'
    mov si, 0x8000
    mov di, filename
    mov cx, 8
    repe cmpsb
    jne notfound

    ; Found! Load file
    mov al, [si]
    mov dl, [si+1]
    mov bx, 0x9000
    call read_sector

    ; Print file
    mov si, 0x9000
    call puts

    ; Ask name
    mov si, newline
    call puts
    mov si, msg_prompt
    call puts

    mov di, 0x9100
    call read_line

    ; Greet user
    mov si, newline
    call puts
    mov si, msg_hello
    call puts
    mov si, 0x9100
    call puts
    mov si, msg_tail
    call puts

hang:
    jmp hang

notfound:
    mov si, msg_nf
    call puts
    jmp hang

; --- Data ---
filename:   db 'HELLO   '
msg_boot:   db 'Booting OS...', 0x0D, 0x0A, 0
msg_nf:     db 'File Not Found!', 0x0D, 0x0A, 0
msg_prompt: db 'Enter your username: ', 0
msg_hello:  db 'Hello ', 0
msg_tail:   db ', welcome to the OS!', 0x0D, 0x0A, 0
newline:    db 0x0D, 0x0A, 0

times 510 - ($ - $$) db 0
dw 0xAA55

🛠️ Compile & Run

nasm -f bin src/main.asm -o build/main.bin
dd if=/dev/zero of=build/floppy.img bs=512 count=2880
dd if=build/main.bin of=build/floppy.img bs=512 count=1 conv=notrunc
printf 'HELLO   \x01\x03' | dd of=build/floppy.img bs=512 seek=2 conv=notrunc
printf 'Welcome to my OS!\r\n\x00' | dd of=build/floppy.img bs=512 seek=3 conv=notrunc
qemu-system-x86_64 -fda build/floppy.img

📘 What You Learned Today

Loading extra sectors (file table & data)
Comparing file names
Reading user input
Printing dynamic messages
Expanding bootloader functionality

🔮 What’s Next?

On Day 3, we’ll:

Add multiple file support
Print a menu of files
Let the user choose which file to read!