Real Mode

Disclaimer

This guide is meant for people with very little experience. If you follow along and have a curious mind, this will teach you extremely important concepts in osdev.

Guide Reference

This guide contain a part of the informations located in the CPU System Management Modes guide.

Real Mode is the first mode the CPU boots into.

It is a simple 16-bit mode with strong practical limitations, but it is also the only mode from which you can completely control every single CPU operation.

Limitations

The limitations are:

Less than 1 MB of RAM available.
There isn't any memory protection or virtual memory.
There aren't security mechanisms to protect against crash or malwares.
The default CPU operand length is only 16 bits.
The memory addressing modes provided are more restrictive than other CPU modes.
Accessing more than 64k requires the use of segment registers that are difficult to work with.

Advantages

The advantages are:

We can use BIOS drivers to control devices and handle interrupts.
We can use an advanced collection of low level API functions provided by the BIOS.
Memory access is really faster.

Code example

[ORG 0x7C00]
[BITS 16]

mov si, msg

print:
    mov ah, 0x0e
    mov al, [si]
    cmp al, 0
    je done
    int 0x10
    inc si
    jmp print

done:
    jmp $

msg:
    db "Hello World!", 0

times 510-($-$$) db 0
db 0x55, 0xAA

Explanation

This example prints "Hello World!" in Real Mode. Let's see every row what it do.

[ORG 0x7C00]    ; Offset at 0x7C00, where the program starts
[BITS 16]       ; Explain that the code is in Real Mode (isn't required to made it works)

mov si, msg     ; Copy in the si register the address of the phrase

print:              ; Function print
    mov ah, 0x0e    ; Say to the CPU that you want to print a value
    mov al, [si]    ; Load in al the value saved in the address stored in si
    cmp al, 0       ; Compare al's value to 0, if true it means that the phrase is ended
    je done         ; je - Jump if Equal, jump to done if al == 0
    int 0x10        ; If here, the phrase isn't ended. Call the interrupt and print the char
    inc si          ; Add 1 to the address saved in si (pass to next char)
    jmp print       ; Jump to print

done:       ; Function done
    jmp $   ; Jump forever here -> while(true) {}

msg:                        ; Save the first address of the phrase in msg
    db "Hello World!", 0    ; Save the text "Hello World!" with a 0 at the end.

times 510-($-$$) db 0   ; Add 0 until the whole file is 510 byte of size
db 0x55, 0xAA           ; Add the last 2 bytes of end program

Limitations

Advantages

Code example

Explanation

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