CODEWITHSUNDEEP
assemblyx86bootloader
grammar
grammar

Reputation: 35

How do these particular assembler directives (using db and dw) cause a computer to reboot?

How can these 3 assembler directives cause a computer to reboot?

db 0x0ea 
dw 0x0000 
dw 0xffff

I found this from http://fisnikhasani.com/building-your-own-bootloader/

As far as my understanding goes, these 3 instructions send you to FFFF:0000, the end of memory which causes a reboot by calling BIOS POST. But shouldn't there be a jmp instruction in order to make that jump?

Also, it seems to me that in db 0x0ea, ea is the machine instruction for jmp. If so, how can db 0x0ea write machine instruction? If db and dw have other functions apart from declaring variables, what are they? Can someone please point me to more literature surrounding db and dw and any of its hidden functions.

Upvotes: 2

Views: 889

Answers (3)

Seva Alekseyev
Seva Alekseyev

Reputation: 61351

Think what does "declaring variables" mean in the context of assembly. db and dw, when a value is provided, write that value straight into memory. Code, on the other hand, is stored in the memory as, well, bytes. You can fill memory with bytes by having an assembler process your assembly source, or you can look up instruction encoding and fill memory with bytes by db/dw/dd commands.

That's what they're doing here. This sequence of bytes - ea 00 00 ff ff - encodes the jmp far 0ffffh:0 command.

Upvotes: 4

Breakthrough
Breakthrough

Reputation: 2534

You can hard-code instructions by simply inserting the proper bits using certain assembler directives (db, dw, etc...). In 16-bit mode, the bytes $EA0000FFFF disassembles to:

ljmp $0xffff,$0x0

Which places the value $FFFF in the CS (code segment) register, and $0000 in the IP (instruction pointer). This effectively starts executing code from the computer's reset vector, which should proceed to boot the system as if you just turned it on.

Upvotes: 1

Doug Currie
Doug Currie

Reputation: 41180

db and dw are not assembly instructions, they are pseudo instructions. Their arguments are simply used to initialize the byte or word allocated by the pseudo instruction. When they are used in a code segment, they can be used to create executable code.

Upvotes: 0

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