CODEWITHSUNDEEP
assemblyusbbootloader
Rikh
Rikh

Reputation: 4222

Simple USB Bootloader

I'm trying to make a simple bootloader that will run from a USB. After several problems, I tried using the following asm code block (that i got from MikeOS): 

BITS 16

start:
mov ax, 07C0h       ; Set up 4K stack space after this bootloader
add ax, 288     ; (4096 + 512) / 16 bytes per paragraph
mov ss, ax
mov sp, 4096

mov ax, 07C0h       ; Set data segment to where we're loaded
mov ds, ax

mov si, msg
call move

jmp $   

msg db 'Whatever!', 0

move:
mov ah, 0Eh

.print:
lodsb
cmp al,0
je .Done
int 10h
jmp .print

.Done:
ret

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

This resulted in L being printed infinite number of times when i tried to boot it from the USB :( Can anyone let me know what i'm doing wrong.

I'm copying it into Sector 0, using dd.

Edit: This somehow doesn't work on my PC but works on my sisters laptop. Anyone have any idea why?

Upvotes: 1

Views: 589

Answers (1)

Brendan
Brendan

Reputation: 37232

There's no bug in the code you've posted that would explain the symptoms.

Note: There are 2 bugs. It assumes the direction flag is clear and doesn't do a cld before the lodsb, but that might cause it to display 'W' followed by strange characters (and not an 'L' repeated). It also doesn't set a value in bh for the page number used by "int 0x10, ah=0x0E", but that might cause nothing to be displayed (and not an 'L' repeated).

Given the absence of a cause in the code itself; the most likely problem is either a problem with how you copy the resulting binary onto the USB flash (more likely), or a problem with how you assemble the code (less likely).

Upvotes: 1

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