Hello World bootloader not working!

Question

Hello. I've been working through the tutorials on this webpage which progressively creates a bootloader that displays Hello World.

The 2nd tutorial (where we attempt to get an "A" to be output) works perfectly, and yet the 1st tutorial doesn't work for me at all! (The BIOS completely ignores the floppy disk and boots straight into Windows). This is less of an issue, although any explanations would be appreciated.

The real problem is that I can't get the 3rd tutorial to work. Instead on outputting "Hello World", I get an unusual character (and blinking cursor) in the bottom-left corner of the screen. It looks a bit like a smiley face inside a rounded rectangle. Does anyone know how to get Hello World to display as it should?

Answer 1

I think the problem is likely to be related to the origin specified.

[ORG 0x7C00]    ;Origin, tell the assembler that where the code will

Based on the conversation we've been having it appears that the address isn't as predicted in some way. It might simply that DS the data segment register is not what you expect. You might actually be able to get the original listing from the web page to work by adding a push and pop of ds before the call to display the string like this,

 push cs
 pop ds

If not the following code works.

 [ORG 0x000]    ; switched to 0 since we are going to try to correct it ourself

 call nextinstruction
 nextinstruction:    ; get the return address of the call into dx
 pop dx              ; which is essentially the start of the code + 3 (3 bytes for the call instruction)
 MOV SI, HelloString ;Store string pointer to SI
 add si, dx          ; add IP from start of program
 sub si, 3           ; subtract the 3 the call instruction probably took
 push cs
 pop ds              ; make ds the same as cs.  
 CALL PrintString   ;Call print string procedure
 JMP $      ;Infinite loop, hang it here.

This code figures out the offset at runtime of the code being run and also makes sure DS is point to the same segment. Unless otherwise noted instructions involving SI generally also use DS as their code segment to reference the memory.

DS is a segment register and you might want to read something like the Art of Assembly to learn more.

Earlz is also doing the same sort of thing, just making sure the registers are correct so that the memory address is referenced correctly. It's just he knows more about the boot sector specifics than me.

Answer 2

You say "boot straight into windows" so I assume you are using a physical PC. Future note to make: Always use an emulator for development! It's just easier. I like Bochs for OSDeving cause it has nice debugging features. Now, onto the possible solution.

There are a lot of buggy BIOSes that break the informal specifications of the IBM PC for the 0x7C00 load address.

This can give a lot of problems with memory addresses and such whenever you are assembling. So make the beginning look like this:

[BITS 16] ;tell the assembler that its a 16 bit code
[ORG 0x7C00] ;this tells the assembler where the code will be loaded at when it runs on your machine. It uses this to compute the absolute addresses of labels and such.

jmp word 0:flush ;#FAR jump so that you set CS to 0. (the first argument is what segment to jump to. The argument(after the `:`) is what offset to jump to)
;# Without the far jmp, CS could be `0x7C0` or something similar, which will means that where the assembler thinks the code is loaded and where your computer loaded the code is different. Which in turn messes up the absolute addresses of labels.
flush: ;#We go to here, but we do it ABSOLUTE. So with this, we can reset the segment and offset of where our code is loaded.
mov BP,0 ;#use BP as a temp register
mov DS,BP ;#can not assign segment registers a literal number. You have to assign to a register first.
mov ES,BP ;#do the same here too
;#without setting DS and ES, they could have been loaded with the old 0x7C0, which would mess up absolute address calculations for data. 

See, some load at 0x07C0:0000 and most load(and its considered proper to) at 0x0000:7C00. It is the same flat address, but the different segment settings can really screw up absolute memory addresses. So let's remove the "magic" of the assembler and see what it looks like (note I don't guarantee addresses to be completely correct with this. I don't know the size of all opcodes)

jmp word 0:0x7C04 ;# 0x7C04 is the address of the `flush` label 
...

So, we jump to an absolute address.

Now then. What happens when we don't do this?

take this program for example:

mov ax,[mydata]
hlt

mydata: dw 500 ;#just some data

This disassembles to something like

mov ax,[0x7C06] 

Oh, well it uses absolute addressing, so how could that go wrong? Well, what if DS is actually 0x7C0 ? then instead of getting the assembler expected 0:0x7C06 it will get 0x7C0:0x7C06 which are not the same flat address.

I hope this helps you to understand. It's really a complicated topic though and takes a while of low level programming to fully understand.