Hello World bootloader not working

Question

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

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.