Hi there, i'm looking at some documents regarding the video memory.
I'm experimenting with a small bootloader/bootsector.

The document author uses nasm, and fasm's syntax is different in that case i think.

In nasm you would write a character like this:


I tried to rewrite it for fasm and ended up with this:


Am i wrong with my rewrite here? The reason i'm asking is that i don't see the character, and i need to know if it's my character output that doesn't work, or if the bootloader stops earlier

If that code is correct, i have a problem elsewhere. Here is the full code if anyone would like to help: http://pastebin.com/m662156c2
It builds without problems

I don't think you SS and DS are correct: You will need to load them with a different descriptor. 0x10 maybe?

BTW: your syntax above is correct.

Changing the value to 10h or similiar does not fix the problem. As the syntax is correct there must be a problem elsewhere. I don't really know where though..

This is document/tutorial i'm reading: http://www.osdever.net/tutorials/view/the-world-of-protected-mode

VelcroMan: There is lots of PM setup/entry code in this forum. Have you tried searching here? Lots of people have posted working code similar to yours.

I fixed the error(s) in the code so it works fine now.
I downloaded the sourcecode for the article and looked through it, and there were a few changes not shown in the article, so i hade a few errors.

If anyone are interested, a working fasm version of http://www.osdever.net/tutorials/view/the-world-of-protected-mode is avliable here: http://pastebin.com/f2c5ee886

And thank you revolution for your help

bootloaders run in real mode, to u have to use segment registers to pass > 65k address.

try:
mov ax,0xb800
mov ds,ax
mov [0],0x1B70

Don't you see the code enabling PM ????



This is good, just the "ds:" prefix is unnecessary as it is the default anyway



This is good but unnecessary as long as you don't use the stack.



A few bits have different meaning



WRONG.

http://www.sudleyplace.com/dpmione/expanddown.html



The MA$M syntax is horrible, again

actually, fasm should add DS override prefix to instruction when writing ds:

ok i didnt even saw this code, my mistake.

db gdt_end - gdt should be - 1, its not size but sizie -1, for unknown reasons.

#
cli ;Disable interrups

you dont disable int 2, it can appear as any other one.
i belive you should do something witn port IO wich APIC to disable routing interrupts to current cpu, interrupt flag is useless, i think so.


you should also check if cpu support protected mode, but only when you write something serious, here you dont have to.


#
mov ax, 08h ;Store the code segment adress in ax
#
mov ss, ax ;Fill the stack segment register with the adress
#
mov ds, ax ;Fill the data segment register with the adress
you store 0x08 segment in ss and ds, it goes only to cs.

#
mov esp, 90000h ;500h to 9FFFFh are free memory. Our bootloader originates at 7C00h, so if we put our stack at 90000h we have enough memory (FFFFh) and out code have a lot of room

why you assume this address holds ram, not memory mapped something?

#
hang: ;Hang the cpu
#
jmp hang


try jmp $, shorter. or cli + hlt, shouldnt be interrupted, if nmi fires just use jmp $.