How to jump to specific location.

[kendfrey]

At the end of my boot sector code, I want to jump to my kernel, located at 0x1000:0x0000. The code is currently executing at about 0x0000:0x7d00. I am stumped. I currently have jmp dword ptr es:0000h (es contains 0x1000). Bochs keeps giving me something about prefetch EIP xxxxxxxx > CS.limit xxxxxxxx. What does the prefetch thing mean, and how do I fix it? I'm using MASM BTW.

[DavidCooper]

I currently have jmp dword ptr es:0000h (es contains 0x1000).

Why would you try to use ES for code? Try a more conventional jump.

[kendfrey]

Like what?

[Coty]

like "jmp 0x1000:0x0000" ??? Nasm won't even let me compile jmp es:0x0000...

[kendfrey]

MASM won't let me compile jmp 1000h:0000h. "error A2096: segment, group, or segment register expected"

[Combuster]

JMP es:0x0000 does not exist as an instruction: you should not load a code segment with a data segment. The closest variant is jmp [es:0x0000] which is a simple indirect jump and does not do what you want. NASM is right in that regard in refusing assembly.

Why people keep trying to use jmp far ds, address as an instruction is beyond me. It's not anywhere in the manuals after all (mov cs, xxx is another one).

[kendfrey]

As far as I know, mov cs, xx is not allowed by MASM either. There are workarounds (my bootloader makes use of one ). Anyway, I found something about segments in MASM, and I did this:

Code: Select all

kernel segment at 1000h
org 0
start label far
kernel ends

and then jmp kernel:start worked. I don't know why MASM is so persnickety.

[rdos]

When the assembler refuses to cooperate, read up on the opcodes and use db, dw, dd instead

[Chandra]

As far as I know, mov cs, xx is not allowed by MASM either.

Becuase that is not a valid way to set CS. Read the Manuals.

There are workarounds (my bootloader makes use of one ). Anyway, I found something about segments in MASM, and I did this:

Code: Select all

kernel segment at 1000h
org 0
start label far
kernel ends

and then jmp kernel:start worked. I don't know why MASM is so persnickety.

A simple workaround can be:

Code: Select all

push word 0x1000
push word 0x0000
retf

That's the Nasm Syntax though.

[DavidCooper]

Coty made a suggestion:-

like "jmp 0x1000:0x0000" ???

Did you actually try that? Your reply was:-

MASM won't let me compile jmp 1000h:0000h. "error A2096: segment, group, or segment register expected"

I don't know the right syntax for a far jump in different assemblers, so you need to look up the MASM manual to see how it should be done. [In machine code terms you should end up with a far jump instruction (the single byte 234, which I think is EA in hex), followed by two bytes of address (if you're in real mode) and two more bytes to be loaded into CS. Unfortunately, that doesn't help you work out how an assembler will require you to formulate the instruction.]

[Gigasoft]

MASM does not have a way to specify a far jump with a numeric segment part. You have to define the instruction using db and dw.

[kendfrey]

Not an easy way, but see my previous post. (Yes it generates the same opcode as jmp xxxx:xxxx)