Re: [Nasm-devel] 64bit mode: Absolute JMP encoding

[Ian S. <is...@gm...>]

Sorry.. what I meant to say was an "absolute indirect" jump.

>From the Intel manual: JMP r/m64 "Jump near, absolute indirect, RIP =
64-Bit offset from register or memory".

Is there a way to not fake it?

The reason is that at the beginning of my "kernel" I have a jump table
for all of the various system calls.

kernel_start:

	jmp start    ; The actual start of the kernel code
	
	; Aligned for simplicity... It does waste a bit of space.
	align 16

	jmp near os_print_string	; 0x00010010 (physical address that apps can call)
	align 8
	jmp near os_print_char
	align 8
	jmp near os_print_char_hex
	align 8
	.... etc, etc.

Thanks,
-Ian


On Mon, Dec 22, 2008 at 11:33 AM, H. Peter Anvin <hp...@zy...> wrote:
> Ian Seyler wrote:
>> Hello,
>>
>> I posted this question in the Expert discussion forum but did not get
>> a working answer. I figured I would try the mailing list too.
>>
>> What is the syntax for a 64-bit absolute jump in NASM?
>
> There isn't one, because there is no such instruction.
>
>> For instance what I want to do is this:
>> [BITS 64]
>> jmp 0x1234567890ABCDEF     ; or FFFF800000000000 as the start of the
>> upper canonical address.
>>
>> According to the Intel manuals this should be encoded with opcode 0xFF
>> (followed by a 64-bit (offset?) address) but NASM keeps using opcode
>> 0xE9 (with a 32-bit offset address).
>
> FF is an *indirect* jump.
>
>> How can I get NASM to encode a 64-bit absolute jump?
>
> You can't.  Again, there is no such instruction.  You can fake it,
> however, with the following code sequence:
>
>        jmp far qword [rel next]
> next:
>        resq 0x123456789abcdef
>
> This is an indirect jump, but puts the offset immediately following the
> instruction, and uses RIP-relative addressing to get a fixed byte sequence.
>
>        -hpa
>
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