Re: [Nasm-users] Complete newbie question

[H. P. A. <hp...@zy...>]

Frank Kotler wrote:
> ce...@ar... wrote:
>> Hello again!
>>
>> I seem to have run into another problem with the nasm-human interface.
>> I've got code loaded into es:bx, courtesy of int 13.
>>
>> My understanding is that I ought to be able to far jump to es:bx, however
>> jmp es:bx gets me the error "invalid combination of opcode and operands".
>>
>> For fun, I replaced es:bx with 0xFFFF:0xFFFF which worked OK (although,
>> FFFFh:FFFFh doesn't -- nasm parses that first FFFFh as a label).  I then
>> tried 0xFFFF:bx and es:0xFFFF, both of which gave me the same invalid
>> combination of opcodes and operands.
>>
>> Here's a listing of things which I've tried while trying to figure out 
>> the
>> problem.
>> jmp cs:0xFFFF
>> jmp ds:0xFFFF
>> jmp es:bx
>> jmp bx:dx
>> jmp ebx:edx
>> jmp ds:dx
>>
>> These two complain about operation size not being specified.  Including
>> dword after jmp brings back the invalid combination error.
>> jmp 0xFFFF:bx  jmp 0xFFFF:ebx
>>
>> Incidentally, [cs:0xFFFF] and [ds:0xFFFF] will work, although that's 
>> nothing even approximating what I want. 
> 
> Hi Phil,
> 
> You've pretty much run the gamut! The only kind of "jmp xxxx:xxxx" is 
> "imm:imm". As you know "jmp [xxx]" is a different thing. If you've got a 
> target address in es:bx, your choices are to put 'em in a memory 
> location "mov [target], bx"/"mov [target + 2], es"/"jmp far [target]", 
> or "push es"/"push bx"/"retf". I don't *know* of any other way to do 
> it.... Well, typically in a bootsector, we know what went into es and bx 
> before the int 13h, so we can do "jmp imm:imm"...
> 

A common way to jump to say ES:BX is:

	push es
	push bx
	retf

	-hpa