Re: [Nasm-users] Complete newbie question

[Frank K. <fbk...@co...>]

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"...

Best,
Frank