How do i use fs:bx in call instruction at Flat Assembler?

Question

I am creating a Kernel and I need some help with the call instruction. I am using Flat Assembler to build the Kernel.

call fs:bx

Is that possible ?
Because flat assembler gives me an "invalid size of operand" Error.

I know I can do

call 1000h:0h

But that is not what I want. Does someone have the answer to this question?

Answer 1

I'm assuming you want call fs:bx to set cs=fs and ip=bx, as a far call.

Indirect far calls require the seg:offset to be in memory, not registers. See the insn ref entry for call to confirm that the only available indirect far call is the call m16:16 form.

So in 16-bit code, you could

push   fs
push   bx
...    ;  push args
far call   [bp-whatever]       ; assuming you know the difference between bp and sp at this point
add    sp, 4 + arg_size     ; clean up args and the far-pointer

Or reserve space on the stack ahead of time so you can do something like

my_function:
    push       bp
    mov        bp, sp
    sub        sp, 16          ; or whatever size you need for locals.  16 and 8 are just for example.

    ...

    mov        [bp - 8], fs
    mov        [bp - 6], bx    ; separately store both parts of fs:bx into [bp-8]
    far call   [bp - 8]

    ...
    leave
    ret

You can't mov cs, fs or anything like that (even using a scratch GP register). Changing cs would be a jump, so you have to do the whole thing as one far call.

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Of course, you probably only put the segment value in fs in the first place to set up for this instruction, so don't do that in the first place.