minor problems

Code:

       format binary
       use16
       org  0
head:  jmp  byte tail
       rb   65536-4    ;gap
tail:  jmp  byte head

In 16-bit mode, these jmps can be coded as 'EB,FC' and 'EB,00'.
Fasm can't assemble them, though x86 processors execute them
just as expected.
Yes, they look something weird, but there is 'mov es,eax' too.

Code:

       format binary
       use16
       org  0
top:   ret
       rb   65536-4    ;gap
       call top

Internally, fasm dose '0-65536' to get the call's operand, I think.
That means fasm accepts -65536 to 65535 as IP-relatives in 16-bit mode.
Displacements are kind of register-relatvies, but fasm rejects them
in the range of -65536 to -32769 like [bx-60000].
Of cource, displacements in the range of -65536 to -65409
and 65408 to 65535 can be optimized by 8-bit values in 16-bit mode.

There are same ploblems in 32-bit mode, though the 'gap' is 4GB.

That's all.