In fasm MOVZX AX,BX compiles error but
In c32asm can get (16B)

Do they do the same thing?

movzx & movsx only makes sense if 1st operand has more bits than last operand like movzx eax,bx

nop,
if you write then you will get

but im saying why would you want movzx ax,bx in the first place? why try to zero extend bits that dont exist maybe thats why fasm wont accept it

Such combination of operands was never listed in Intel manuals, the only ones listed for 32-bit processors were: You should be careful with over-interpreting combinations of operands that are not officially defined for instructions, though encodable. For example LDS/LES instructions have only "r,m" combinations defined, though you could theoretically encode "r,r" with the same instruction code. But such combination was never officially defined and years later Intel used the sequences of codes that would correspond to theoretical "LDS r,r" instruction for a very different purpose: these codes are now part of the VEX prefix space for AVX instructions.

yes a very wise precaution tomasz

So they behave same now but not later?
p.s. I remember debug.com uses les sp,ax(or anything, i forgot) to have an interrupt?