Any combination of registers of different sizes are allowed when they shouldn't be.

    asm
    {
        movl $eax, [$ax+$ecx];
    }

error: <inline asm>:1:8: invalid 16-bit base register
        movl (%ax,%ecx,1), %eax

All registers in an address must be the same size.

• In 64-bit mode, they must be either all 64 bit or 32 bit.
• In 32-bit mode, they must be either all 32 bit or 16 bit.
• In 16-bit mode, they must be either all 16 bit or 32 bit.