"syscall" and "sysret" are the new instructions (introduced by both AMD64 and Intel EM64T).

"more strict checking of operands for few instructions" added - "mov ds,eax" is in fact invalid instruction, since such MOV always uses 16-bit size (use "mov ds,ax" instead, there will be no any prefix added).

... MOV DS, EAX ...
When the processor executes the instruction with a 32-bit general-purpose register, it assumes that the 16 least-significant bits of the general-purpose register are the destination or source operand.

However "mov eax,ds" is still allowed, because in this exceptional case the all 32 bits of target are affected (the value is zero-extended).

... MOV EAX, DS ...
If the register is a destination operand, the resulting value in the two high-order bytes of the register is implementation dependent. For the Pentium 4, Intel Xeon, and P6 family processors, the two high-order bytes are filled with zeros; for earlier 32-bit IA-32 processors, the two high order bytes are undefined.