The .X64 directive isn't needed usually, because for output formats WIN64 (see -win64) and ELF64 (see -elf64), .X64 is the default.
The .X64p directive is useful for mixed-model binaries or system software (see example DOS64 ).
When the cpu is set to 64-bit, the SEGMENT directive accepts a new 'size' value: USE64. It tells the assembler that this segment's offset is 64-bit wide and uses 64-bit instructions.
The SYSCALL calling convention is renamed to SYSCALL_ when 64-bit is on, because there exists a SYSCALL instruction mnemonic in this mode.