They can be defining, in that they say what the writer strictly can or cannot do, or they can leave the door open for different implementations. These documents usually include a test to see if the implementation conforms to the specifications. JVM uses a defining document so that interpreters will all be able to execute legal JVM commands. The Pentium II s ISA level is not defined because Intel fears that its chips will be cloned. There are two levels to the ISA, Kernel mode, which runs the operating system and allows all instructions to be executed, and User mode, which runs applications and does not permit certain sensitive instructions, such a direct cache manipulation.
All computers divide memory up into cells with consecutive addresses.
Usually these cells are 8 bits but can range from 1 to 60 bits. The ability to read words at arbitrary addresses requires extra logic on the chip, which makes it bigger and more expensive. This cannot be remedy however, because its is an issue of compatibility relating back to the 8088. However, some machines have separate spaces for instructions and data. Also programs cannot be overwritten by a write because all writes go to the data space.