Chapter 2: Hardware Platforms Supporting OpenVMS

As described previously, the OpenVMS operating system was originally designed for the VAX computer. Later, the system was ported to the Alpha series of processors, and, as this book is being written, it is being ported to the Intel Itanium line of processors. Let's have a brief comparison of these three architectures, highlighting the most important characteristics of each.

The VAX

As mentioned earlier, the VAX architecture was designed hand-in-hand with the VAX/VMS operating system. This is believed to have been the first time this approach had ever been used for an interactive computer system. Several hardware features were designed specifically to assist with the VMS operating system functions, such as selecting which task to run next ("context switching") and manipulating linked lists of data structures.

The VAX is a 32-bit CISC architecture. The term "32-bit" means that the natural size of a datum used by the CPU is 32 bits (4 bytes) in size. CISC stands for "complex instruction set computing." In short, this means that the VAX has numerous instructions of different in-memory sizes, and they vary in their execution times.

True to its scalability goals, the VAX processor lies at the heart of computers varying enormously in size, speed, and capabilities. The fastest VAX machine is several hundred times quicker than the slowest, while storage capacity and physical size vary by corresponding amounts. The biggest, fastest VAX systems achieve performance comparable to mid-range IBM 3090 mainframe systems.

The VAX is truly a well-engineered architecture, one that is well thought out, well planned, complete, and well executed. Its adherence to its design and reliability are legendary. But even the best-engineered products eventually become obsolete, and the VAX is no longer manufactured. Let's take a look at why this is and why it means little to the future of OpenVMS.

The VAX is a 32-bit machine. In practical terms, this boils down to one fact that is perhaps more important than any other: a 32-bit machine can effectively use a maximum of 4 gigabytes of memory space. ^[1]

Generous when it was designed in the mid-1970s, its 32-bit nature is now becoming a liability for a small, but increasing, number of applications.

Early in the lifespan of the VAX, engineers at Digital examined the rate at which computer memory requirements were growing and predicted that 32-bit architectures would begin to run out of steam in the mid-1990s. This turned out to be a very accurate estimate.

The VAX also has characteristics that do not naturally lend themselves to executing several instructions simultaneously, an increasingly popular technique.

These factors, coupled with a relatively high cost, are the primary reasons the VAX is no longer being carried forward. Digital foresaw these events well in advance and began work on a successor to the VAX, the Alpha AXP architecture, which overcomes many limitations of the VAX.

"CISC" versus "RISC" architectures

"CISC" stands for "complex instruction set computer" and "RISC" for "reduced instruction set computer." Although there is some overlap between the two categories, some generalizations apply. CISC machines have a large number of built-in instructions, whereas RISC machines have a smaller number of simpler instructions. RISC machines usually execute a larger number of instructions in a given time, but each one performs less work, negating some or all of the implied performance benefit. However, the simpler instruction set of a RISC architecture has characteristics that make it easier to have many instructions in progress at one time. With proper programming techniques, a serious performance advantage can be had over a CISC design by virtue of executing several instructions simultaneously.

^[1]There are techniques that allow a computer to use a memory address space greater than its natural data size would imply. In fact, some of these techniques were successfully applied to the VAX's predecessor, the PDP-11.