Early Intel® Architecture

Chapter Contents

The power button is pressed, causing the #RESET pin to be asserted, that is, the voltage is driven into the predefined threshold that represents the active state logic level. After a predefined number of clock cycles pass, the #RESET pin is then deasserted, invoking the hardware reset. The hardware reset is responsible for initializing the processor to a known state. This includes setting the registers, including the general purpose, control and status registers, to their documented initial values.

This first “breath” leaves the processor in an execution state significantly different from the one which most developers are familiar. The processor begins execution in 16-bit Real-Address mode. This journey between 16-bit Real-Address mode and 64-bit, or 32-bit, Protected Virtual-Address mode occurs every time the processor is powered on. In doing so, this process mirrors the historical evolution of the Intel® Architecture, from the 16-bit 8086 of 1978 to the 64-bit Intel® Core™ processor family of the twenty-first century. As the computer is booted, more than 40 years of advancements in processor technology unfold in the blink of an eye.

The author imagines that some readers are wondering what benefit could possibly come from the retention of all these relics of computing antiquity in modern processor design. Backwards compatibility, which by definition involves maintaining older designs, directly benefits the reader. As readers, hopefully, continue through this book, they will find references to many architectural features. Education on and usage of these features is an investment of the reader’s time. By strictly maintaining backward compatibility, Intel ensures that the investments made by the reader today continue to pay dividends for the foreseeable future. Consider that original bytecode compiled in the late 1970s will still run unmodified on a modern Intel® processor fabricated in 2014.

Other detractors argue that Intel Architecture is inelegant. Since elegance is in the eyes of the beholder, this claim is hard to refute. The author is not a hardware engineer; however, as a software engineer, the author sees an interesting parallel between hardware and software in this claim. Many software projects are architected to be elegant solutions to a well-defined problem. This elegance erodes over time as new features, which were unimaginable during the design phase, are requested and bug fixes and workarounds are introduced. After enduring these unavoidable harsh realities of the software lifecycle, a software’s worth comes not from elegance, but from whether that software still works, is still reliable, and can still be extended to meet future needs. The author submits that x86 still works, is still reliable, and can still be extended.

Covering every aspect of Intel Architecture would require an entire book, as opposed to a couple of chapters. The goal of the next three chapters is to introduce the fundamentals of the architecture, in order to provide just enough knowledge to enable the reader to understand the later content in this book, as well as begin to reason effectively about power consumption and performance.

Because Intel Architecture has been in a constant state of evolution for the last 40 years, and has been steadily increasing in complexity, the author has divided the introduction to Intel Architecture into three separate chronologically ordered chapters, sampling the general timeline of Intel products. Despite following the timeline, history is only being used as a convenient tool. Not all processors or architectural details will be covered in the sections that follow. Also, unless explicitly stated, not all technologies are discussed in the section corresponding to the processor that introduced them. Instead, each section will utilize the given processor as a vehicle for simplifying and explaining important concepts of the modern x86 architecture. This has the dual benefit of gradually introducing the concepts of the modern x86 architecture in a natural and logical progression, while also providing the context necessary to understand the reasoning behind the technical decisions made through the years.

This chapter examines some of the earliest members of the x86 family. The majority of these processors are 16-bit. The chapter ends with the first 32-bit x86 processor, and the first processor to run Linux, the Intel® 80386.

1.1 Intel® 8086

The 8086, introduced in 1978 and shown in Figure 1.1, was not the first processor developed by Intel, however it was the first processor to implement what would be known as the x86 architecture. In fact, the two digits in the name x86 correspond to the fact that the first x86 processor designations all ended with 86, such as 8086, 80186, 80286, 80386, and so on. This trend was continued by Intel until the Pentium® processor family. Despite being introduced in 1978, the 8086 was continually produced throughout the 1980s and 1990s. Famously, NASA had been using the 8086 in their shuttles until 2011.

In stark contrast to the thousands of pages in the modern Intel® Software Developer Manual (SDM), the 8086 user manual, including a full reference on the system behavior, programming tools, hardware pinout, instructions and opcodes, and so on, fits within about two hundred pages. This simplified nature makes the 8086 an excellent introduction to the fundamental concepts of the x86 architecture. The rest of this book will build further on the concepts introduced here.

In the architecture of the 8086, there are three fundamental design principles (Intel Corporation, 1979). The first principle is the usage of specialized components to isolate and distribute functionality. In other words, rather than one large monolithic system architecture, building an 8086 system provided flexibility in the peripherals selected. This allowed system builders to customize the system to best accommodate the needs of the given situation. The second principle is the inherent support for parallelism within the architecture. At the time of the 8086, this parallelism came in the form of combining multiple processors and coprocessors together, in order to perform multiple tasks in parallel. The third principle was the hierarchical bus organization, designed to support both complex and simple data flows.

To illustrate these principles, consider that along with the 8086, there were two other Intel processors available, the 8088 and 8089. The 8088 was an identical processor to the 8086, except that the external data bus was 8 bits, instead of 16 bits. The 8088 was chosen by IBM® for the heart of the “IBM compatible PC.” The 8089 was a separate coprocessor, with it’s own instruction set and assembler, designed for offloading data transfers, such as Direct Memory Access (DMA) requests....