Lecture_15_The Intel x86 Architecture

22. The Intel x86 Architecture
22.1. Introduction
In 1978 Intel had a 16-bit successor to the 8080, the 8086/8088. The 8088 was internally identical to the 8086 with the exception of an 8-bit external data bus, rather than 16-bit. Certainly this would limit the performance of the device. The growth of the PC industry essentially tracked Intel’s continued development and ongoing refinement of the x86 architecture.
Intel introduced microprocessors way back in 1969. Their first 4-bit microprocessor was the 4004. This was followed by the 8080 and 8085. The work on these early microprocessors led to the development of the Intel architecture (IA). The first processor in the IA family was the 8086 processor. It has a 20-bit address bus and a 16-bit data bus. Thus, it could address up to 1 MB of memory.
The 8088 is a less expensive version of the 8086 processor. The cost reduction is obtained by using an 8-bit data bus. Except for this difference, the 8088 is identical to the 8086 processor. Intel introduced segmentation with these processors. These processors can address up to four segments of 64 KB each. This IA segmentation is referred to as the real mode segmentation.
The 80186 is a faster version of the 8086. It also has a 20-bit address bus and 16-bit data bus, but has an improved instruction set. The 80186 was never widely used in computer systems. The real successor to the 8086 is the 80286, which was introduced in 1982. It has a 24-bit address bus, which implies 16 MB of memory address space. The data bus is still 16 bits wide, but the 80286 has some memory protection capabilities.
Intel introduced its first 32-bit CPU—the 80386—in 1985. It has a 32-bit data bus and 32-bit address bus. The memory address space has grown substantially (from 16 MB address space to 4 GB). This processor introduced paging into the IA architecture. It also allowed definition segments as large as 4 GB. Like the 80286, it can run all the programs written for 8086 and 8088 processors.
The Intel 80486 was introduced in 1989. This is an improved version of the 80386. While maintaining the same address and data buses, it combined the coprocessor functions for performing floating-point arithmetic. The 80486 processor has added more parallel execution capability to instruction decode and execution units to achieve a scalar execution rate of one instruction per clock. It has an 8 KB onchip L1 cache. Furthermore, support for the L2 cache and multiprocessing has been added.
The latest in the family is the Pentium series. It is not named 80586 because Intel found belatedly that numbers couldn’t be trademarked! The first Pentium was introduced in 1993. The Pentium is similar to the 80486 but uses a 64-bit wide data bus. Internally, it has 128- and 256-bit wide datapaths to speed internal data transfers. However, the Pentium instruction set supports 32-bit operands like the 80486. The Pentium has added a second execution pipeline to achieve superscalar performance by having the capability to execute two instructions per clock. It has also doubled the onchip L1 cache, with 8 KB for data and another 8 KB for the instructions.
The Pentium Pro processor has a three-way superscalar architecture. That is, it can execute three instructions per CPU clock. The address bus has been expanded to 36 bits, which gives it an address space of 64 GB. In addition to the L1 caches provided by the Pentium, the Pentium Pro has a 256 KB L2 cache in the same package as the CPU.
The Pentium II processor has added multimedia (MMX) instructions to the Pentium Pro architecture. It has expanded the L1 data and instruction caches to 16 KB each. It has also added more comprehensive power management features including Sleep and Deep Sleep modes to conserve power during idle times. Table 7.1 summarizes the key characteristics of the IA family of processors.
Intel’s 64-bit Itanium processor is targeted for server applications. For these applications, the Pentium’s memory address space is not adequate. The Itanium uses a 64-bit address bus to provide substantially large address space. Its data bus is 128 bits wide. In a major departure, Intel has moved from the CISC designs of Pentium processors to RISC orientation for their Itanium processors.