ISA 1. Overview Most of Intel CPU use x86 instruction set architecture. The x86 architecture is a variable instruction length, which primarily "CISC" design. There are some characteristics of x86 architecture. Firstly, byte-addressing is enabled. Secondly, words are stored in memory with little-endian byte order. Thirdly, memory access to unaligned addresses is allowed for all valid word sizes. In 1978, the x86 instruction set architectures was first introduced in 1978 with the Intel 8086 CPU. Intel named processors in digital formats such as 80x86, including Intel 8086, 80186, 80286, 80386, Intel 8086 CPU and 80486. The instruction set architecture is known as "x86" because it ends in "86." Since then, x86 became the standard platform for personal computers and the most successful CPU architecture in history. Take 2018 for example, almost all of the laptops and personal computers are based on the x86 architecture. On the contrary, the portable electronic devices such as mobile phone and tablets are based on ARM architecture, such as Apple A serious microprocessors, Qualcomm Snapdragon, Huawei Kylin and so on (Brandon, 2015). Actually, not only Intel use x86 instruction set architecture, AMD, Cyrix (now acquired by VIA), NEC, IBM, IDT, and Transmeta also use x86 instruction set. 2. History There are many extensions and additions have been added to the x86 instruction set in the history. The x86 architecture remained 16-bit until the 32-bit processor Intel 80386 was developed in 1985. A series of processors then 64-bit multicore processor represented minor improvements to the 32-bit architecture, introducing several extensions, until 2003, when AMD developed a 64-bit extension to the architecture called AMD64. Intel later introduced a compatible processor called Intel 64. Commonly known as x86-64 or x64, they ushered in the 64-bit era of x86. The following table lists the Intel processor models that have evolved in chronological order for the x86 instruction set. Each row is a significant improvement in the processor or a commercially successful design of the processor's microarchitecture. 3.Operating modes (a) Real mode Real mode refers to real address mode which is the early mode of x86 architecture. Real mode appears in the early 8088 CPU period. Due to limited CPU performance, there were only 20 address lines (so the address space was only 1MB) and 8 16-bit general purpose register. All of the x86 architecture have real mode. In real mode, access to memory is partitioned. To get the last 20-bit memory address, move the segment address 4 bits (making a 20-bit number with four least-significant zeros) to the left and plus the offset address. Therefore, the total addressable space in real mode is 2 bytes, or 1MB. There are two addressing modes in real mode: near and far. In far mode, both extents and offsets need to be specified; In the near mode, only the offset mode needs to be specified, and the memory extent is obtained by the appropriate extent register. (b) Protected mode With the development of the CPU, the number of address lines of the CPU has changed from 20 to 32, so the accessible memory space has changed from 1MB to 4GB, and the number of registers has changed to 32 bits (Intel,1983). As a result, the real mode memory address calculation method is no longer suitable. Therefore, the current protection mode is introduced to achieve larger space and more flexible memory access. In the protected mode, all 32 address lines of the CPU are valid, and up to 4G bytes of physical address space can be addressed. The extended memory segment management mechanism and optional memory paging management mechanism not only provide hardware support for memory sharing and protection, but also provide hardware support for virtual memory. Intel introduced protected mode in x86 architecture, in order to operating system that user will have a totally new modes of operation. Because there is a big difference between real mode and protected mode, programs written for real mode cannot run directly in protected mode. Thus, to abandon the widely existing problem in real mode, it needs to switch from protected mode to real mode. In order to be compatible with early x86 CPU, the method of resetting the processor can be switched back to real mode. (b) long mode Actually, long mode is an addition of the x86(32-bit) instruction set. There are many instructions were deleted in the long mode which unlike the switches from 16-bit to 32-bit. In this mode, actual binary backward compatibility also effective. It retains the old mode to execute the old code which cannot execute in long mode, but its enablement of long mode changes the way assemblers and compilers of new code work This was an important additional mode of the x86 instruction set architecture was introduced and applied by a processor company other than Intel for the first time. It was also infrequent that Intel accepted technology and idea from their rival AMD.