Intel x86 Microprocessor

This history illustrates the impact of the “golden handcuffs” of compatibility on the x86, as the existing software base at each step was too important to jeopardize with significant architectural changes. If you looked over the life of the x86, on average the architecture has been extended by one instruction per month!

Whatever the artistic failures of the x8 6, keep in mind that there are more instances of this architectural family on desktop computers than of any other architecture, increasing by more than 250 million per year. Nevertheless, this checkered ancestry has led to an architecture that is difficult to explain and impossible to love.

Brace yourself for what you are about to see! Do «of try to read this section with the care you would need to write x86 programs; the goal instead is to give you familiarity with the strengths and weaknesses of the world’s most popular desktop architecture.

Rather than show the entire 16-bit and 32-bit instruction set, in this section we concentrate on the 32-bit subset that originated with the 80386, as this portion of the architecture is what is used today. We start our explanation with the registers and addressing modes, move on to the integer operations, and conclude with an examination of instruction encoding.


x86 Registers and Data Addressing Modes

The registers of the 80386 show the evolution of the instruction set. The 80386 extended all 16-bit registers (except the segment registers) to 32 bits, prefixing an E to their name to indicate the 32-bit version. We’ll refer to them generically as GPRs (general-purpose registers). The 80386 contains only eight GPRs. This means MIPS programs can use four times as many and ARM twice as many.

Arithmetic, logical, and data transfer instructions are twooperand instructions. There are two important differences here. The x86 arithmetic and logical instructions must have one operand act as both a source and a destination; ARM and MIPS allow separate registers for source and destination.

This restriction puts more pressure on the limited registers, since one source register must be modified. The second important difference is that one of the operands can be in memory. Thus, virtually any instruction may have one operand in memory, unlike ARM and MIPS.

Data memory-addressing modes, described in detail below, offer two sizes of addresses within the instruction. These so-called displacements can be 8 bits or 32 bits.
Although a memory operand can use any addressing mode, there are restrictions on which registers can be used in a mode. x86 addressing modes and which GPRs cannot be used with each mode, as well as how to get the same effect using MIPS instructions.


x86 Integer Operations

The 8086 provides support for both 8-bit {byte) and 16-bit (word) data types. The 80386 adds 32-bit addresses and data (double words) in the x86. (AMD64 adds 64-bit addresses and data, called quad words; we’ll stick to the 80386 in this section.) The data type distinctions apply to register operations as well as memory accesses.

Almost every operation works on both 8-bit data and on one longer data size. That size is determined by the mode and is either 16 bits or 32 bits.

Clearly, some programs want to operate on data of all three sizes, so the 80386 architects provided a convenient way to specify each version without expanding code size significantly. They decided that either 16-bit or 32-bit data dominates most programs, and so it made sense to be able to set a default large size. This default data size is set by a bit in the code segment register. To override the default data size, an 8-bit prefix is attached to the instruction to tell the machine to use the other large size for this instruction.

The prefix solution was borrowed from the 8086, which allows multiple prefixes to modify instruction behavior. The three original prefixes override the default segment register, lock the bus to support synchronization (see Section 2.11), or repeat the following instruction until the register ECX counts down to 0. This last prefix was intended to be paired with a byte move instruction to move a variable number of bytes. The 80386 also added a prefix to override the default address size.

The x86 integer operations can be divided into four major classes:

1. Data movement instructions, including move, push, and pop
2. Arithmetic and logic instructions, including test, integer, and decimal arithmetic operations
3. Control flow, including conditional branches, unconditional jumps, calls,and returns
4. String instructions, including string move and string compare

Conditional branches on the x86 are based on condition codes or flags, like ARM. Condition codes are set as a side effect of an operation; most are used to compare the value of a result to 0. Branches then test the condition codes. PCrelative branch addresses must be specified in the number of bytes, since unlike ARM and MIPS, 80386 instructions are not all 4 bytes in length.

String instructions are part of the 8080 ancestry of the x86 and are not commonly executed in most programs. They are often slower than equivalent software routines.


x86 Instruction Encoding

Saving the worst for last, the encoding of instructions in the 80386 is complex, with many different instruction formats. Instructions for the 80386 may vary from 1 byte, when there are no operands, up to 15 bytes.

The opcode byte usually contains a bit saying whether the operand is 8 bits or 32 bits. For some instructions, the opcode may include the addressing mode and the register; this is true in many instructions that have the form “register = register op immediate.” Other instructions use a “postbyte” or extra opcode byte, labeled “mod, reg, r/m,” which contains the addressing mode information. This postbyte is used for many of the instructions that address memory. The base plus scaled index mode uses a second postbyte, labeled “sc, index, base.”


x86 Conclusion

Intel had a 16-bit microprocessor two years before its competitors’ more elegant architectures, such as the Motorola 68000, and this head start led to the selection of the 8086 as the CPU for the IBM PC. Intel engineers generally acknowledge that the x86 is more difficult to build than computers like ARM and MIPS, but the large market means AMD and Intel can afford more resources to help overcome the added complexity. What the x86 lacks in style, it makes up for in quantity, making it beautiful from the right perspective.

Its saving grace is that the most frequently used x86 architectural components are not too difficult to implement, as AMD and Intel have demonstrated by rapidly improving performance of integer programs since 1978. To get that performance, compilers must avoid the portions of the architecture that are hard to implement fast.


Source: Computer Organization and Design, David A. Patterson, John L. Hennessy