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Chapter One System Organization

To write even a modest 80x86 assembly language program requires considerable familiarity with the 80x86 family. To write good assembly language programs requires a strong knowledge of the underlying hardware. Unfortunately, the underlying hardware is not consistent. Techniques that are crucial for 8088 programs may not be useful on Pentium systems. Likewise, programming techniques that provide big performance boosts on the Pentium chip may not help at all on an 80486. Fortunately, some programming techniques work well no matter which microprocessor you're using. This chapter discusses the effect hardware has on the performance of computer software.

1.1 Chapter Overview

This chapter describes the basic components that make up a computer system: the CPU, memory, I/O, and the bus that connects them. Although you can write software that is ignorant of these concepts, high performance software requires a complete understanding of this material. This chapter also discusses the 80x86 memory addressing modes and how you access memory data from your programs.

This chapter begins by discussing bus organization and memory organization. These two hardware components will probably have a bigger performance impact on your software than the CPU's speed. Understanding the organization of the system bus will allow you to design data structures and algorithms that operate at maximum speed. Similarly, knowing about memory performance characteristics, data locality, and cache operation can help you design software that runs as fast as possible. Of course, if you're not interested in writing code that runs as fast as possible, you can skip this discussion; however, most people do care about speed at one point or another, so learning this information is useful.

With the generic hardware issues out of the way, this chapter then discusses the program-visible components of the memory architecture - specifically the 80x86 addressing modes and how a program can access memory. In addition to the addressing modes, this chapter introduces several new 80x86 instructions that are quite useful for manipulating memory. This chapter also presents several new HLA Standard Library calls you can use to allocate and deallocate memory.

Some might argue that this chapter gets too involved with computer architecture. They feel such material should appear in an architectural book, not an assembly language programming book. This couldn't be farther from the truth! Writing good assembly language programs requires a strong knowledge of the architecture. Hence the emphasis on computer architecture in this chapter.

1.2 The Basic System Components

The basic operational design of a computer system is called its architecture. John Von Neumann, a pioneer in computer design, is given credit for the architecture of most computers in use today. For example, the 80x86 family uses the Von Neumann architecture (VNA). A typical Von Neumann system has three major components: the central processing unit (or CPU), memory, and input/output (or I/O). The way a system designer combines these components impacts system performance (See Figure 1.1).

Figure 1.1 Typical Von Neumann Machine

In VNA machines, like the 80x86 family, the CPU is where all the action takes place. All computations occur inside the CPU. Data and machine instructions reside in memory until required by the CPU. To the CPU, most I/O devices look like memory because the CPU can store data to an output device and read data from an input device. The major difference between memory and I/O locations is the fact that I/O locations are generally associated with external devices in the outside world.

1.2.1 The System Bus

The system bus connects the various components of a VNA machine. The 80x86 family has three major busses: the address bus, the data bus, and the control bus. A bus is a collection of wires on which electrical signals pass between components in the system. These busses vary from processor to processor. However, each bus carries comparable information on all processors; e.g., the data bus may have a different implementation on the 80386 than on the 8088, but both carry data between the processor, I/O, and memory.

A typical 80x86 system component uses standard TTL logic levels1. This means each wire on a bus uses a standard voltage level to represent zero and one2. We will always specify zero and one rather than the electrical levels because these levels vary on different processors (especially laptops). The Data Bus

The 80x86 processors use the data bus to shuffle data between the various components in a computer system. The size of this bus varies widely in the 80x86 family. Indeed, this bus defines the "size" of the processor.

Every modern x86 CPU from the Pentium on up employs a 64-bit wide data bus. Some of the earlier processors used 8-bit, 16-bit, or 32-bit data busses, but such machines are sufficiently obsolete that we do not need to consider them here..

You'll often hear a processor called an eight, 16, 32, or 64 bit processor. While there is a mild controversy concerning the size of a processor, most people now agree that the minimum of either the number of data lines on the processor or the size of the largest general purpose integer register determines the processor size. The modern x86 CPUs all have 64-bit busses, but only provide 32-bit general purpose integer registers, so most people classify these devices as 32-bit processors.

Although the 80x86 family members with eight, 16, 32, and 64 bit data busses can process data up to the width of the bus, they can also access smaller memory units of eight, 16, or 32 bits. Therefore, anything you can do with a small data bus can be done with a larger data bus as well; the larger data bus, however, may access memory faster and can access larger chunks of data in one memory operation. You'll read about the exact nature of these memory accesses a little later (see "The Memory Subsystem" on page 140). The Address Bus

The data bus on an 80x86 family processor transfers information between a particular memory location or I/O device and the CPU. The only question is, "Which memory location or I/O device? " The address bus answers that question. To differentiate memory locations and I/O devices, the system designer assigns a unique memory address to each memory element and I/O device. When the software wants to access some particular memory location or I/O device, it places the corresponding address on the address bus. Circuitry associated with the memory or I/O device recognizes this address and instructs the memory or I/O device to read the data from or place data on to the data bus. In either case, all other memory locations ignore the request. Only the device whose address matches the value on the address bus responds.

With a single address line, a processor could create exactly two unique addresses: zero and one. With n address lines, the processor can provide 2n unique addresses (since there are 2n unique values in an n-bit binary number). Therefore, the number of bits on the address bus will determine the maximum number of addressable memory and I/O locations. Early x86 processors, for example, provided only 20 bit address busses. Therefore, they could only access up to 1,048,576 (or 220) memory locations. Larger address busses can access more memory.
Table 12: 80x86 Family Address Bus Sizes
Address Bus Size
Max Addressable Memory
In English!
8088, 8086, 80186, 80188
One Megabyte
80286, 80386sx
Sixteen Megabytes
Four Gigabytes
80486, Pentium
Four Gigabytes
Pentium Pro, II, III, IV
64 Gigabytes

Future 80x86 processors (e.g., the AMD "Hammer") will probably support 40, 48, and 64-bit address busses. The time is coming when most programmers will consider four gigabytes of storage to be too small, much like they consider one megabyte insufficient today. (There was a time when one megabyte was considered far more than anyone would ever need!). The Control Bus

The control bus is an eclectic collection of signals that control how the processor communicates with the rest of the system. Consider for a moment the data bus. The CPU sends data to memory and receives data from memory on the data bus. This prompts the question, "Is it sending or receiving?" There are two lines on the control bus, read and write, which specify the direction of data flow. Other signals include system clocks, interrupt lines, status lines, and so on. The exact make up of the control bus varies among processors in the 80x86 family. However, some control lines are common to all processors and are worth a brief mention.

The read and write control lines control the direction of data on the data bus. When both contain a logic one, the CPU and memory-I/O are not communicating with one another. If the read line is low (logic zero), the CPU is reading data from memory (that is, the system is transferring data from memory to the CPU). If the write line is low, the system transfers data from the CPU to memory.

The byte enable lines are another set of important control lines. These control lines allow 16, 32, and 64 bit processors to deal with smaller chunks of data. Additional details appear in the next section.

The 80x86 family, unlike many other processors, provides two distinct address spaces: one for memory and one for I/O. While the memory address busses on various 80x86 processors vary in size, the I/O address bus on all 80x86 CPUs is 16 bits wide. This allows the processor to address up to 65,536 different I/O locations. As it turns out, most devices (like the keyboard, printer, disk drives, etc.) require more than one I/O location. Nonetheless, 65,536 I/O locations are more than sufficient for most applications. The original IBM PC design only allowed the use of 1,024 of these.

Although the 80x86 family supports two address spaces, it does not have two address busses (for I/O and memory). Instead, the system shares the address bus for both I/O and memory addresses. Additional control lines decide whether the address is intended for memory or I/O. When such signals are active, the I/O devices use the address on the L.O. 16 bits of the address bus. When inactive, the I/O devices ignore the signals on the address bus (the memory subsystem takes over at that point).

1Actually, newer members of the family tend to use lower voltage signals, but these remain compatible with TTL signals.

2TTL logic represents the value zero with a voltage in the range 0.0-0.8v. It represents a one with a voltage in the range 2.4-5v. If the signal on a bus line is between 0.8v and 2.4v, it's value is indeterminate. Such a condition should only exist when a bus line is changing from one state to the other.

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