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The Processing Environment

The processing environment for MASM 6.1 includes the processor on which your programs run, the operating system your programs use, and the aspects of the segmented architecture that influence the choice of programming models. This section summarizes these elements of the environment and how they affect your programming choices.

8086-Based Processors

The 8086 "family" of processors uses segments to control data and code. The later 8086-based processors have larger instruction sets and more memory capacity, but they still support the same segmented architecture. Knowing the differences between the various 8086-based processors can help you select the appropriate target processor for your programs.

The instruction set of the 8086 processor is upwardly compatible with its successors. To write code that runs on the widest number of machines, select the 8086 instruction set. By using the instruction set of a more advanced processor, you increase the capabilities and efficiency of your program, but you also reduce the number of systems on which the program can run.

Table 1.1 lists modes, memory, and segment size of processors on which your application may need to run. Each processor is discussed in more detail following.

Table 1.1 8086 Family of Processors

Processor Available Modes Addressable Memory Segment Size
8086/8088 Real 1 megabyte 16 bits
80186/80188 Real 1 megabyte 16 bits
80286 Real and Protected 16 megabytes 16 bits
80386 Real and Protected 4 gigabytes 16 or 32 bits
80486 Real and Protected 4 gigabytes 16 or 32 bits

Processor Modes

Real mode allows only one process to run at a time. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. The MS-DOS operating system runs in real mode.

Windows 3.1 operates only in protected mode, but runs MS-DOS programs in real mode or in a simulation of real mode called virtual-86 mode. In protected mode, more than one process can be active at any one time. The operating system protects memory belonging to one process from access by another process; hence the name protected mode.

Protected-mode addresses do not correspond directly to physical memory. Under protected-mode operating systems, the processor allocates and manages memory dynamically. Additional privileged instructions initialize protected mode and control multiple processes. For more information, see "Operating Systems," following.

8086 and 8088

The 8086 is faster than the 8088 because of its 16-bit data bus; the 8088 has only an 8-bit data bus. The 16-bit data bus allows you to use EVEN and ALIGN on an 8086 processor to word-align data and thus improve data-handling efficiency. Memory addresses on the 8086 and 8088 refer to actual physical addresses.

80186 and 80188

These two processors are identical to the 8086 and 8088 except that new instructions have been added and several old instructions have been optimized. These processors run significantly faster than the 8086.


The 80286 processor adds some instructions to control protected mode, and it runs faster. It also provides protected mode services, allowing the operating system to run multiple processes at the same time. The 80286 is the minimum for running Windows 3.1 and 16-bit versions of OS/2®.


Unlike its predecessors, the 80386 processor can handle both 16-bit and 32-bit data. It supports the entire instruction set of the 80286, and adds several new instructions as well. Software written for the 80286 runs unchanged on the 80386, but is faster because the chip operates at higher speeds.

The 80386 implements many new hardware-level features, including paged memory, multiple virtual 8086 processes, addressing of up to 4 gigabytes of memory, and specialized debugging registers. Thirty-two-bit operating systems such as Windows NT and OS/2 2.0 can run only on an 80386 or higher processor.


The 80486 processor is an enhanced version of the 80386, with instruction "pipelining" that executes many instructions two to three times faster. The chip incorporates both a math coprocessor and an 8K (kilobyte) memory cache. (The math coprocessor is disabled on a variation of the chip called the 80486SX.) The 80486 includes new instructions and is fully compatible with 80386 software.

8087, 80287, and 80387

These math coprocessors work concurrently with the 8086 family of processors. Performing floating-point calculations with math coprocessors is up to 100 times faster than emulating the calculations with integer instructions. Although there are technical and performance differences among the three coprocessors, the main difference to the applications programmer is that the 80287 and 80387 can operate in protected mode. The 80387 also has several new instructions. The 80486 does not use any of these coprocessors; its floating-point processor is built in and is functionally equivalent to the 80387.

Operating Systems

With MASM, you can create programs that run under MS-DOS, Windows, or Windows NT - or all three, in some cases. For example, ML.EXE can produce executable files that run in any of the target environments, regardless of the programmer’s environment. For information on building programs for different environments, see "Building and Running Programs" in Help for PWB.

MS-DOS and Windows 3.1 provide different processing modes. MS-DOS runs in the single-process real mode. Windows 3.1 operates in protected mode, allowing multiple processes to run simultaneously.

Although Windows requires another operating system for loading and file services, it provides many functions normally associated with an operating system. When an application requests an MS-DOS service, Windows often provides the service without invoking MS-DOS. For consistency, this book refers to Windows as an operating system.

MS-DOS and Windows (in protected mode) differ primarily in system access methods, size of addressable memory, and segment selection. Table 1.2 summarizes these differences.

Table 1.2 The MS-DOS and Windows Operating Systems Compared

Operating System System Access Available Active Processes Addressable Memory Contents of Segment Register Word Length
MS-DOS and Windows real mode Direct to hardware and OS call One 1 megabyte Actual address 16 bits
Windows virtual-86 mode Operating system call Multiple 1 megabyte Segment selectors 16 bits
Windows protected mode Operating system call Multiple 16 megabytes Segment selectors 16 bits
Windows NT Operating system call Multiple 512 megabytes Segment selectors 32 bits


In real-mode programming, you can access system functions by calling MS-DOS, calling the basic input/output system (BIOS), or directly addressing hardware. Access is through MS-DOS Interrupt 21h.


As you can see in Table 1.2, protected mode allows for much larger data structures than real mode, since addressable memory extends to 16 megabytes. In protected mode, segment registers contain selector values rather than actual segment addresses. These selectors cannot be calculated by the program; they must be obtained by calling the operating system. Programs that attempt to calculate segment values or to address memory directly do not work in protected mode.

Protected mode uses privilege levels to maintain system integrity and security. Programs cannot access data or code that is in a higher privilege level. Some instructions that directly access ports or affect interrupts (such as CLI, STI, IN, and OUT) are available at privilege levels normally used only by systems programmers.

Windows protected mode provides each application with up to 16 megabytes of "virtual memory," even on computers that have less physical memory. The term virtual memory refers to the operating system’s ability to use a swap area on the hard disk as an extension of real memory. When a Windows application requires more memory than is available, Windows writes sections of occupied memory to the swap area, thus freeing those sections for other use. It then provides the memory to the application that made the memory request. When the owner of the swapped data regains control, Windows restores the data from disk to memory, swapping out other memory if required.

Windows NT

Windows NT uses the so-called "flat model" of 80386/486 processors. This model places the processor’s entire address space within one 32-bit segment. The section "Defining Basic Attributes with .MODEL" in Chapter 2 explains how to use the flat model. In flat model, your program can (in theory) access up to 4 gigabytes of virtual memory. Since code, data, and stack reside in the same segment, each segment register can hold the same value, which need never change.

Segmented Architecture

The 8086 family of processors employs a segmented architecture - that is, each address is represented as a segment and an offset. Segmented addresses affect many aspects of assembly-language programming, especially addresses and pointers.

Segmented architecture was originally designed to enable a 16-bit processor to access an address space larger than 64K. (The section "Segmented Addressing," later in this chapter, explains how the processor uses both the segment and offset to create addresses larger than 64K.) MS-DOS is an example of an operating system that uses segmented architecture on a 16-bit processor.

With the advent of protected-mode processors such as the 80286, segmented architecture gained a second purpose. Segments can separate different blocks of code and data to protect them from undesirable interactions. Windows takes advantage of the protection features of the 16-bit segments on the 80286.

Segmented architecture went through another significant change with the release of 32-bit processors, starting with the 80386. These processors are compatible with the older 16-bit processors, but allow flat model 32-bit offset values up to 4 gigabytes. Offset values of this magnitude remove the memory limitations of segmented architecture. The Windows NT operating system uses 32-bit addressing.

Segment Protection

Segmented architecture is an important part of the Windows memory-protection scheme. In a "multitasking" operating system in which numerous programs can run simultaneously, programs cannot access the code and data of another process without permission.

In MS-DOS, the data and code segments are usually allocated adjacent to each other, as shown in Figure 1.1. In Windows, the data and code segments can be anywhere in memory. The programmer knows nothing about, and has no control over, their location. The operating system can even move the segments to a new memory location or to disk while the program is running.

Figure 1.1 Segment Allocation

Segment protection makes software development easier and more reliable in Windows than in MS-DOS, because Windows immediately detects illegal memory accesses. The operating system intercepts illegal memory accesses, terminates the program, and displays a message. This makes it easier for you to track down and fix the bug.

Because it runs in real mode, MS-DOS contains no mechanism for detecting an improper memory access. A program that overwrites data not belonging to it may continue to run and even terminate correctly. The error may not surface until later, when MS-DOS or another program reads the corrupted memory.

Segmented Addressing

Segmented addressing refers to the internal mechanism that combines a segment value and an offset value to form a complete memory address. The two parts of an address are represented as


The segment portion always consists of a 16-bit value. The offset portion is a 16-bit value in 16-bit mode or a 32-bit value in 32-bit mode.

In real mode, the segment value is a physical address that has an arithmetic relationship to the offset value. The segment and offset together create a 20-bit physical address (explained in the next section). Although 20-bit addresses can access up to 1 megabyte of memory, the BIOS and operating system on International Standard Architecture (IBM PC/AT and compatible) computers use part of this memory, leaving the remainder available for programs.

Segment Arithmetic

Manipulating segment and offset addresses directly in real-mode programming is called "segment arithmetic." Programs that perform segment arithmetic are not portable to protected-mode operating systems, in which addresses do not correspond to a known segment and offset.

To perform segment arithmetic successfully, it helps to understand how the processor combines a 16-bit segment and a 16-bit offset to form a 20-bit linear address. In effect, the segment selects a 64K region of memory, and the offset selects the byte within that region. Here’s how it works:

1. The processor shifts the segment address to the left by four binary places, producing a 20-bit address ending in four zeros. This operation has the effect of multiplying the segment address by 16.

2. The processor adds this 20-bit segment address to the 16-bit offset address. The offset address is not shifted.

3. The processor uses the resulting 20-bit address, called the "physical address," to access an actual location in the 1-megabyte address space.

Figure 1.2 illustrates this process.

Figure 1.2 Calculating Physical Addresses

A 20-bit physical address may actually be specified by 4,096 equivalent
segment:offset addresses. For example, the addresses 0000:F800, 0F00:0800, and 0F80:0000 all refer to the same physical address 0F800.

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