The original processors developed by Intel were unable to use more than 1 MB of RAM, and the original IBM PC allowed only the first 640 KB of memory for direct use. MS-DOS applications were written to conform to this limitation. As application requirements grew, programmers needed to optimize the use of memory to make the most of the available space. This 1 MB of memory was divided into two sections. The first 640 KB was reserved for the operating system and applications (designated as conventional memory). The remaining 384 KB of RAM (designated as upper memory) was earmarked for running the computer's own housekeeping needs (BIOS, video RAM, ROM, and so on). Although some early PC clones had firmware that could make direct use of the upper memory block available to programmers, actually doing so would result in hardware and software incompatibility issues (see Figure 7.9)
Figure 7.9 IBM PC/MS-DOS Memory map
Under MS-DOS and Windows 3.x, the 640-KB area must be kept as free as possible for program use. MS-DOS memory optimization ensures that MS-DOS applications have as much of this memory as possible. The MS-DOS limitations no longer apply to Windows 95 in 32-bit mode and newer operating systems. However, they are still an important part of running MS-DOS, Windows 3.x-based programs on older machines or in MS-DOS compatibility mode with the more advanced operating systems.
Types of Memory Access
When we speak of memory in a computer, we are generally speaking of its RAM, because ROM cannot be written to by either the system or applications. Although we have only one source of RAM, under MS-DOS-based operating systems, it is divided into smaller groups depending on how it is used.
Extended Memory Specification (XMS)
RAM above the 1-MB address is called extended memory. With the introduction of the 80286 processor, memory was addressable up to 16 MB. Starting with the 80386DX processor, memory was addressable up to 4 GB. Extended memory is accessed through an extended memory manager (HIMEM.SYS for MS-DOS or a third-party utility such as 386MAX).
Conventional memory is the amount of RAM, typically 640 KB, that is addressable by an IBM PC or compatible machine operating in real mode. (Real mode is the only operating mode supported by MS-DOS.) Conventional memory is located in the area between 0 and 640 KB. Without the use of special techniques, conventional memory is the only kind of RAM accessible to MS-DOS programs.
MS-DOS Protected Mode Interface (DPMI)
MS-DOS Protected Mode Interface (DPMI) is a specification that allows multiple applications to access extended memory at the same time and has been endorsed by most memory-manager producers and applications developers. Windows uses the DPMI specification.
Expanded Memory Specification (EMS)
This technique, developed by Lotus/Intel/Microsoft (LIM), uses a 64-KB section of memory (usually in upper memory) to provide a "window" in which data can be written. Once in this area, the data can be transferred to the expanded memory. The memory chips are located on an expansion card installed inside the computer. The data is paged or swapped to and from the CPU through this window (see Figure 7.10).
Figure 7.10 Expanded memory
Expanded memory can provide up to 32 MB of additional memory, and because it is loaded from a 64-KB section, it is below the 1-MB limit and therefore MS-DOS can recognize it.
MS-DOS applications must be specifically written to take advantage of expanded memory. Windows applications do not use expanded memory; 80386 and newer processors can emulate expanded memory by using memory managers such as EMM386.EXE and HIMEM.SYS.
High Memory Area (HMA)
An irregularity was found in the Intel chip architecture that allowed MS-DOS to address the first 64 KB of extended memory on machines with 80286 or higher processors. This special area is called the high memory area. A software driver called an "A20 handler" must be run to allow the processor to access the HMA. MS-DOS uses HIMEM.SYS for this purpose. The only limitation is that HIMEM.SYS can load only a single program into this area. Typically, MS-DOS is loaded into HMA to free conventional memory (see Figure 7.11).
Figure 7.11 High memory area
Beginning with 80286 processors using an operating system such as OS/2 or Windows, a computer can create "virtual machines," providing all the functionality of a standard computer in real mode but allowing multiple tasks to take place at the same time. This is called protected mode because the processor, memory, and other hardware are "protected" from the software application taking direct control of the system by the operating system, which allocates memory and processor time.
In real mode (MS-DOS), a computer can perform only one operation at a time and an application expects full control of the system. Real mode operates within the MS-DOS 1-MB limitation.
Many high-speed expansion boards use shadow RAM to improve the performance of a computer. Shadow RAM rewrites (or shadows) the contents of the ROM BIOS and/or video BIOS into extended RAM (between the 640-KB boundary and 1 MB). This allows systems to operate faster when application software calls any BIOS routines. In some cases, system speed can be increased up to 400 percent (see Figure 7.12).
Figure 7.12 Shadow RAM
Upper Memory Area (UMA)
The upper memory area (UMA), the memory block from 640 KB to 1024 KB, is designated for hardware use, like video RAM, BIOS, and memory-mapped hardware drivers that are loaded into high memory. Refer to Tutorial 15, "Software: MS-DOS and Windows 3.x," for details.
Upper Memory Blocks (UMB)
The unused addresses in upper memory, known as upper memory blocks (UMB), can be divided into blocks. These empty blocks have no RAM associated with them and are simply reserved space. This unused space is valuable because, unlike expanded and extended memory, MS-DOS can run programs in UMB.
Virtual Control Program Interface (VCPI)
The memory-management specification, known as Virtual Control Program Interface (VCPI), accesses extended memory for MS-DOS-based applications. It allows only one application to control extended memory and does not support multitasking. Windows is not compatible with the VCPI specification.