Computer Memory The DOS, Win 3X, & Win 9X Model Richard L. Goldman ©February 1, 2001

RAM and ROM Chips The PC uses two basic types of memory – Random Access Memory (RAM) and Read Only Memory (ROM). Both RAM and ROM are found in a variety of chips throughout the computer as well as the peripheral device connected to the computer.

ROM ROM chips contain information about the device that they are located on. The ROM chips may also contain small programs that the device may use to operate or communicate with other devices. The information stored on a ROM chip is programmed into them when the device that they are located on is manufactured.

RAM RAM chips store information that is easily changed by the user. RAM is the type of memory used by the computer to store programs and data that it is currently processing. The information stored in RAM is lost when the power to it is removed.

Memory Map During the booting process all types of physical memory, RAM and ROM, is located and assigned a memory address.

Power Supply Floppy Drive Hard Drive System Board Sys. BIOS CMOS RAM CPU Chip Set Battery Memory Keyboard Monitor Printer ROM RAM

Memory Addresses Some of the physical memory chips have specific memory addresses reserved for them. Other memory chips, both RAM and ROM, that are located during the booting process are assigned a memory address by the CPU. Within the DOS Memory Map there are several areas – each reserved for a specific purpose.

The original PC could support up to 1MB of RAM (1024KB) 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Each dot = 1KB Each row = 64KB

The first 640KB of memory was designated for the user’s use and called Conventional Memory (or Base Memory) 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Each dot = 1KB Each row = 64KB Upper Memory Conventional Memory The remainder was reserved for the computer's use and called Upper Memory 640KB 1024KB (1MB)

Specific areas of the Upper Memory is reserved for specific purposes. 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Upper Memory Conventional Memory VGA RAM 32KB Video ROM 128KB Adapter Board ROM & RAM - 160MB System ROM 128KB Free Area 128KB

Expanded Memory Expanded Memory was a technique used for a short time to increase memory. 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Upper Memory Conventional Memory Expanded Memory Additional memory chips were added using a Memory Expansion Board. A Memory Manager used a 64KB page frame in Upper Memory to map to 16KB memory pages on the Expanded Memory Board.

Extended Memory. Expanded Memory was replaced with Extended Memory **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Upper Memory Conventional Memory Extended Memory Extended Memory continues the linear address model used by Conventional and Upper Memory.

The first 64KB of Extended Memory is called High Memory **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** 0F0000 **************************************************************** 0E0000 **************************************************************** 0D0000 **************************************************************** 0C0000 **************************************************************** 0B0000 **************************************************************** 0A0000 **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** **************************************************************** A B C D E F Upper Memory Conventional Memory Extended Memory High Memory

Memory Management To access memory above 640KB the operating system uses a Memory Manager program. Since memory is considered a device – programs called device drivers are used to manage memory. DOS uses the device drivers HIMEM.SYS & EMM386.EXE to manage memory. To start the Memory Managers put the following commands at the beginning of the CONFIG.SYS file. DEVICE=C:\DOS\HIMEM.SYS DEVICE=C:\DOS\EMM386.EXE NOEMS (NOEMS means No Expanded Memory) Win 9X has a built in memory manager and does not require a CONFIG.SYS file, HIMEM.SYS, or EMM386.EXE.

High Memory The DOS OS uses the High Memory area to store part of its programs in order to free up space in the Conventional Memory area. To load part of DOS into High Memory add the following line to the CONFIG.SYS file after the EMM386 driver line: DOS=HIGH

Upper Memory Blocks (UMB) The UMB Command allows unused areas of Upper Memory to be used by other drivers. To enable Upper Memory Blocks enter the following in the CONFIG.SYS file after the DOS=HIGH command: DOS=UMB Devices may then be loaded with the DEVICEHIGH (in CONFIG.SYS) or the LOADHIGH (in AUTOEXEC.BAT) commands. EXAMPLE: DEVICEHIGH=C:\MOUSE.SYS