A+ Hardware Service Technician: System Architecture

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RAM and Your Computer

DOS

RAM is broken up into DOS memory “sections.” The first 640 Kb of memory is Conventional Memory, which is used to load DOS, run programs, and load drivers. DOS is loaded in the first 64K of memory.

Above 640 Kb, but below 1024 Kb, is Upper Memory, also called Reserved Memory. 640 Kb – 768 Kb is Video RAM, 768 Kb – 960 Kb is for BIOS and RAM buffers, and 960 Kb – 1024 Kb is Motherboard BIOS.

Above 1024 Kb is Extended memory. High Memory Area (HMA) is the first 64 Kb of Extended memory. Extended memory is mainly used by Windows programs.

Expanded memory is sometimes used by DOS programs and is available up to 32 MB. It is swapper in 16 Kb pages (addresses in pages of 16 Kb).

In DOS, the MEM.EXE program allows you to determine how memory is being used by DOS. It has several switches, the important ones are highlighted here: /? the help switch, /C the classify switch - gives a report of how memory is used, /D the debug switch - details the first 640KB of memory, /F the free memory switch, shows all free memory blocks in the first 640KB of memory, /M (module) the module switch, shows the starting addresses of the data, program, and how much memory is allocated, and /P the pause switch, displays the output one page at a time.

To use upper memory, you need to make sure the upper memory device driver, EMM386.EXE is invoked in CONFIG.SYS and DOS=UMB is there also.

DOS=HIGH loads DOS into HMA.

Device=HIMEM.SYS is the extended memory device driver.

A Parity Error indicates a problem with RAM. Parity is a simple form of error checking and is used to check RAM.

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