Memory Layout and Memory Map

Memory Layout Overview:

As a result of the design decisions made in the earliest PCs, memory is broken into the following four basic pieces (with some of the pieces being divided further):

• Conventional Memory: The first 640 KB of system memory is the (in)famous conventional memory that every PC user has over time grown to know and hate. This is the area that is available for use by standard DOS programs, along with many drivers, memory-resident programs, and most anything else that has to run under standard DOS. It is found at addresses 00000h to 9FFFFh.
• Upper Memory Area (UMA): This is the upper 384 KB of the first megabyte of system memory (immediately above conventional memory). It is reserved for use by system devices and for special uses such as ROM shadowing and drivers. It uses addresses A0000h to FFFFFh.
• High Memory Area (HMA): This is the first 64 KB (less 16 bytes) of the second megabyte of system memory. Technically this is the first 64 KB of extended memory, but it can be accessed when the processor is in real mode, which makes it different from the rest of extended memory. It is usually used for DOS, to allow more conventional memory to be preserved. It occupies addresses 100000h to 10FFEFh.
• Extended Memory: This is all the memory above the high memory area until the end of system memory. It is used for programs and data when using an operating system running in protected mode, such as any version of Windows. Extended memory is found from address 10FFF0h to the last address of system memory. (Technically, the high memory area is part of extended memory, it all depends on how you are looking at things).

Memory Map

After the BIOS transfers control to boot sector, the first megabyte of memory looks like this:

Here is another detailed version of memory map.

A Complete Map: (from http://my.execpc.com/~geezer/osd/ram/)

The table of interrupt vectors begins at the very start of the microprocessors memory address 00000. (each interrupt vector is a pointer that tells the microprocessor the location where the code associated with the interrupt is located).

BIOS Data Area: This BIOS data area comprises of 256 bytes of memory starting at absolute memory location 000400.

Standard VGA Video ROM Code: C0000 - C7FFF (32K)

Rom Code Memory (BIOS Extension): 0C8000-0F4000.

System BIOS: F0000 - FFFFF (64K)

System Identification Bytes: 0FFFFE - 0FFFFF

There are 2 bytes assigned to identify the system. These are known as Model Byte and the Submodel Byte. The model byte is located at absolute memory address 0FFFFE (hex) and the Submodel Byte follows it.

SYSTEM MODEL BYTE SUBMODEL BYTE