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  On the original IBM PC, the 1 MB address space of the 8088 was split
  into several functional areas. (See Figure 1-6.) This memory map has
  been carried forward for compatibility in all subsequent PC and PS/2
  models.

               +------------------------------------+
               |   PC/AT and PS/2 extended memory   |
               +------------------------------------+
  100000H ----.+------------------------------------+
               |      Reserved for ROM BIOS         |
   E0000H ----.|------------------------------------|
               |    Reserved for installable ROM    |
   C0000H ----.|------------------------------------|
               |           Video buffers            |
   A0000H ----.+------------------------------------+
               +------------------------------------+--+
               |      Transient portion of DOS      |  |
               |------------------------------------|  |
               |                                    |  |
               |       Transient Program Area       |  |
               |      (user programs and data)      |  |
               |------------------------------------|  |
               |       Resident portion of DOS      |  |-- System
               |------------------------------------|  |   RAM
               |  Data area for ROM BIOS and BASIC  |  |
   00500H ----.|------------------------------------|  |
               |       Data area for ROM BIOS       |  |
   00400H ----.|------------------------------------|  |
               |          Interrupt vectors         |  |
   00000H ----.+------------------------------------+--+

  Figure 1-6.  An outline of memory usage in PCs and PS/2s.

  Some of the layout of the PC and PS/2 memory map is a consequence of the
  design of the 8086 microprocessor. For example, the 8086 always maintains
  a list of interrupt vectors (addresses of interrupt handling routines) in
  the first 1024 bytes of RAM. Similarly, all 8086-based microcomputers have
  ROM memory at the high end of the 1 MB address space, because the 8086,
  when first powered up, executes the program that starts at address FFFF0H.

  The rest of the memory map follows this general division between RAM at
  the bottom of the address space and ROM at the top. A maximum of 640 KB of
  RAM can exist between addresses 00000H and 9FFFFH. (This is the memory
  area described by the DOS CHKDSK program.) Subsequent memory blocks are
  reserved for video RAM (A0000H through BFFFFH), installable ROM modules
  (C0000H through DFFFFH), and permanent ROM (E0000H through FFFFFH). We'll
  explore each of these memory areas in greater detail in the chapters that
  follow.