The System Memory Map
            (from PC World June 1984 by Karl Koessel)

     For any particular computer system, the amount of memory 
that can be accessed directly is limited by the addressing
technique of the system's processor.  The PC's 8088 microprocessor
is able to access 1,048,576 (1024K, or 1MB) individual addresses.
     The PC's directly addressable memory is mapped into variously
sized sections.  Each section is dedicated to one function -- 
system RAM and system ROM (memory used by the system), user RAM 
(for languages, applications and other programs), monochrome
display RAM, color graphics display RAM, fixed disk control ROM
(for the XT), and application cartridge ROM (for the PCjr).
Also, some sections are reserved for future use.
     The System Memory Map shows the PC's memory addresses and 
their assigned functions.  The first three-quarters of memory is 
intended for functions that require RAM, and the last quarter is 
reserved for those that require ROM.  The first 655,360 (640K)
addresses (from 0 to 655,359, or 0 to 9FFFF in hex) are allocated 
to system and user RAM.  Of this 640K, the first 4K to 24K or more 
(depending on the operating system used) is used as system RAM. 
The remainder is allocated to user RAM.
     On the System Memory Map this 640K of RAM is divided into 
two sections.  The first 256K is installed directly on the system 
board.  The rest (up to 384K) is installed in one or more of the 
PC's slots.
     The next 131,072 (128K) addresses (from 655,360 to 786,431, 
or A0000 to BFFFF in hex) are reserved for the RAM that supports 
the PC's monochrome display and color graphics display.
Specifically, 4096 (4K) addresses (720,896 to 724,991, or B8000
to BBFFF in hex) are assigned to color graphics display.  The
unlabeled parts of this 128K are reserved for display functions
that may be adds in the future, such as extra high resolution
graphics or additional screen colors.
     The last 262,144 addresses (256K) of the PC's memory are
devoted to ROM.  The first 196,608 (192K) addresses (786,432 to 
983,039, or C0000 to EFFFF in hex), labeled "Expansion and
Con-trol," are reserved for ROM-based control of I/O devices,
such as the fixed disk controller of the XT or expanded PC.
These addresses are also used by ROM-based languages and
programs, such as Cartridge BASIC for the PCjr.  After the system
is loaded, it scans this section of memory for expansion board ROM.
     The last 65,536 (64K) addresses of this section (983,040 to 
1,048,575, or F0000 to FFFFF in hex) -- the final section of
directly addressable memory -- are alloted to the Base System ROM. 
The current version resides in the last 40K addresses (64K for 
the PCjr).  This ROM controls the basic functions of the computer. 
It contains the Cassette BASIC interpreter, cassette operating 
system, Power-on Self Test, I/O drivers, dot patterns for 128 
characters in graphics mode, and a floppy disk bootstrap loader. 
PCjr's larger ROM also contains dot patterns for an additional 
128 characters in graphics mode, and user-selectable diagnostic 
routines.
     The PC's system board has six 8K ROM sockets.  The Base
System ROM occupies the last five sockets.  The empty socket is
provided for the addition of 8K of user ROM, beginning at address 
999,424 (F4000 in hex).  The system boards of the XT, the Portable 
PC and the PCjr have two ROM sockets, which may contain either 
32K ROM chips or 8K ROM chips.  The XT and the Portable PC contain 
one of each type of chip, for a total of 40K Base System ROM.  The 
PCjr uses two 32K ROM chips for a total of 64K Base System ROM. 
Although routines and information within the Base System ROM may 
be accessed directly, they are better accessed indirectly through 
DOS calls, since according to IBM the Base System's "intermal 
workings and absolute addresses are subject to change without 
notice."
- - - - -
Start Address                    Start Address
Decimal   Hex      Function      Decimal   Hex      Function    
-----------------------------------------------------------------
|   0    00000 |                 | 512K   80000 |               |
|  16K   04000 |                 | 528K   84000 |               |
|  32K   08000 |                 | 544K   88000 |               |
|  48K   0C000 |                 | 560K   8C000 |   384K RAM    |
|--------------|                 |--------------|  on Expansion |
|  64K   10000 |                 | 576K   90000 |     Boards    |
|  80K   14000 |                 | 592K   94000 |               |
|  96K   18000 |    128-256K     | 608K   98000 |               |
| 112K   1C000 |     RAM on      | 624K   9C000 |               |
|--------------|     System      |------------------------------|
| 128K   20000 |     Board       | 640K   A0000 |               |
| 144K   24000 |                 | 656K   A4000 |  128K Display |
| 160K   28000 |                 | 672K   A8000 |      RAM      |
| 176K   2C000 |                 | 688K   AC000 |               |
|--------------|                 |------------------------      |
| 192K   30000 |                 | 704K   B0000 - Monochrome    |
| 208K   34000 |                 | 720K   B4000 |               |
| 224K   38000 |                 | 736K   B8000 - Color Graphics|
| 240K   3C000 |                 | 752K   BC000 |               |
|--------------------------------|------------------------------|
| 256K   40000 |                 | 768K   C0000 |               |
| 272K   44000 |                 | 784K   C4000 |               |
| 288K   48000 |                 | 800K   C8000 |               |
| 304K   4C000 |                 | 816K   CC000 |               |
|--------------|                 |--------------|               |
| 320K   50000 |                 | 832K   D0000 |   192K ROM    |
| 336K   54000 |                 | 848K   D4000 |   Expansion   |
| 352K   58000 |                 | 864K   D8000 |      and      |
| 368K   5C000 |    384K RAM     | 880K   DC000 |    Control    |
|--------------|  on Expansion   |--------------|               |
| 384K   60000 |     Boards      | 896K   E0000 |               |
| 400K   64000 |                 | 912K   E4000 |               |
| 416K   68000 |                 | 928K   E8000 |               |
| 432K   6C000 |                 | 944K   EC000 |               |
|--------------|                 |------------------------------|
| 448K   70000 |                 | 960K   F0000 |   64K Base    |
| 464K   74000 |                 | 976K   F4000 |  System ROM   |
| 480K   78000 |                 | 992K   F8000 |   BIOS and    |
| 496K   7C000 |                 |1008K   FC000 |    BASIC      |
|--------------------------------|------------------------------|
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