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Summary: Memory maps (not to be confused with memory mapped I/O) are diagrams or charts that show how an operating system divides up main memory.
| high memory | |||
| segment | offset | size | contents |
|---|---|---|---|
| 0000h | 0000h | 1024 | interrupt vector table |
| 0400h | 172 | BIOS communication area | |
| 04ACh | 68 | reserved by IBM | |
| 04F0h | 16 | user communication area | |
| 0500h | 256 | DOS communication area | |
| 0600h | varies | operating system: IBMBIO.COM (DOS interface to the BIOS) IBMDOS.COM (DOS interrupt handlers and interrupt 21h service routines DOS buffers, control areas, and device drivers COMMAND.COM (resident portion), interrupt 22h, 23h, and 24h handlers, routine to reload transient portion of COMMAND.COM memory-resident utilities transient applications transient portion of COMMAND.COM | |
| 9000h | FFFFh | highest address of user memory | |
| A000h | 0000h | 128K | reserved area of ROM |
| A000h | 0000h | beginning of area used by EGA and PS/2 video systems | |
| B000h | 0000h | 4000 | monochrome video memory |
| 0800h | 16K | color graphics adaptor (CGA) video memory | |
| C000h | 0000h | 192K | ROM expansion and control area |
| 8000h | ROM for hard disk | ||
| D000h | ROM expansion and control area | ||
| E000h | ROM expansion and control area | ||
| F000h | 0000h | 16K | reserved ROM |
| 4000h | 40K | base system ROM, ROM BASIC | |
| E000h | 8K | ROM BIOS | |
| low memory | |||
If a TSR is created in assembly langauge, the programmer has exact control over the use of memory and can omit the heap area and place the initialization code at the end of the segment (where it can be released after initialization).
If a TSR is created in C, the small-memory model must be used. Because of the severe memory limitations, the programmer should not use C allocation functions (such as malloc and calloc), as these add to the heap. A small heap must be assigned because some C library routines (such as fprintf) allocate memory from the heap.
Variables required by MS-DOS interrupt handlers must be placed at the beginning of the code segment.
| high memory | |
| address | contents |
|---|---|
| $EFFFFF | end of VIA space |
| $E80000 | beginning of VIA space |
| $DFFFFF | end of IWM space |
| $D00000 | beginning of IWM space |
| $BFFFFF | end of SCC write |
| $B00000 | beginning of SCC write |
| $9FFFFF | end of SCC read |
| $900000 | beginning of SCC read |
| $5FFFFF | end of SCSI |
| $580000 | beginning of SCSI |
| $41FFFF | end of ROM |
| $400000 | beginning of ROM |
| $3FFFFF | end of RAM for 4MB Mac Plus |
| $27FFFF | end of RAM for 2.5MB Mac Plus |
| $1FFFFF | end of RAM for 2MB Mac Plus |
| $0FFFFF | end of RAM for 1MB Mac Plus |
| varies | sound buffer |
| varies | System Error Handler area |
| varies | main screen (ScrnBase) |
| varies | MacsBugs (optional) |
| varies | RAM cache (optional) |
| varies | Jump Table (A5 relative) |
| varies | application and QuickDraw globals (A5 relative) |
| varies | stack (A7) |
| varies | Application Heap (ApplZone) |
| $001600 | beginning of System Heap (SysZone) |
| $000E00 | beginning of Toolbox A-trap table |
| $000800 | additional low-memory global area |
| $000400 | beginning of OS A-trap table |
| $000100 | beginning of low-memory global area |
| $000000 | beginning of RAM Motorola 680x0 vectors |
| low memory | |
| system | main screen | alternate screen |
|---|---|---|
| Macintosh 128K | $1A700 | $12700 |
| Macintosh 512K | $7A700 | $72700 |
| Macintosh Plus, 1 Mb | $FA700 | $F2700 |
| Macintosh Plus, 2Mb | $1FA700 | $1F2700 |
| Macintosh Plus, 2.5Mb | $27A700 | $272700 |
| Macintosh Plus, 4 Mb | $3FA700 | $3F2700 |
| system | main sound buffer | alternate sound buffer |
|---|---|---|
| Macintosh 128K | $1FD00 | $1A100 |
| Macintosh 512K | $7FD00 | $7A100 |
| Macintosh Plus, 1Mb | $FFD00 | $FA100 |
| Macintosh Plus, 2Mb | $1FFD00 | $1FA100 |
| Macintosh Plus, 2.5Mb | $27FD00 | $27A100 |
| Macintosh Plus, 4 Mb | $3FFD00 | $3FA100 |
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Structured Computer Organization, 4th edition; by Andrew S. Tanenbaum; Prentice Hall; October 1998; ISBN 0130959901; Paperback; 669 pages; $95.00; used by CS 308-273A (Principles of Assembly Languages) at McGill University School of Computer Science
Computers: An Introduction to Hardware and Software Design; by Larry L. Wear, James R. Pinkert (Contributor), William G. Lane (Contributor); McGraw-Hill Higher Education; February 1991; ISBN 0070686742; Hardcover; 544 pages; $98.60; used by CS 308-273A (Principles of Assembly Languages) at McGill University School of Computer Science
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Copyright © 2001 Milo
Last Updated: March 28, 2001
Created: March 28, 2001
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