Summary: Memory systems incomputers. Access to memory (for storage of programs and data) is one of themost basic and lowest level activities of an operating system.
Memory maps
Summary: Memory maps (not tobe confused with memory mapped I/O) are diagrams or charts that show how an operatingsystem divides up main memory.
PC-DOS and MS-DOS memory map
| 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 | |||
MS-DOS TSR MEMORY MAP
Ifa TSR is created in assembly langauge, the programmer has exact control overthe use of memory and can omit the heap area and place the initialization codeat the end of the segment (where it can be released after initialization).
Ifa TSR is created in C, the small-memorymodel must be used. Because of the severe memory limitations, the programmershould 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 libraryroutines (such as fprintf) allocate memory from the heap.
Variablesrequired by MS-DOS interrupt handlers must be placed at the beginning of thecode segment.
Mac Plus memory map
| 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 | |
Mac Plus video memory locations
| 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 |
Mac Plus sound memory locations
| 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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