Basics of computer memory

Summary: Memory systems in computers. Access to memory (for storage of programs and data) is one of the most basic and lowest level activities of an operating system.

• memory hardware issues
• main storage
• external storage
• buffers
• basic memory software approaches
• static and dynamic approaches
• absolute addressing
• overlay
• relocatable software
• demand paging and swapping
• program counter relative
• base pointers
• indirection, pointers, and handles
• stack frames
• virtual memory
• OS memory services
• memory maps
• PC-DOS and MS-DOS memory map
• MS-DOS TSR memory map
• Mac Plus memory map
• Mac Plus video memory locations
• Mac Plus sound memory locations
• low memory
• PC-DOS and MS-DOS low memory
• BIOS Communication Area
• Reserved
• Inter-Application (User) Communication Area
• DOS Communication Area  

Memory maps

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.

• PC-DOS and MS-DOS memory map
• MS-DOS TSR memory map
• Mac Plus memory map
• Mac Plus video memory locations
• Mac Plus sound memory locations

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

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. 

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