IS:112 HARDWARE & SOFTWARE CONCEPTS

PC MEMORY

 

 

F000

   

E000

   

D000

Upper

384K

C000

Memory

System ROM

B000

 

Disk Controller ROM

A000

 

Other Uses

9000

   

8000

   

7000

   

6000

   

5000

Conventional

640K

4000

Memory

Addressable RAM

3000

   

2000

   

1000

   

0000

   

 

 

 

-An address in memory is made up two 16-bit values: segment & offset

-There are 16 segments of 64K each (the largest value using 16 bits)

-A 20-bit address is formed by adding the segment (shifted left one bit) to the offset

Segment: 50000

Offset: FFF8

Address: 5FFF8

 

-Total addressable memory is 1,024K (1 Meg): 16 x 64K

-Newer PCs are not limited to 20-bit address, have 24-bit or 32-bit address bus

-DOS still uses segment-offset scheme to remain compatible with older machines

-To use more than 640K of memory, have other types of memory

 

 

CONVENTIONAL MEMORY

-First 640K of memory

-Used by DOS and running DOS programs

-Can move majority of DOS into high memory, freeing up conventional memory for programs

UPPER MEMORY

-Upper 384K of memory

-Reserved for special purposes (system ROM, hard disk controller ROM, video RAM, etc)

-Not all upper memory is used

-80386 processors and higher have Virtual 86 mode

-DOS 5.0 and higher have program to utilize upper memory

-Allows use of upper memory for RAM, device drivers, terminate-and-stay-resident programs

EXAMPLE:

F000

System ROM

E000

 

D000

Unused

C000

Video ROM

B000

Unused

A000

Video Graphics

 

 

EXPANDED MEMORY

-Allows up to 32Meg RAM on expanded memory card

-Requires driver called expanded memory manager (EMM)

-Programs must be written to take advantage of expanded memory

-EMS page frame (64K made up of 4 16K physical pages) is used in upper memory

-Actual expanded memory is on expansion card and divided into 16K logical pages

-Physical pages swapped to and from logical pages

 

EXAMPLE:

     

Page 8

F000

 

16K

 

Page 7


EMS Page Frame

16K

 

Page 6

   

16K

 

Page 5

 

16K

 

Page 4

E000

     

Page 3

       

Page 2

       

Page 1

Physical Pages Logical Pages (EMS)

 

 

 

 

EXTENDED MEMORY

-Most common form found on 80386 and higher processors

-Lies above 1Meg boundary

-Requires driver (HIMEM)

-Uses protected mode for processor

EXAMPLE:

 

Extended

Above

Memory

1MB

   
   

Upper

384K

Memory

 
   

Conventional

640K

Memory

 

 

 

HIGH MEMORY AREA

-First 64K above 1Meg boundary

-If segment is FFFF and offset is FFFF, result is 10FFEF (1,114,095)

-Last valid address in first megabyte is 1,048,575 - difference is 65,520

-Advantage is not having to be in protected mode to address, must use HIMEM to enable address line