3.3 Real Mode

of DOS-accessible RAM that are not used by hardware. These unused slots

are referred to as upper memory blocks, or UMBs.

Memory above the real-mode limit of 1 MB is called extended memory.

When processors like the 80386 were released, there was an entire industry

of vendors who sold products called DOS extenders that allowed real-mode

programs to access extended memory.

You can get a general overview of how DOS maps its address space using the

mem.exe:

C:\> mem.exe

555360 bytes total conventional memory

555360 bytes available to MS-DOS

582080 largest executable program size

1048576 bytes total contiguous extended memory

0 bytes available contiguous extended memory

941056 bytes available XMS memory

MS-DOS resident in High Memory Area

You can get a more detailed view by using the coinmand with the debug

switch:

C:\> mem.exe

Convent!onal

Segment

/ti

Memory Detai1:

Total

Name

Type

00000

1,039

(IK)

Interrupt Vector

00040

271

(OK)

ROM Communication Area

00050

527

(IK)

DOS Communication Area

00070

2,656

(3K)

10

System Data

CON

System Device Driver

AUX

System Device Driver

PRN

System Device Driver

CLOCK$

System Device Driver

A: - C:

System Device Driver

COMl

System Device Driver

LPTl

System Device Driver

LPT2

System Device Driver

LPT3

System Device Driver

COM2

System Device Driver

COM3

System Device Driver

COM4

System Device Driver

00116

42,816

(42K)

MSDOS

System Data

00B8A

10,832

(UK)

10

System Data

192

(OK)

FILES=8

256

(OK)

FCBS=4

7,984

(8K)

BUFFERS=15

Parti I 63