Expert C Programming: Deep C Secrets by Peter van Der Linden

Expert C Programming: Deep C Secrets by Peter van Der Linden

Author:Peter van Der Linden [Linden, Peter van Der]
Language: eng
Format: mobi, epub, pdf
Tags: Software Programming, C Programming Software, C, Computers, Programming Languages
ISBN: 9780131774292
Google: 9f9uAQAAQBAJ
Amazon: 0131774298
Publisher: Prentice Hall
Published: 1994-06-24T00:00:00+00:00


Different-Looking Pointers, Same Address

An address on the Intel 8086 is formed by combining a 16-bit segment with a 16-bit offset.

The segment is shifted left four places before the offset is added. This means that many

different segment/offset pairs can point to the same one address.

segment

offset

resulting address

(shifted left 4 bits)

A0000

+ FFFF

= AFFFF

:

AFFF0

+ 000F

= AFFFF

In general, there will be 0x1000 (4096) different segment/offset combinations that point to

the same one address.

A C compiler-writer needs to make sure that pointers are compared in canonical form on a PC,

otherwise two pointers that have different bit patterns but designate the same address may wrongly compare unequal. This will be done for you if you use the "huge" keyword, but does not occur for the

"large" model. The far keyword in Microsoft C indicates that the pointer stores the contents of the segment register and the offset. The near keyword means the pointer just holds a 16-bit offset, and it will use the value already in the data or stack segment register.

Handy Heuristic

A Guide to Memory Prefix Use

Prefix Power of Two Meaning

Number of Bytes

Kilo 210 One

thousand

bytes

1,024

Mega 220 One

million

bytes 1,048,576

Giga 230 One

billion

bytes 1,073,741,824

Tera 240

One trillion bytes

1,099,511,627,776

Bubba 264

Eighteen billion billion bytes 18,446,744,073,709,551,616



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