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(shifted left 4 bits)offset resulting address
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 2^10 One thousand bytes 1,024
Mega 2^20 One million bytes 1,048,576
Giga 2^30 One billion bytes 1,073,741,824
Tera 2^40 One trillion bytes 1,099,511,627,776