Intel 64 and IA-32 Architectures Software Developers Manual Volume 2A, Instruction Set Reference, A-M

1-6 Vol. 2
ABOUT THIS MANUAL
Base 2 (binary) numbers are represented by a string of 1s and 0s, sometimes
followed by the character B (for example, 1010B). The “B” designation is only used in
situations where confusion as to the type of number might arise.
1.3.5 Segmented Addressing
The processor uses byte addressing. This means memory is organized and accessed
as a sequence of bytes. Whether one or more bytes are being accessed, a byte
address is used to locate the byte or bytes in memory. The range of memory that can
be addressed is called an address space.
The processor also supports segmented addressing. This is a form of addressing
where a program may have many independent address spaces, called segments.
For example, a program can keep its code (instructions) and stack in separate
segments. Code addresses would always refer to the code space, and stack
addresses would always refer to the stack space. The following notation is used to
specify a byte address within a segment:
Segment-register:Byte-address
For example, the following segment address identifies the byte at address FF79H in
the segment pointed by the DS register:
DS:FF79H
The following segment address identifies an instruction address in the code segment.
The CS register points to the code segment and the EIP register contains the address
of the instruction.
CS:EIP
1.3.6 Exceptions
An exception is an event that typically occurs when an instruction causes an error.
For example, an attempt to divide by zero generates an exception. However, some
exceptions, such as breakpoints, occur under other conditions. Some types of excep-
tions may provide error codes. An error code reports additional information about the
error. An example of the notation used to show an exception and error code is shown
below:
#PF(fault code)
This example refers to a page-fault exception under conditions where an error code
naming a type of fault is reported. Under some conditions, exceptions which produce
error codes may not be able to report an accurate code. In this case, the error code
is zero, as shown below for a general-protection exception:
#GP(0)