Bit Manipulation
Bit Manipulation
Bit Manipulation
5.1 Chapter Overview
Chapter Five
Manipulating bits in memory is, perhaps, the thing that assembly language is most famous for. Indeed, one of the reasons people claim that the “C” programming language is a “medium-level” language rather than a high level language is because of the vast array of bit manipulation operators that it provides. Even with this wide array of bit manipulation operations, the C programming language doesn’t provide as complete a set of bit manipulation operations as assembly language. This chapter will discuss how to manipulate strings of bits in memory and registers using 80x86 assembly language. This chapter begins with a review of the bit manipulation instructions covered thus far and it also introduces a few new instructions. This chapter reviews information on packing and unpacking bit strings in memory since this is the basis for many bit manipulation operations. Finally, this chapter discusses several bit-centric algorithms and their implementation in assembly language.
5.2
What is Bit Data, Anyway?
Before describing how to manipulate bits, it might not be a bad idea to define exactly what this text means by “bit data.” Most readers probably assume that “bit manipulation programs” twiddle individual bits in memory. While programs that do this are definitely “bit manipulation programs,” we’re not going to limit this title to just those programs. For our purposes, bit manipulation refers to working with data types that consist of strings of bits that are non-contiguous or are not an even multiple of eight bits long. Generally, such bit objects will not represent numeric integers, although we will not place this restriction on our bit strings. A bit string is some contiguous sequence of one or more bits (this term even applies if the bit string’s length is an even multiple of eight bits). Note that a bit string does not have to start or end at any special point. For example, a bit string
Bit Manipulation
5.1 Chapter Overview
Chapter Five
Manipulating bits in memory is, perhaps, the thing that assembly language is most famous for. Indeed, one of the reasons people claim that the “C” programming language is a “medium-level” language rather than a high level language is because of the vast array of bit manipulation operators that it provides. Even with this wide array of bit manipulation operations, the C programming language doesn’t provide as complete a set of bit manipulation operations as assembly language. This chapter will discuss how to manipulate strings of bits in memory and registers using 80x86 assembly language. This chapter begins with a review of the bit manipulation instructions covered thus far and it also introduces a few new instructions. This chapter reviews information on packing and unpacking bit strings in memory since this is the basis for many bit manipulation operations. Finally, this chapter discusses several bit-centric algorithms and their implementation in assembly language.
5.2
What is Bit Data, Anyway?
Before describing how to manipulate bits, it might not be a bad idea to define exactly what this text means by “bit data.” Most readers probably assume that “bit manipulation programs” twiddle individual bits in memory. While programs that do this are definitely “bit manipulation programs,” we’re not going to limit this title to just those programs. For our purposes, bit manipulation refers to working with data types that consist of strings of bits that are non-contiguous or are not an even multiple of eight bits long. Generally, such bit objects will not represent numeric integers, although we will not place this restriction on our bit strings. A bit string is some contiguous sequence of one or more bits (this term even applies if the bit string’s length is an even multiple of eight bits). Note that a bit string does not have to start or end at any special point. For example, a bit string