locations in main memory to be referenced by descriptive names rather than by numeric addresses

Computer Science An Overview _J. Glenn Brookshear _11th Edition

Chapter 6 Programming Languages

As suggested in Section 6.1, high-level programming languages allow locations in main memory to be referenced by descriptive names rather than by numeric addresses. Such a name is known as a variable, in recognition of the fact that by changing the value stored at the location, the value associated with the name changes as the program executes.

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Early Generations

As we learned in Chapter 2, programs for modern computers consist of sequences of instructions that are encoded as numeric digits. Such an encoding system is known as a machine language. Unfortunately, writing programs in a machine language is a tedious task that often leads to errors that must be located and corrected (a process known as debugging) before the job is finished.

In the 1940s, researchers simplified the programming process by developing notational systems by which instructions could be represented in mnemonic rather than numeric form.

For example, the instruction

Move the contents of register 5 to register 6

would be expressed as

4056

using the machine language introduced in Chapter 2, whereas in a mnemonic system it might appear as

MOV R5, R6.

As a more extensive example, the machine language routine

156C

166D

5056

306E

C000

which adds the contents of memory cells 6C and 6D and stores the result at location 6E (Figure 2.7 of Chapter 2) might be expressed as

LD R5,Price

LD R6,ShippingCharge

ADDI R0,R5 R6

ST R0,TotalCost

HLT

using mnemonics. (Here we have used LD, ADDI, ST, and HLT to represent load, add, store, and halt. Moreover, we have used the descriptive names Price, ShippingCharge, and TotalCost to refer to the memory cells at locations 6C, 6D, and 6E, respectively. Such descriptive names are often called identifiers.)

Once such a mnemonic system was established, programs called assemblers were developed to convert mnemonic expressions into machine language instructions. Thus, rather than being forced to develop a program directly in machine language, a human could develop a program in mnemonic form and then have it converted into machine language by means of an assembler.

A mnemonic system for representing programs is collectively called an assembly language. At the time assembly languages were first developed, they represented a giant step forward in the search for better programming techniques. In fact, assembly languages were so revolutionary that they became known as second-generation languages, the first generation being the machine languages themselves.

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