Graham Lee

Input and Output Devices

Input and output (I/O) devices, also called peripherals, are used for transferring programs and data into the computer, or for transferring results back to the user. Such devices convert between the external means of recording information required by the user, such as punched holes or printed characters, and the internal means of storing information inside the computer, such as patterns of electric signals.

Logic Functions and Expressions

There is a close link between the design of computer components such as the Function Unit, and the mathematical study of logic. So much so, in fact, that the computer circuits which perform operations such as addition are often called logic circuits, or quite simply logic, and their design is called logic design. The link exists because, on the one hand, logic circuits are actually devices for manipulating bit patterns, since these are used to represent information inside the computer, and, on the other hand, mathematical logic provides three basic logic operators (and, or and not) for manipulating bits. These operators can be used to describe, or express, using a logic expression1, the workings of a logic circuit, and their properties can be exploited when designing circuits.

High-Level Programming Languages

There are obvious differences between the programming language Pascal and the assembly code of SDC, and the assembly codes of other computers. The differences arise because assembly codes are low-level, machine-dependent languages which are used for coding programs to be executed on a particular computer. They reflect the architectural details of that computer. Pascal, on the other hand, is an example of a high-level programming language.

Syntax Analysis and Parsing Algorithms

The syntax-analysis phase of a compiler or an interpreter checks to see if the source program is correctly formed according to the syntax of the high-level language used. It produces appropriate error messages for any syntactically illegal part of the program. Syntax analysis is also known as parsing, and is done using a parsing algorithm.

A Simple Digital Computer

The automatic, digital computer was invented in 1833 by Charles Babbage (1791–1871), a mathematician at Cambridge University, England. For various financial and technological reasons, Babbage’s machine, which he called the Analytical Engine, was never built.

Floating-Point Arithmetic

The arithmetic instructions of SDC deal only with integers, and the Function Unit of chapter 10 can only perform arithmetic operations on integers. The calculations which computers are required to carry out may also involve real numbers, which are numbers with integral and fractional parts.

Parallel, Binary Subtractors

The Function Unit is capable of subtracting one binary integer from another, using a circuit called a subtractor, so that the SUB instruction of table 1.4 can be executed.

The Function Unit

The Function Unit is also called the Arithmetic and Logic Unit (ALU) because it is typically capable of performing arithmetic operations and logic operations. The Function Unit of SDC can perform the operations necessary to enable SDC to execute the ADD, SUB, MPY, DIV, AND and OR instructions of table 1.4. Thus the Function Unit is able to perform the following operations between the integer or bit pattern in the ACC and another brought from the Memory: addition, subtraction, multiplication, division and, or on pairs of corresponding bits of their operands Figure 10.1 shows the inputs and outputs of the Function Unit.

The Control Unit

The Control Unit has to generate control signals to cause information to flow from one register to another along the bus, and function-select signals to operate the components. By generating the right series of control signals, it can cause the computer to go through the instruction cycle, and to execute the instructions of table 1.4.

Macros and Macro Assemblers

Macros are related to subroutines. A macro can be defined in a similar way to a subroutine. It can also be called, but the effect of a macro call is different to the effect of a subroutine call.