Programming Paradigms

Computer Science A2 · Further Programming. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A programming paradigm is a style of programming, a way of thinking about how to build a solution. Low-level programming uses the instruction set of the processor, and each instruction finds its data through an addressing mode.

Imperative (procedural) programming gives the steps in order, using variables, selection, loops, procedures and functions. Object-oriented programming (OOP) builds a program from objects, which hold data (attributes) together with the methods that work on that data.

Declarative programming states facts and rules. The programmer asks a query (a goal) and the language finds the values that satisfy it, without being told the steps.

Rules to remember

Immediate: the operand is the value itself (LDM #n). Direct: the operand is the address of the value (LDD). Indirect: the operand is an address that holds the address of the value (LDI).
Indexed: address used = operand + contents of the index register IX (LDX). Relative: address used = current address + offset.
A function returns a value; a procedure does not. BYVAL passes a copy; BYREF passes the address, so changes affect the original variable.
A class is a template; an object is an instance of a class. A constructor sets up a new object.
Encapsulation: attributes are kept private and reached only through public methods (getters read a value, setters change it).
Inheritance: a subclass takes the attributes and methods of its parent class. Polymorphism: a subclass redefines an inherited method so the same call behaves differently. Containment (aggregation): one class contains objects of another class.
Declarative: a fact such as parent(amir, sara). A rule such as grandparent(X, Z) IF parent(X, Y) AND parent(Y, Z). Names starting with a capital letter are variables.

Common mistake

Students stop one step early with indirect addressing and load the address instead of the data. With LDI, go to the operand address, read the address stored there, then go to that second address for the value.

Worked example

Address 40 holds 62, address 62 holds 17 and address 63 holds 90. The index register IX holds 1. What is in the accumulator after each of LDM #40, LDD 40, LDI 40 and LDX 62?

LDM #40 is immediate: the number 40 itself is loaded.
LDD 40 is direct: the contents of address 40 are loaded, which is 62.
LDI 40 is indirect: address 40 holds 62, so the contents of address 62 are loaded, which is 17.
LDX 62 is indexed: 62 + 1 = 63, so the contents of address 63 are loaded, which is 90.

Answer: 40, 62, 17 and 90.

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