Refactor, add extensive README.md, add future planning

src/Machine/Memory/ProgramMemory.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Machine\Memory;
+
+use InvalidArgumentException;
+use Program\Program;
+
+class ProgramMemory
+{
+	/** @var Program[] */
+	private array $memory = [];
+
+
+	public function write(Program $program): void
+	{
+		$this->guardInvalidProgramNumber($program->N);
+		$this->memory[$program->N] = $program;
+	}
+
+
+	public function read(int $N): ?Program
+	{
+		return @$this->memory[$N];
+	}
+
+
+	private function guardInvalidProgramNumber(int $N): void
+	{
+		if ($N > 9998 || $N < 9001) {
+			throw new InvalidArgumentException('Program number must be in range 9001 - 9998');
+		}
+	}
+}