Tom Computer Simulator 2.8 - User Guide and FAQ

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Introduction

Main window

The File menu options allow loading and saving of TOM programs. The Edit option has delete, copy and paste functions. The Views options allow you to display TOMs other screens. The Options menu allows you to select various display settings, such as Hexadecimal. The Help menu option gives you access to this on-line Windows help facility.

TOM's Architecture - Memory store

Each memory location has a numeric label to the left of it which is the address of that memory location. The address of a memory location is used in our programs to identify which memory location we mean in the same way as we might use a postal address to identify someone's house.

Each memory location consists of two words. The most significant word (high word) is the left of the two and the least significant (low word) is the right hand one.

Each word is exactly one byte (eight bits) which is precisely enough memory to store any of the numbers 0 to 255. See number systems for why this is.

TOM's Architecture - A TOM Program

What we mean by a program is nothing other than a collection of instructions that can be entered into the memory locations and then executed. With TOM you will usually enter a program directly into memory by clicking on the memory cells and entering values using the keypad.

TOM's Architecture - Program Counter

TOM, and any computer for that matter, works by repeatedly fetching instructions from memory and executing them. It keeps track of which instruction to execute next by storing its address in the Program Counter. That is, the Program Counter always contains the address of the next instruction to be executed. We usually start with the Program Counter set to 0, so the first instruction executed is that stored at memory location 0. Instructions are then normally executed in sequential order, i.e. execute instruction at location 0, followed by that at location 1, and followed by that at location 2, and so on. The exception to this is when one of the jump instructions is executed which alter the flow of control of the program.

TOM's Architecture - Accumulator

The accumulator used to store values that the computer is currently using. It is entirely general purpose and may contain a data value, an address or even an instruction number. It all depends on what the person who designed the program wants to store in it. TOM has several instructions which act on the Accumulator, including inputting a number which is stored in the Accumulator (see below), and adding to and subtracting from the Accumulator.

TOM's Architecture - Input Unit

The input unit allows values to be input to the executing program. In a real computer, there would be many input units capable of supporting all kinds of input device (mouse, keyboard etc.). In TOM we have a simple form, which looks a little like a calculator.

You may enter a number by clicking on the buttons marked with digits, when you have completed your number, type OK. The last character input can be deleted by using the key with the arrow.

Whenever the instruction number (or opcode) 11 is executed, the dialogue box will appear on the screen and a number can be entered. If the number is too large, the number will be rejected.

TOM's Architecture - Output Unit

The output unit allows the program being executed to output values. In a real computer there would be output units corresponding to all the different kinds of output devices, such as the screen and the printer. In TOM we have one simple scrolling output area in the bottom right of TOM's window represented by a printer icon. The output unit will output a number from the accumulator to the output area of TOM's screen.

TOM's Architecture - Processor

The processor is the part of the computer that actually effects the execution of the instructions. It contains various internal registers, in particular the Program Counter and the Accumulator. To program TOM we don't need to know how this part works, but we do need to know the effects each of the possible instructions has. These effects are described in the section on TOM's instruction set.

For those interested in how the processor works, TOM provides an interactive simulation of the internal workings of its processor.



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