The digits or text in blue represent the Operation Code (opcode): a binary number which represents the operation to perform. A binary number is naturally interpreted by a CPU, yet it is difficult to remember for a human. Hence, programmers can use its mnemonic (assembly) equivalent code to identify the desired operation, such as "LOD", "ADD", and "STO".

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The digits or text in red represent the operand(s) of the instruction. The operand has different interpretations according to the so-called addressing mode (which is also specified in the opcode). In the case of the simulator, the available addressing modes are:

• Immediate (or numeric) addressing, as in ADD #8:  in this case the operand is a number (add the number 8)

• Direct addressing, as in ADD 8: in this case the operand represents a memory address (add the number stored at the memory address 8). Rather than using directly numeric memory addresses, it is more convenient for humans to associate them with meaningful labels, such as Y or COUNTER. Hence a memory address can be more easily identified by a label, as in ADD Y. Important notice: the labels do not physically exist in the CPU or in memory, because they are  translated to the corresponding numerical addresses by the assembler, which can parse the whole assembly program offline. Yet, as the simulator translates the instructions one by one "on the fly", it is necessary first to define the labels by entering them in the grayed column, before they can be used in any instruction.