AMOS Interface

To perform more complex calculation's some theoretical understanding is needed. Every time a value is entered into an expression, it is laced on the top of a list of numbers known as a stack. Supposing a stack is represented by this:

3
2
1
0

The stack grows from the bottom upwards, so 3 sits on the top and the zero is sitting at the bottom. Whenever the Interface encounters a number or a string, it is pushed straight to the top of the current stack. But operators are treated differently. When an operator or a function is spotted, the Interface will execute it immediately, grabbing the required values directly from the stack. After the calculation has been performed, the result is replaced back onto the current stack. This process continues until the Interface reaches the end of the expression.

Here is a theoretical calculation of one of these expressions, in this case how the result of 21 is evaluated from the expression 23+ 4*1+

23+4*1+=54*1+
54*1+=20 1+
20 1+=21

That expression is evaluated strictly from left to right. The 2 and the 3 are first placed onto the stack and are then added together by the plus sign, giving a result of 5. The 4 is now loaded onto the stack to be multiplied by the item immediately below it, which is the 5 that resulted from the last operation. This gives a value of 20, and this new value is loaded onto the stack to be added to the item below, generating a final value of 21.

It is important to realise that the stack is only retained during the current calculation. It has no permanent existence whatsoever! This means that after the expression has been calculated, exactly one value must be left in the stack, otherwise a syntax error will be generated.

Here is a table of the available arithmetic operators:

A separate set of logical operators that can be used to perform tests is explained in Chapter 9.2.