What is the minimum instruction set required for any Assembly language to be considered useful?
Control structures comprise the basic feature without which there is no language. This means that your language must provide arithmetic operations on two variables; and then allow a program to change the program counter -- that is, to branch -- based on the result of an operation. Very often, the crucial operation is SUB, for subtract one operand from another. And the conditions on which you would allow a branch are:
- result is zero;
- result is greater than zero;
- result is less than zero.
- no condition, i.e., branch unconditional
You also need instructions to move data around: LOAD and STORE, say.
These three conditions and their corresponding branches (or skips, which is another way to do it) are necessary for any program. Not only that, but just these three simple operations plus data-moving instructions, are sufficient to do anything in a program except I/O. If you wanted to, and given a cooperating memory organization, you could rewrite Linux using just LOAD, STORE, ADD, SUB, and the three conditional branches.
The PDP-8 was a much more powerful machine than this: it had a rich set of eight instructions, including I/O.
HTH
Well, this is a very broad subject. I suppose you need to get familiar with Random Access Machine. I'm not an expert, but it's difficult to tell which instructions should be supported by this very basic microprocessor. For example: Subtraction and multiplication may be simulated by Addition operation. Multiplication is possible if microprocessor supports jumps and conditional instructions and subtraction is possible by adding negative number.
Surprisingly, there is such a thing as a one instruction set computer.