Machine Problem 3

22C:18, Spring 1996

Due Tuesday Mar. 12, 1995, in discussion

Given the M68000 program that emulates an RPN calculator, assigned in machine problem 2, add the following new single-character commands:

Each character on the line of input has meaning as an instruction to the calculator:

% -- the mod operator:

As with division, this combines the top two elements on the stack, but it returns the remainder, not the quotient.

L -- the load operation:

This uses the top element on the stack as a memory address to select from one of 1000 memory locations, indexed by the numbers 0 to 999. If the top item on the stack is outside this range, it is an error. Otherwise, the top item on the stack is replaced by the value from memory. The locations in memory each hold one stack item.

S -- the store operation:

This uses the top element on the stack as a memory address to select from one of 1000 memory locations, indexed by the numbers 0 to 999. If the top item on the stack is outside this range, it is an error. Otherwise, the item under the top item on the stack stored in the indicated memory location, and the top two items on the stack are popped.

J -- the jump operation:

The top item on the stack is popped and used as a branch displacement, and popped off the stack. If it is zero, it is an error (the branch is to the J instruction itself). If negative, the branch goes backwards. If positive, the branch goes forward.

< = > -- the conditional jump operations:

These are identical to the J operation, except that they test the sign of the item below the branch displacement and only branch if the item is less than, equal to, or greater than zero. In addition, they always pop the item off the stack, whether or not they branch.

    Input: E12ESELEL+P ELE1-ES ELP ELEE19->
    Output: 24
    Output: 11
    Output: 10