Q. Illustrate program on hypothetical machine?

The program given in figure above is a hypothetical program which performs addition of numbers stored from locations 2001 onwards until a zero is encountered. Consequently X is initialized to 2001 whereas MBR which stores the result is initialized to zero. We have presumed that in this machine all operations are performed by CPU. The programs will execute instructions as:

SKIP instruction is a zero-address instruction and skips next instruction to be executed in sequence. Or we can say it increments the value of PC by one instruction length. The SKIP also can be conditional. For illustration the instruction ISZ skips the next instruction only if the result of most recent operation is zero. 

CALL and RETN are used for CALLing subprograms and RETurning from them. Presume that a memory stack has been built such that stack pointer points to a non-empty location stack and expand in the direction of zero address.