IN      read an integer from stdin and place result in r1; ignore operands r2 and r3
OUT     write contents of r1 to stdout; ignore operands r2 and r3
ADD     add contents of r2 and r3 and place result in r1
SUB     subtract contents of r3 from contents of r2 and place result in r1
MUL     multiply contents of r2 and contents of r3 and place result in r1
DIV     divide contents of r2 by contents of r3 and place result in r1
HALT    ignore operands and terminate the machine