11.7. Real Stack Machines.   

In PL0 if we want to increment a variable: 


                              LOD  0,ADR

                              LIT  0,1

                              ADD

                              STO  0,ADR    i  i+1

On a real STACK MACHINE such as PDP11 we could simply use 


      INC  ADR2(SP)  i  i+1

i.e. increment by 1 the value of variable offset from the top of the STACK by value ADR2. Or 

      INC  ADR(B)

All types of arithmetic operations can be done on the stack with or without changing the top of stack register. 

      j  i;     LOD  0,ADRi

            --> STO  0,ADRj


      k  i;     LOD  0,ADRi

                STO  0,ADRk

Variable i is used both times so it would be nice if we don't "destroy" it 

 LOD  0,ADRi

 COPY 0,ADRj

 COPY 0,ADRk  LOD  0,ADRk


       MOV  ADRi(B),ADRj(B)

      MOV  ADRi(B),ADRk(B)


           on PDP 11
No manipulation of top of STACK which practically is too limited.