stack.asm

// Kuba 'lambdaofgod' Bartczuk
// stack implemented in assembly language from Elements of Computing Systems (Nisan & Schocken)
// http://www.nand2tetris.org
// didnt even read the whole book (actually a bummer)

@R0 // initialize 0th register to 0
M=0

(STACK)
   
    // D = (keyboard) input
    @KBD
    D=M

    @STACK // if D = 0 do nothing
    D;JEQ

    @80
    D=D-A

    @POP
    D;JEQ
    //if D = 80 (D = 'p') pop else push

// push nonzero input
    (PUSH)
        @0
        M=M+1 // increment 0th register

        @0
        A=M // set address to content of 0th register
        M=D // register[A] = D : set register[A] to keyboard input
        
        @0 
        A=M 

        @STACK
        0;JMP
//pop stack
    (POP)
        @0 // if register[0] = 0 it means the stack is empty - can't pop it
        D=M
        M;JEQ

        @0 // set address
        A=M
        M=0

        @0 // modify stack size accordingly
        M=M-1
        @STACK
        0;JM