cstk    EXTDEF  sinit, spush, spop, stkp

sinit   STA     stkA
        LDA     #stk
        STA     stkp
        LDA     stkA
        RSUB

spush   . stkp++
        STA     stkA
        LDA     stkp
        ADD     #3
        STA     stkp
        LDA     stkA
        RSUB

spop    . stkp--
        STA     stkA
        LDA     stkp
        SUB     #3
        STA     stkp
        LDA     stkA
        RSUB
        
stkp    WORD    0
stk     RESW    1000
stkA    WORD    0  
        END     cstk