;C02 stack module assembly language subroutines
;Requires external zero page variables 
;STKLO, STKHI, DSTLO, DSTHI, SRCLO, SRCHI
;external variables 
;STKSLO, STKSHI, STKELO, STKEHI, TEMP0, TEMP1, TEMP2
;external routines 
;MEMCPL, SETDST, SETSRC

;Implementation Notes:
;Stack starts at STKSLO,STKSHI and builds upward tp STKELO, STKEHI
;Each entry on the stack consists of the entry data followed 
;by a single byte containing the length of the entry data

;stkbgn(&b) - Set Stack Start Address
;Args: X,Y = Address
;Sets: STKSLO, STKSHI = Stack Start Address
;Affects: Z, N
STKBGN: STX STKSLO  ;Save Stack Start LSB
        STY STKSHI  ;Save Stack Start MSB
        RTS

;stkend(&b) - Set Stack End Address (+1)
;Args: X,Y = Address
;Sets: STKELO, STKEHI = Stack End Address
;Affects: Z, N
STKEND: STX STKELO  ;Save Stack End LSB
        STY STKEHI  ;Save Stack End MSB
        RTS

;stkptr() - Get Stack Pointer
;Uses: STKLO,STKHI = Stack Pointer
;Affects: N,Z
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKPTR: LDX STKLO     ;Load Stack Pointer LSB
        LDY STKHI     ;Load Stack Pointer MSB
        RTS

;stkssp() - Save Stack Pointer 
;Uses: STKLO,STKHI = Stack Pointer
;Sets: TEMP1.TEMP2 = Stack Pointer
;Affects: N,Z
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKSSP: JSR STKPTR    ;Get Stack Pointer
        JMP SAVRXY    ;Save in TEMP1, TEMP2 

;stksrc() - Set Source Pointer to Stack Pointer
;Uses: STKLO,STKHI = Stack Pointer
;Sets: SRCLO,SRCHI = Source Array Pointer
;Affects: X,N,Z
;Returns: Y = 0
STKSRC: JSR STKPTR    ;Get Stack Pointer Address
        JMP SETSRC    ;Set Source/ an/d Return

;stkdst() - Set Destination Pointer to Stack Pointer
;Uses: STKLO,STKHI = Stack Pointer
;Sets: DSTLO,DSTHI = Destination Array Pointer
;Affects: N,Z
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKDST: JSR STKPTR    ;Get Stack Pointer Address
        JMP SETDST    ;Set Destination and Return

;stkrst() - Reset Stack Pointer to Start Address
;Uses: STKSLO, STKSHI = Stack Start Address
;Sets: STKLO, STKHI = Stack Pointer
;Affects: Z, N
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKRST: LDX STKSLO    ;Load X with Stack Start LSB
        LDY STKSHI    ;Load X with Stack Start MSB
        JMP STKSET    ;Store in Stack Pointer 

;stkrsp() - Restore Stack Pointer 
;Uses: TEMP1.TEMP2 = Stack Pointer
;Sets: STKLO,STKHI = Stack Pointer
;Affects: N,Z
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKRSP: JSR RESRXY    ;Get TEMP1, TEMP2
                      ;and fall into STKSET

;stkset() - Set Stack Pointer Address
;Args: X,Y = Address
;Sets: STKLO, STKHI = Stack Pointer Address
;Affects: Z, N
;Returns: X = Stack Pointer LSB
;         Y = Stack Pointer MSB
STKSET: STX STKLO     ;Store X in Stack Pointer LSB
        STY STKHI     ;Store Y in Stack Pointer MSB
        RTS           ;Exit Routine

;stkpsh(n, &m) - Push n Bytes of m onto Stack
;Args: A = Number of Bytes to Append
;      Y,X = Address of Source Array
;Uses: STKELO, STKEHI = Stack End Address
;Sets: DSTLO, DSTHI = Pointer to Stack Entry
;      SRCHI, SRCLO = Pointer to Source Array
;      TEMP0 = Number of Bytes to Append
;Updates: STKLO, STKHI = Stack Pointer
;Returns: A=Number of Bytes Pushed
;           0 = Error: Stack Underflow
STKPSH: JSR SETSRC    ;Set Source Address
STKPSA: STA TEMP0     ;Save Number of Bytes
        JSR STKDST    ;Set Destination to Stack Pointer
        JSR STKALC    ;Allocate Space on Stack
        JMP MEMCPL    ;Copy Bytes and Return

;stkadd(n) - Add N Bytes to Stack Pointer
;Args: A=Number of Bytes to Allocate
;Updates: STKLO, STKHI = Stack Pointer
;Affects: A,C,N,Z
STKADD: CLC 
STKADC: ADC STKLO       ;Add to Stack Pointer LSB
        STA STKLO       ;and Save
        LDA STKHI       ;Add Carry
        ADC #0          ;to Stack Pointer MSB
        STA STKHI       ;and Save
        RTS

;stkalc(n) - Allocate Space on Stack
;Args: A=Number of Bytes to Allocate
;Updates: STKLO, STKHI = Stack Pointer
;Affects: C,N,Z
;Returns: A = Number of Bytes Allocated
;             0 if Error: Pointer Overflow or Length 0 
;         Y=0
STKALC: JSR STKSSP     ;Save Stack Pointer
        LDA TEMP0       ;If No Bytes
        BEQ STKABT      ;  Abort Calling Routine
        JSR STKADD      ;Add to Stack Pointer
        LDA STKEHI      ;If Stack End MSB
        CMP STKHI       ;  < Stack Pointer MSB
        BCC STKRPA      ;  Abort Calling Routine
        BNE STKALX      ;Else if Not Equal
        LDA STKELO      ;and Stack End LSB
        CMP STKLO       ;  < Stack Pointer LSB
        BCC STKRPA      ;  Abort Calling Routine
STKALX: LDA TEMP0       ;Get Number of Bytes from X Register
        LDY #0          ;Initialize Index
        STA (STKLO),Y   ;Store after Allocated Area
                        ;and fall inro STKINC

;stkinc(n) - Increment Stack Pointer
;Sets: STKLO, STKHI = Stack Pointer Address
;Affects: Y,Z,N
STKINC: INC STKLO       ;Increment LSB 
        BNE STKINX      ;If Zero
        INC STKHI       ;  Increment MSB
STKINX: RTS

;stkcmp - Compare Stack Pointer to Stack Start Address
;Uses: STKLO, STKHI = Stack Pointer Address
;      STKSLO, STKSHI = Stack Start Address
;Returns: C=1,Z=1 if Stack Pointer = Stack Start 
;         C=1,Z=0 if Stack Pointer > Stack Start
;         C=0,Z=0 if Stack Pointer < Stack Start
STKCMP: LDA STKHI       ;Compare MSB
        CMP STKSHI
        BCC STKRTS      ;If Pointer<Start Return C=0, Z=1
        BNE STKRTS      ;If Pointer>Start Return C=1. Z=0
        LDA STKLO       ;If MSB Equal
        CMP STKSLO      ;Compare LSB
STKRTS: RTS

;stkdec(n) - Decrement Stack Pointer
;Sets: STKLO, STKHI = Stack Pointer Address
;Affects: X,Z,N
STKDEC: LDX STKLO       ;If LSB
        BNE STKDEL      ;  is Zero
        DEC STKHI       ;  Decrement MSB
STKDEL: DEC STKLO       ;  Decrement LSB
STKDER: RTS

;stksiz() - Get Stack Size in Bytes
;Uses: STKLO,STKHI = Stack Pointer
;Sets: STKSLO, STKSHI = Stack Start Address
;Returns: A = $FF if Stack Size is Non-Zero
;             $00 if Stack is Empty
;         Y,X = Stack Size (MSB, LSB)
STKSIZ: LDA STKLO   ;Subtract 
        SEC         ;Start Address LSB
        SBC STKSLO  ;from Pointer LSB
        TAX         ;and Copy to X
        LDA STKHI   ;Subtract Start MSB
        SBC STKSHI  ;from Pointer MSB
        TAY         ;and Copy to Y
        BCC STKZRO  ;Return FALSE if <=0
        BNE STKTRU  ;Return TRUE if MSB <> 0
        TXA         ;Else
        BEQ STKRTS  ;Return FALSE if LSB = 0 
STKTRU: LDA #$FF    ;Else Return TRUE
        RTS

;stkdrp(&m) - Drop Top Entry from Stack
;Note: Aborts Calling Routine if Error
;Uses: STKSLO, STKSHI = Stack Start Address
;Sets: TEMP1, TEMP2 = Original Stack Pointer
;Updates: STKLO, STKHI = Stack Pointer
;Affects: Y,C,N,Z
;Returns: A=Number of Bytes in Entry
;           0=Error: Stack Underflow
;         Y=0
STKDRP: JSR STKSSP      ;Save Stack Pointer
STKDRN: JSR STKCMP      ;If Stack Pointer
        BCC STKRPA      ;  <= Stack Start
        BEQ STKRPA      ;  Return 0
        JSR STKDEC      ;Decrement Stack Pointer
        LDY #0          ;Initialize Index
        LDA (STKLO),Y   ;Get Number of Bytes in Entry
                        ;and fall into STKSUB

;stksub(n) - Reduce Stack Pointer by A
STKSUB: STA TEMP0       ;Save Number in TEMP0
        LDA STKLO
        SEC             ;Subtract A from LSB
        SBC TEMP0
        STA STKLO
        LDA STKHI       ;Subtract 0 from LSB
        SBC #0          ;For Borrow
        STA STKHI
        JSR STKCMP      ;If Stack Pointer < Start 
        BCC STKRPA      ;  Restore Pointer and Return 0
STKSUE: LDA TEMP0       ;Retrieve Number of Bytes
        RTS
STKRPA: JSR STKRSP      ;Restore Stack Pointer
STKABT: PLA             ;Drop Return Address from Stack
        PLA             ;  Aborting Calling Routine
STKZRO: LDA #0          ;Set A to 0
        RTS

;stkpop(&m) - Pop Top Entry off Stack
;Args: Y,X = Address of Source Array
;Uses: STKSLO, STKSHI = Stack Start Address
;Updates: STKLO, STKHI = Stack Pointer
;Affects: C,N,Z
;Returns: A,Y = Number of Bytes in Entry
;           0 = Error: Stack Underflow
STKPOP: JSR SETDST      ;Set Destination Address
        JSR STKDRP      ;Deallocate Stack Entry
STKPOS: JSR STKSRC      ;Set Source Address to Stack Pointer
        JMP MEMCPL      ;Copy Bytes and Return