C02/include/pointer.a02

;C02 library pointer.h02 assembly language subroutines
;Requires External Zero Page Variables PTRLO, PTRHI
;External Routines MSETSRC

;ptrset(&a) - Set Pointer Address
;Args: X,Y = Address
;Sets: PTRLO, PTRHI = Pointer
PTRSET: STX PTRLO  ;Save Block Start Low Byte
        STY PTRHI  ;Save Block Start High Byte
        RTS

;ptrcmp(&a) - Compare Pointer to Address
;Requires: PTRLO, PTRHI - Pointer
;Args: X,Y = Address to Compare Against
;Affects N,Z
;Returns A=$01 and C=? if Pointer > Address  
;        A=$00 and Z=1, C=? if Pointer = Address  
;        A=$FF and C=? if Pointer < Address
PTRCMP: CPY PTRHI  ;Compare High Bytes
        BCC PTRCMN ;If Pointer > Address, Return 1
        BNE PTRCMF ;If Pointer < Address Return 1
        CPX PTRLO  ;Compare Low Byte
        BCC PTRCMN ;If Pointer > Address, Return 1
        BNE PTRCMF ;If Pointer < Address Return 1
PTRCMZ: LDA #$00   ;Return 0
        RTS
PTRCMN: LDA #$01   ;Return 1
        RTS
PTRCMF: LDA #$FF   ;Return 255
        RTS

;ptrget() - Read Byte from Pointer Address
;           and Increment Pointer 
;Sets: PTRLO, PTRSHI = New pointer address
;Returns: A = Byte at pointer address
;Affects:  Y, N, Z
PTRGET: LDY #0         ;Set Offset to 0
        LDA (PTRLO),Y  ;Load Value at Pointer

;ptrinc() - Increment Pointer Address
;Sets: PTRLO, PTRSHI = New pointer address
;Affects:  N, Z
PTRINC: INC PTRLO   ;Increment Pointer Low Byte
        BNE PTRRET  ;If Zero
        INC PTRHI   ;  Increment Pointer High Byte
PTRRET: RTS         ;Return from function

;ptrput() - Write Byte from Pointer Address
;           and Increment Pointer 
;Args: A = Byte to write 
;Sets: PTRLO, PTRSHI = New pointer address
;Affects: Y, N, Z
PTRPUT: LDY #0         ;Set Offset to 0
        STA (PTRLO),Y  ;Load Value at Pointer
        JMP PTRINC     ;Increment Pointer

;ptrdec() - Decrement Pointer Address
;Sets: PTRLO, PTRSHI = New pointer address
;Affects:  Y, N, Z
PTRDEC: LDY PTRLO   ;Get Pointer Low Byter
        BNE PTRDEL  ;If Zero
        DEC PTRHI   ;  Deccrement Pointer High Byte
PTRDEL: DEC PTRLO   ;Decrement Pointer Low Byte
        RTS         ;Return from function

;ptradd() - Add Offset to Pointer
;Args: A = Number of bytes to add
;Sets: PTRLO, PTRSHI = New pointer address
;Affects: A, N, Z
PTRADD: CLC            ;Add Offset to Low Byte
        ADC PTRLO
        STA PTRLO      ;And Save It
        BCC PTRRET     ;If Carry 
        INC PTRHI      ;  Increment High Byte
        RTS

;ptrsub() - Subtract Offset from Pointer
;Args: A = Number of bytes to subtract
;Sets: PTRLO, PTRSHI = New pointer address
;Affects: A, N, Z
PTRSUB: STA TEMP0      ;Save Offset
        LDA PTRLO      ;Get Pointer Low Byte
        SEC            ;Subtract Offset
        SBC TEMP0      ;
        STA PTRLO      ;And Save New Low Byte
        BCS PTRRET     ;If Borrow
        DEC PTRHI      ;  Decrement High Byte
        RTS