C02/include/pointer.a02

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

;ptrini - Pointer Initialization code
;Sets: TEMP0 = Memory Pointer Address
PTRINI: STX TEMP0       ;Save Address LSB
        TAX             ;Copy Memory Pointer to X register
        LDA TEMP0       ;Retrieve Address LSB
        RTS

;ptradr(ptr, &addr) - Return Address Stored in Pointer
;Args: A = Zero Page Pointer Address
;Sets: TEMP0 = Pointer Address
;Affects: N,Z
;Returns: A = Zero Page Pointer Address
;         Y,X = New Pointer Contents
PTRADR: JSR PTRINI      ;Swap A and X Registers
PTRRTN: LDY (1,X)       ;Load Pointer MSB into Y
        LDA (1,X)       ;Load Pointer LSB into A
        TAX             ;and transfer to X
        LDA TEMP0       ;Load Pointer Addres into A
        RTS

        
;ptrset(ptr, &addr) - Store Address in Pointer
;Args: A = Zero Page Pointer Address
;      X,Y = Address to Store
;Sets: TEMP0 = Pointer Address
;Affects: N,Z
;Returns: A = Zero Page Pointer Address
;         Y,X = New Pointer Contents
PTRSET: JSR PTRINI      ;Swap A and X Registers
        STA 0,X         ;Store in Memory Pointer LSB
        STY 1,X         ;Store Address MSB in Memory Pointer MSB
        TXA
        LDX TEMP0
        RTS             ;Return Pointer Address as Result

;ptrcmp(ptr, &addr) - Compare Pointer to Address
;Args: A = Zero Page Pointer Address
;      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: JSR PRTINI      ;Swap A and X Registers
        CPY 1,X         ;Compare High Bytes
        BCC PTRCMN      ;If Pointer > Address, Return 1
        BNE PTRCMF      ;If Pointer < Address Return 1
        CPA 0,X         ;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(ptr) - Read Byte from Pointed to Address
;              and Increment Pointer 
;Args: A = Zero Page Pointer Address
;Affects: N, Z
;Returns: A = Byte at pointer address
;         X = Zero Page Pointer Address
PTRGET: TAX             ;Transfer Pointer Address to X 
        LDA (0,X)       ;Load Value at Pointer
        JMP PTRINN      ;Increment Pointer Value

;ptrinc(ptr) - Increment Pointer Address
;Args: A = Zero Page Pointer Address
;Affects:  N, Z
;Returns: A,X = Zero Page Pointer Address
PTRINC: TAX             ;Transfer Pointer Address to X 
PTRINN: INC (0,X)       ;Increment Pointer Low Byte
        BNE PTRRET      ;If Zero
        INC (1,X)       ;  Increment Pointer High Byte
PTRRET: RTS             ;Return from function

;ptrput(ptr,byte) - Write Byte from Pointer Address
;           and Increment Pointer 
;Args: A = Zero Page Pointer Address
;      Y = Byte to write 
;Affects: N, Z
;Returns: A,X = Zero Page Pointer Address
PTRPUT: TAX             ;Transfer Pointer Address to X 
        STY (0,X)       ;Store Value at Pointed to Address
        JMP PTRINN      ;Increment Pointer

;ptrdec() - Decrement Pointer Address
;Args: A = Zero Page Pointer Address
;Affects:  Y, N, Z
;Returns: A = Zero Page Pointer Address
;         Y,X = New Pointer Contents
PTRDEC: TAX             ;Transfer Pointer Address to X 
        LDY (0,X)       ;Get Pointer Low Byter
        BNE PTRDEL      ;If Zero
        DEC (1,X)       ;  Decrement Pointer High Byte
PTRDEL: DEC (0,X)       ;Decrement Pointer Low Byte
        JMP PTRRTN      ;Set Registers and Return

;ptradd(ptr, &offset) - Add Offset to Pointer
;Args: A = Zero Page Pointer Address
;      Y,X = Number of bytes to add
;Sets: TEMP0 = Pointer Address
;Affects: N, Z
;Returns: A = Zero Page Pointer Address
;         Y,X = New Pointer Contents
PTRADD: JSR PRTINI      ;Swap A and X Registers
        CLC             ;Add Offset Low Byte
        ADC (0,X)       ;to Pointer Low Byte
        STA (0,X)       ;and Save It
        TYA             ;Add Offset High Byte
        ADC (1,X)       ;to Pointer High Byte        
        STA (1,X)       ;and Save It
        JMP PTRRTN      ;Set Registers and Return

;ptrsub(offset) - Subtract Offset from Pointer
;Args: A = Zero Page Pointer Address
;      Y,X = Number of bytes to add
;Sets: TEMP0 = Pointer Address
;      TEMP1 = Offset LSB
;      TEMP2 = Offset MSB
;Affects: N, Z
;Returns: A = Zero Page Pointer Address
;         Y,X = New Pointer Contents
PTRSUB: STY TEMP2       ;Save Offset MSB
        STX TEMP1       ;Save Offset LSB
        STA TEMP0       ;Save Pointer Address
        TAX             ;Transfer Pointer Address to X  
        LDA (0,X)       ;Get Pointer Low Byte
        SEC             ;Subtract Offset LSB
        SBC TEMP1       ;from Pointer LSB
        STA PTRLO       ;and Save It
        LDA (1,X)       ;Subtract Offset MSB
        SBC TEMP1       ;from Pointer MSB
        STA (1,X)       ;and Save It
        JMP PTRRTN      ;Set Registers and Return