; C02 library stdlib.h02 assembly language subroutines
; Requires 
; external zero page locations SRCLO and srchi
; and external locations RANDOM, RDSEED, TEMP0, TEMP1, and TEMP2. 

;Return ABSolute Value of Accumulator
ABS:    CMP #$80        ;If Negative (High Bit Set)
        BCC ABSX        ;  Carry will Already be Set
        EOR #$FF        ;  One's Complement
        ADC #$00        ;  and Increment (Carry set by CMP)
ABSX:   RTS

;Return Higher of A and Y
;Uses TEMP0
;Affects A,N,Z,C
MAX:    STY TEMP0       ;Save Second Parameter
        CMP TEMP0       ;If First Parameter
        BCC MAXX        ;  Greater than Second Parameter
        TYA             ;Copy Second Parameter into Accumulator  
MAXX:   RTS

;Return Lower of A and &
;Uses TEMP0
;Affects A,N,Z,C
MIN:    STY TEMP0       ;Save Second Parameter
        CMP TEMP0       ;If First Parameter
        BCS MINX        ;  Less than Second Parameter
        TYA             ;Copy Second Parameter into Accumulator  
MINX:   RTS

;Multiply A times Y
;Uses TEMP0, TEMP1
;Affects A,N,Z,C
MULT:   STA TEMP0       ;Store Multiplicand
        STY TEMP1       ;Store Multiplier
;Multiply TEMP0 times TEMP1
MULTT:  LDA #$00        ;Initialize Accumulator
        BEQ MULTE       ;Enter Loop
MULTA:  CLC
        ADC TEMP0       ;Add Multiplicand
MULTL:  ASL TEMP0       ;Shift Multiplicand Left
MULTE:  LSR TEMP1       ;Shift Multiplier Right
        BCS MULTA       ;If Bit Shifted Out, Add Multiplicand
        BNE MULTL       ;Loop if Any 1 Bits Left
        RTS

;Divide A by Y
;Used TEMP0,TEMP1
;Affects A,X,N,Z,C
DIV:    STA TEMP0       ;Store Dividend
        STY TEMP1       ;Store Divisor
;Divide TEMP0 by TEMP1
DIVT:   LDA #$00        ;Clear Accumulator
        LDX #$07        ;Load Loop Counter
        CLC
DIVL:   ROL TEMP0       ;Shift Bit Out of Dividend
        ROL             ;  into Accumulator
        CMP TEMP1       ;If Accumulator 
        BCC DIVS        ;  >= Divisor
        SBC TEMP1       ;Subtract Divisor
DIVS:   DEX             ;Decrement Counter
        BPL DIVL        ;  and Loop
        ROL TEMP0       ;Shift Result into Dividend
        LDA TEMP0       ;Load Result into Accumulator
        RTS

;Generate Pseudo-Random Number between 1 and 255
;Uses RANDOM (must be non-zero on entry)
;Affects A,N,Z,C
RAND:   LDA RANDOM      ;Load Last Result
        ASL             ;Shift the Seed
        BCC RANDX       ;If a one was shifted out
        EOR #$1D        ;  Twiddle the bite
RANDX:  STA RANDOM      ;Save the Seed
        RTS

;Seed Pseudo-Random Number Generator
;Uses RDSEED (if A is zero)
;Affects A,N,Z,C
;Sets RANDOM
RANDS:  ORA #$00        ;If Passed Value not 0
        BNE RANDX       ;  Store in Seed and Return
        LDA RDSEED      ;Load System Generated Seed
        BNE RANDX       ;If Not 0, Store and Return
        ADC #$01        ;Else Add 1 or 2 
        BNE RANDX       ;  then Store and Return

;Return A Shifted Y Bytes to the Left
;Affects A,Y,N,Z,C
SHIFTL: ASL             ;Shift Byte to Left
        DEY             ;Decrement Counter
        BNE SHIFTL      ;  and Loop if Not 0
        RTS

;Return A Shifted Y Bytes to the Right
;Affects A,Y,N,Z,C
SHIFTR: LSR             ;Shift Byte to Right
        DEY             ;Decrement Counter
        BNE SHIFTL      ;  and Loop if Not 0
        RTS

;Convert ASCII to Byte
;Uses TEMP0, TEMP1
;Affects A,X,Y,N,Z,C
ATOC:   JSR SETSRC      ;Initialize Source String
        STY TEMP0       ;Initialize Result
ATOCL:  LDA (SRCLO),Y   ;Get Next Character
        CMP #$30        ;If Less Than '0'
        BCC ATOCX       ;  Exit
        CMP #$3A        ;If Greater Than '9'
        BCS ATOCX       ;  Exit
        AND #$0F        ;Convert to Binary Nybble
        STA TEMP1       ;  and Save It
        LDA TEMP0       ;Load Result
        ASL             ;Multiply by 5 by
        ASL             ;  Multiplying by 4
        ADC TEMP0       ;  And Adding Itself
        ASL             ;Multiply that by 2  
        ADC TEMP1       ;Add Saved Nybble
        STA TEMP0       ;  and Store Result
        INY             ;Increment Index
        BPL ATOCL       ;  and Loop
ATOCX:  LDA TEMP0       ;Load Result
        RTS             ;And Return

;Convert Byte to ASCII
;Uses SRCLO, srchi, TEMP0, TEMP1, TEMP2
;Affects A,X,Y,N,Z
CTOA:   JSR SETSRC      ;Initialize Source String
        JSR CVBCD      ;Convert Accumulator to BCD
        ;LDY #$00       ;Set String Offset to 0
        LDA TEMP2      ;Get High Byte
        BEQ CTOAL      ;If Not Zero
        JSR CTOAN       ;  Convert Low Nybble
CTOAL:  LDA TEMP1      ;Get Low Byte
        LSR            ;Shift High Nybble
        LSR            ;  into Low Nybble
        LSR
        LSR
        BNE CTOA2      ;If Not Zero
        CMP TEMP2      ;  and High Byte 
        BEQ CTOA3      ;  not Zero   
CTOA2:  JSR CTOAN      ;  Convert It
CTOA3:  LDA TEMP1      ;Get Low Byte
        JSR CTOAN      ;and Convert Low Nybble
        LDA #$00
        BEQ CTOAX      ;Terminate String
CTOAN:  AND #$0F       ;Strip High Nybble
        ORA #$30       ;Convert to ASCII digit
CTOAX:  STA (SRCLO),Y  ;Store in String
        INY            ;and Increment Offset
        RTS  

;Convert Binary Number in Accumulator to Binary Coded Decimal
;Uses TEMP0
;Returns BCD in TEMP1,temp and A,X = 0
CVBCD:  STA TEMP0  ;Save Binary Value
        LDA #0     ;Clear BCD Bytes
        STA TEMP1
        STA TEMP2
        LDX #8     ;Process 8 bits of Binary
        SEI        ;Disable Interupts
        SED        ;Set Decimal Mode
CVBCDL: ASL TEMP0  ;Shift High Bit Into Carry
        LDA TEMP1  ;Add BCD Low Byte to Itself 
        ADC TEMP1  ;  Plus Bit Shifted out of Binary
        STA TEMP1  ;  Effectively Multiplying It by 2
        LDA TEMP2  ;Add BCD High Byte to Itself
        ADC TEMP2  ;  Plus Bit Shifted out of Low Byte
        STA TEMP2  ;  Effectively Multiplying It by 2
        DEX        ;Decrement Counter and
        BNE CVBCDL ;  Process Next Bit
        CLD        ;Clear Decimal Mode
        CLI        ;Enable Interrupts
        RTS