*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=*
*                              *
* MATH.LIB                     *
*                              *
* AUTHOR:  NATHAN RIGGS        *
* CONTACT: NATHAN.RIGGS@       *
*          OUTLOOK.COM         *
*                              *
* VERSION:   0.2.0             *
* DATE:      12-DEC-2018       *
* ASSEMBLER: MERLIN 8 PRO      *
* OS:        DOS 3.3           *
* LICENSE:   APACHE 2.0        *
*                              *
* STANDARD INTEGER MATH        *
* LIBRARY FOR 16-BIT VALUES,   *
* SIGNED OR UNSIGNED, AS WELL  *
* AS 16-BIT PRNG ROUTINES.     *
*                              *
*------------------------------*
*                              *
* LIST OF ROUTINES             *
*                              *
* ADDIT16: 16-BIT ADD          *
* SUBT16:  16BIT SUBTRACT      *
* MULT16 : 16BIT MULTIPLY      *
* SDIVD16: 16BIT DIVIDE (SIGN) *
* UDIVD16: 16BIT DIVIDE (UNSN) *
* SREMD16: 16BIT REMNDR (SIGN) *
* UREMD16: 16BIT REMNDR (UNSN) *
* COMP16 : 16BIT COMPARE       *
* RAND16 : RANDOM # 0..65535   *
*                              *
*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=*
*
*``````````````````````````````*
* ADDIT16 :: ADD 16-BIT VALUES *
*-                            -*
* ADDS TWO SIXTEEN BIT VALUES  *
* AND RETURNS A 16 BIT VALUE   *
* IN .Y (LOW) AND .X (HIGH).   *
* CAN BE SIGNED OR UNSIGNED.   *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA  #>1000                 *
*  PHA                         *
*  LDA  #<1000                 *
*  PHA                         *
*  LDA  #>1200 ; ADD2          *
*  PHA                         *
*  LDA  #<1200                 *
*  PHA                         *
*  JSR ADDIT16                 *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* ADDEND 2 LOW BYTE            *
* ADDEND 2 HIGH BYTE           *
* ADDEND 1 LOW BYTE            *
* ADDEND 1 HIGH BYTE           *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
* .Y = LOW BYTE OF RESULT      *
* .X = HIGH BYTE OF RESULT     *
* .A = LOW BYTE OF RET ADDR    *
*                              *
* [RETURN] = RESULT (2 BYTES)  *
* [RETLEN] = 2 (RESULT LENGTH) *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
ADDIT16
*
** GET RETURN ADDR
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :ADD2
         PLA
         STA   :ADD2+1
         PLA
         STA   :ADD1
         PLA
         STA   :ADD1+1
*
         LDA   :ADD1
         CLC
         ADC   :ADD2
         TAY              ; LOW BYTE
         STY   RETURN
         LDA   :ADD1+1
         ADC   :ADD2+1
         TAX              ; HIGH BYTE
         STX   RETURN+1
         LDA   #2
         STA   RETLEN
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         RTS
*
** DATA
*
:ADD1    DS    2
:ADD2    DS    2
*
*``````````````````````````````*
* SUBT16 ::     SUBTRACT WORDS *
*-                            -*
* SUBTRACT ONE 16BIT NUMBER    *
* FROM ANOTHER, RETURNING THE  *
* RESULT IN .Y (LOW), .X (HI). *
* SIGNED OR UNSIGNED.          *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #>1000 ; MINUEND        *
*  PHA                         *
*  LDA #<1000                  *
*  PHA                         *
*  LDA #>500 ; SUBTRAHEND      *
*  PHA                         *
*  LDA #<500                   *
*  PHA                         *
*  JSR SUBT16                  *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF SUBTRAHEND       *
* HIGH BYTE OF SUBTRAHEND      *
* LOW BYTE OF MINUEND          *
* HIGH BYTE OF MINUEND         *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
* .Y = RESULT LOW BYTE         *
* .X = RESULT HIGH BYTE        *
* .A = LOW BYTE OF RET ADDR    *
*                              *
* [RETURN] = RESULT (2 BYTES)  *
* [RETLEN] = 2 (RESULT LENGTH) *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUBT16
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :SUBT
         PLA
         STA   :SUBT+1    ; SUBTRAHEND
         PLA
         STA   :MINU
         PLA
         STA   :MINU+1    ; MINUEND
*
** MINUEND - SUBTRAHEND
*
         LDA   :MINU
         SEC
         SBC   :SUBT
         TAY              ; LOW BYTE
         STY   RETURN
         LDA   :MINU+1
         SBC   :SUBT+1
         TAX              ; HIGH BYTE
         STX   RETURN+1
         LDA   #2
         STA   RETLEN
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         RTS
*
** DATA
*
:MINU    DS    2
:SUBT    DS    2
*
*``````````````````````````````*
* MULT16 ::     MULTIPLY WORDS *
*-                            -*
* PERFORM MULTIPLICATION USING *
* THE SHIFT AND ADD ALGORITHM  *
*                              *
* NOTE: WHILE THIS DOES PASS   *
* THE FULL 32BIT PRODUCT BACK  *
* VIA X AND Y, THIS WOULD      *
* RETURN FALSE RESULTS IF ONE  *
* OF THE PARAMETERS HAD BEEN   *
* SIGNED. THUS, USUALLY ONLY   *
* THE LOW WORD IS USED.        *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #>1000                  *
*  PHA                         *
*  LDA #<1000 ; MCAND          *
*  PHA                         *
*  LDA #>3                     *
*  PHA                         *
*  LDA #<3 ; MLIER             *
*  PHA                         *
*  JSR MUL16 ; = 3000          *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF MULTIPLIER       *
* HIGH BYTE OF MULTIPLIER      *
* LOW BYTE OF MULTIPLICAND     *
* HIGH BYTE OF MULTIPLICAND    *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF PRODUCT          *
* HIGH BYTE OF PRODUCT (16BIT) *
*                              *
* .Y = LOW BYTE OF HIGH WORD   *
* .X = HIGH BYTE OF HIGH WORD  *
* .A = LOW BYTE OF RET ADDR    *
*                              *
* [RETURN] = RESULT (2 BYTES)  *
* [RETLEN] = 2 (RESULT LENGTH) *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
MULT16
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :MLIER
         PLA
         STA   :MLIER+1
         PLA
         STA   :MCAND
         PLA
         STA   :MCAND+1
*
** PERFORM MULTIPLICATION USING THE SHIFT
** AND ADD ALGORITHM, WHICH PRODUCES AN
** UNSIGNED 32-BIT PRODUCT IN :HIPROD AND
** :MLIER WITH :HIPROD BEING THE HIGH WORD.
*
         LDA   #0
         STA   :HPROD     ; ZERO HIGH WORD
         STA   :HPROD+1
         LDX   #17        ; # OF BITS IN MPLIER
                          ; PLUS 1. EXTRA LOOP IS
                          ; TO MOVE LAST CARRY INTO
                          ; THE PRODUCT.
         CLC              ; CLEAR CARRY FOR 1ST TIME
                          ; THROUGH LOOP.
:MULLP
*
** IF NEXT BIT = 1 THEN
** HIPROD = HIPROD+MCAND
*
         ROR   :HPROD+1
         ROR   :HPROD
         ROR   :MLIER+1
         ROR   :MLIER
         BCC   :DECCNT    ; BR IF NEXT BIT OF
                          ; MULTIPLIER IS 0
         CLC              ; NEXT BIT=1 SO ADD MCAND
                          ; TO PRODUCT
         LDA   :MCAND
         ADC   :HPROD
         STA   :HPROD
         LDA   :MCAND+1
         ADC   :HPROD+1
         STA   :HPROD+1   ; CARRY = OVERFLOW
                          ; FROM ADD
:DECCNT
         DEX
         BNE   :MULLP     ; CONTINUE UNTIL DONE

         LDY   :HPROD     ; LOW BYTE OF HIGH WORD
         LDX   :HPROD+1   ; HIGH BYTE OF HIGH WORD
*
** PUSH LOW WORD OF PRODUCT ONTO THE STACK
*
         LDA   :MLIER+1
         TAX
         STX   RETURN+1
         LDA   :MLIER
         TAY
         STY   RETURN
         LDA   :HPROD
         STA   RETURN+2
         LDA   #2
         STA   RETLEN
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         LDA   :HPROD     ;24BIT
         RTS
*
** DATA
*
:MCAND   DS    3
:MLIER   DS    3
:HPROD   DS    2
*
*``````````````````````````````*
* 16-BIT DIVISION:             *
*                              *
* SDIV16, UDIV16, SREM16, AND  *
* UREM16.                      *
*-                            -*
* SDIV16: DIVIDED 2 SIGNED BIT *
* WORDS AND RETURN A 16-BIT    *
* SIGNED QUOTIENT.             *
*                              *
* UDIV16: DIVIDE 2 UNSIGNED    *
* 16BIT WORDS AND RETURN A     *
* 16BIT UNSIGNED QUOTIENT.     *
*                              *
* SREM16: DIVIDE 2 SIGNED      *
* 16BIT WORDS AND RETURN A     *
* 16BIT SIGNED REMAINDER.      *
*                              *
* UREM16: DIVIDE 2 UNSIGNED    *
* 16BIT WORKDS AND RETURN A    *
* 16BIT UNSIGNED REMAINDER.    *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
** ALL ROUTINES USE THE SAME   *
** FORMAT.                     *
*                              *
*  LDA #>10000                 *
*  PHA                         *
*  LDA #<10000 ; DIVIDEND      *
*  PHA                         *
*  LDA #>1000 ; DIVISOR        *
*  PHA                         *
*  LDA #<1000                  *
*  PHA                         *
*  JSR UDIV16                  *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF DIVISOR          *
* HIGH BYTE OF DIVISOR         *
* LOW BYTE OF DIVIDEND         *
* HIGH BYTE OF DIVIDEND        *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
* IF NO ERRORS, CARRY = 0,     *
* ELSE CARRY=1, QUOTIENT=0,    *
* AND REMAINDER=0              *
*                              *
* .Y = COUNTER; TRASH          *
* .X = COUNTER; TRASH          *
* .A = LOW BYTE OF RET ADDR    *
*                              *
* [RETURN] = RESULT (2 BYTES)  *
* [RETLEN] = 2 (RESULT LENGTH) *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, IT MAY FALL UNDER A *
* DIFFERENT LICENSE UNTIL IT   *
* IS RADICALLY REWORKED.       *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
UDIVD16
         LDA   #0
         BEQ   UDIVMD
UREMD16
         LDA   #2
UDIVMD
         STA   _RSLTI
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   _DVSOR
         PLA
         STA   _DVSOR+1
         PLA
         STA   _DVEND
         PLA
         STA   _DVEND+1
*
         JSR   UDIV
         BCC   DIVOK      ; BR IF NO ERR
DIVERR   JMP   EREXIT
DIVOK    JMP   OKEXIT
*
** SIGNED DIVISION
*
SDIVD16
         LDA   #0         ; RESULT IS QUOTIENT
         BEQ   SDIVMD      ;   (INDEX=0)
*
** SIGNED REMAINDER
*
SREMD16
         LDA   #2         ; RES = REMAINDER (I=2)
         BNE   SDIVMD
*
SDIVMD
         STA   _RSLTI     ;RESULT INDEX;0=Q,2=R
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   _DVSOR
         PLA
         STA   _DVSOR+1
         PLA
         STA   _DVEND
         PLA
         STA   _DVEND+1
*
*
** DETERMINE SIGN OF QUOTIENT BY
** PERFORMING AN EXCLUSIVE OR OF
** THE HIGH BYTES. IF THE SIGNS
** ARE THE SAME THEN BIT 7 WILL
** BE 0 AND THE QUOTIENT IS
** POSITIVE. IF THE SIGNS ARE
** DIFFERENT THEN THE QUOTIENT
** IS NEGATIVE.
*
         LDA   _DVEND+1
         EOR   _DVSOR+1
         STA   _SQUOT
*
** SIGN OF REMAINDER IS THE SIGN
** OF THE DIVIDEND
*
         LDA   _DVEND+1
         STA   _SREMD
*
** TAKE THE ABSOLUTE VALUE OF
** THE DIVISOR
*
         LDA   _DVSOR+1
         BPL   CHKDE       ; BR IF ALREADY POS
         LDA   #0         ; SUB DVSOR FROM ZERO
         SEC
         SBC   _DVSOR
         STA   _DVSOR
         LDA   #0
         SBC   _DVSOR+1
         STA   _DVSOR+1
*
** TAKE ABS VALUE OF THE DIVIDEND
*
CHKDE
         LDA   _DVEND+1
         BPL   DODIV      ; BR IF DVEND IS POS
         LDA   #0         ; SUB DVEND FROM ZERO
         SEC
         SBC   _DVEND
         STA   _DVEND
         LDA   #0
         SBC   _DVEND+1
         STA   _DVEND+1
*
** DIVIDE ABS VALUES
*
DODIV
         JSR   UDIV
         BCS   EREXIT     ; EXIT IF DIV BY 0
*
** NEGATE QUOTIENT IF IT IS NEGATIVE
*
         LDA   _SQUOT
         BPL   DOREM      ; BR IF Q IS POS
         LDA   #0
         SEC
         SBC   _DVEND
         STA   _DVEND
         LDA   #0
         SBC   _DVEND+1
         STA   _DVEND+1
*
DOREM
*
** NEGATE REMAINDER IF IT IS NEG
*
         LDA   _SREMD
         BPL   OKEXIT     ; BR IF REM IS POS
         LDA   #0
         SEC
         SBC   _DVEND+2
         STA   _DVEND+2
         LDA   #0
         SBC   _DVEND+3
         STA   _DVEND+3
         JMP   OKEXIT
*
** ERROR EXIT (CARRY=1, RSLTS ARE 0)
*
EREXIT
         LDA   #0
         STA   _DVEND
         STA   _DVEND+1   ;QUOTIENT = 0
         STA   _DVEND+2
         STA   _DVEND+3   ; REMAINDER=0
         STA   RETURN
         STA   RETURN+1
         LDA   #2
         STA   RETLEN
         SEC              ; CARRY=1 IF ERROR
         BCS   DVEXIT
*
** GOOD EXIT (CARRY = 0)
*
OKEXIT
         CLC              ; CARRY = 0, NO ERRORS
*
DVEXIT
*
** STORE RESULT
*
         LDX   _RSLTI     ;GET INDEX TO RESULT
                          ; 0=QUOTIENT, 2=REMAINDER
*
** STORE RESULT
*
         LDA   _DVEND,X
         TAY
         LDA   _DVEND+1,X
         TAX
         STY   RETURN
         STX   RETURN+1
         LDA   #2
         STA   RETLEN
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         RTS
*
********************************
* UDIV ROUTINE
********************************
UDIV
*
** ZERO UPPER WORD DIVIDEND
** THIS WILL BE CALLED
** DIVIDEND(1) BELOW
*
         LDA   #0
         STA   _DVEND+2
         STA   _DVEND+3
*
** FIRST CHECK FOR DIV BY 0
*
         LDA   _DVSOR
         ORA   _DVSOR+1
         BNE   OKUDIV     ; BR IF DVSOR NOT 0
         SEC
         RTS
*
** PERFORM THE DIVISION BY
** TRIAL SUBTRACTIONS
*
OKUDIV
         LDX   #16        ; LOOP THROUGH 16 BITS
DIVLP
         ROL   _DVEND     ;SHFT CARRY INTO BIT 0 OF DVEND
         ROL   _DVEND+1   ;WHICH WILL BE THE QUOTIENT AND
         ROL   _DVEND+2   ;SHFT DVEND AT THE SAME TIME
         ROL   _DVEND+3
CHKLT
         SEC
         LDA   _DVEND+2
         SBC   _DVSOR
         TAY              ; SAVE LOW BYTE IN Y
         LDA   _DVEND+3
         SBC   _DVSOR+1   ;SUB HIGHBYTES W RES IN A
         BCC   DECCNT     ; BR IF DVEND < DVSOR AND CARRY
         STY   _DVEND+2   ; ELSE
         STA   _DVEND+3   ;VEN(1)=DVEND(1)-DVSOR
*
DECCNT
         DEX
         BNE   DIVLP
*
         ROL   _DVEND     ;SHFT IN LAST CAR FOR QUOT
         ROL   _DVEND+1
         CLC              ; NO ERRORS, CLEAR CARRY
         RTS
*
** DATA
*
_DVSOR   DS    2          ; DIVISOR
_DVEND   DS    4          ; DIVIDEND[0] AND QUOTIENT
                          ; DIVIDEND[1] AND REMAINDER
_SQUOT   DS    1          ; SIGN OF QUOTIENT
_SREMD   DS    1          ; SIGN OF REMAINDER
_RSLTI   DS    1          ; RESULT INDEX
*
*``````````````````````````````*
* COMP16 ::     16-BIT COMPARE *
*-                            -*
* COMPARE TWO 16BIT SIGNED OR  *
* UNSIGNED WORDS AND RETURN    *
* THE C,Z,N FLAGS AS SET OR    *
* CLEARED.                     *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  CZN-----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #>123                   *
*  PHA                         *
*  LDA #<123                   *
*  PHA         ; WORD 1        *
*  LDA #>1023                  *
*  PHA                         *
*  LDA #<1023                  *
*  PHA         ; WORD 2        *
*  JSR CMP16                   *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF WORD 2 (SUBTRA)  *
* HIGH BYTE OF WORD 2          *
* LOW BYTE OF WORD 1 (MINU)    *
* HIGH BYTE OF WORD 1          *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* FLAGS RETURNED BASED ON WORD *
* 1 - WORD 2.                  *
*                              *
* IF W1 & W2 ARE 2S COMPLEMENT *
*    IF W1 = W2 Z=1,N=0        *
*    IF W1 > W2 Z=0,N=0        *
*    IF W1 < W2 Z=0,N=1        *
* ELSE                         *
*    IF W1 = W2 Z=1,C=1        *
*    IF W1 > W2 Z=0,C=1        *
*    IF W1 < W2 Z=0,C=0        *
*                              *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THE SAME LICENSE    *
* MAY NOT APPLY UNTIL RADICAL  *
* ALTERATION.                  *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
COMP16
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :SUBT      ; SUBTRAHEND
         PLA
         STA   :SUBT+1
         PLA
         STA   :MINU      ; MINUEND
         PLA
         STA   :MINU+1
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         LDA   :MINU
         CMP   :SUBT      ; COMPARE LOW BYTES
         BEQ   :EQUAL
*
** LOW BYTES ARE NOTE EQUAL
** COMPARE HIGH BYTES
*
         LDA   :MINU+1
         SBC   :SUBT+1    ; COMPARE HIGH BYTES
         ORA   #1         ; MAKE Z=0, SINCE LOW
                          ; BYTES ARE NOT EQUAL
         BVS   :OVFLOW    ; MUST HANDLE OVERFLOW
                          ; FOR SIGNED MATH
         RTS              ; EXIT
*
** LOW BYTES ARE UQAL -- COMPARE HIGH
*
:EQUAL
         LDA   :MINU+1
         SBC   :SUBT+1    ; UPPER BYTES
         BVS   :OVFLOW
         RTS              ; RETURN W FLAGS SET
*
**
** OVERFLOW WITH SIGNED ARITHMETIC SO
** COMPLEMENT THE NEGATIVE FLAG.
** DO NO CHANGE THE CARRY FLAG AND
** MAKE THE ZERO FLAG EQUAL TO 0.
** COMPLEMENT NEG FLAG BY EORING
** #$80 AND ACCUMULATOR.
**
*
:OVFLOW
         EOR   #$80       ; COMPLEMENT N FLAG
         ORA   #1         ; IF OVERFLOW THEN THE
                          ; WORDS ARE !=. Z= 0
                          ; CARRY UNCHANGED
         RTS
*
** DATA
*
:MINU    DS    2
:SUBT    DS    2
*
*``````````````````````````````*
* RAND16 : 16BIT RANDOM NUMBER *
*-                            -*
* GENERATE A 16BIT PSEUDO-     *
* RANDOM NUMBER AND RETURN IT  *
* IN Y,X (LOW, HIGH).          *
*                              *
* ORIGINAL AUTHOR IS WHITE     *
* FLAME, AS SHARED ON          *
* CODEBASE64.                  *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*-                            -*
* ENTRY                        *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HIGH BYTE OF RETURN ADDRESS  *
*-                            -*
* EXIT                         *
*                              *
* .Y = HIGH BYTE OF PRODUCT    *
* .X = LOW BYTE OF PRODUCT     *
* .A = LOW BYTE OF PRODUCT     *
*                              *
* [RETURN] = RESULT (2 BYTES)  *
* [RETLEN] = 2 (RESULT LENGTH) *
*-                            -*
* NOTE: THERE ARE 2048 MAGIC   *
* NUMBERS THAT COULD BE EOR'D  *
* TO GENERATE A PSEUDO-RANDOM  *
* PATTERN THAT DOESN'T REPEAT  *
* UNTIL 65535 ITERATIONS. TOO  *
* MANY TO LIST HERE, BUT SOME  *
* ARE: $002D, $1979, $1B47,    *
* $41BB, $3D91, $B5E9, $FFEB   *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
RAND16
*
         LDA   RNDL
         STA   :SEED
         LDA   RNDH
         STA   :SEED+1
*
         LDA   :SEED
         BEQ   :LOW0
*
** DO A NORMAL SHIFT
*
         ASL   :SEED
         LDA   :SEED+1
         ROL
         BCC   :NOEOR
:DOEOR                    ; HIGH BYTE IN A
         EOR   #>$0369
         STA   :SEED+1
         LDA   :SEED
         EOR   #<$0369
         STA   :SEED
         JMP   :EXIT
:LOW0
         LDA   :SEED+1
         BEQ   :DOEOR
         ASL
         BEQ   :NOEOR
         BCS   :DOEOR
:NOEOR
         STA   :SEED+1
:EXIT    LDX   :SEED+1
         LDY   :SEED
         STY   RETURN
         STX   RETURN+1
         LDA   #2
         STA   RETLEN
         RTS
*
** DATA
*
:SEED    DS    2
*
*``````````````````````````````*
* RANDW :: RANDOM WORD         *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
*
** THIS WILL BE INCLUDED IN FUTURE UPDATES
*
*