Sweet 16 Apple2#

General Information

Author: Steve "Woz" Wozniak
Assembler: generic
Published: 1977

See tutorial and reference in the main article at Sweet 16.

********************************
*                             *
*  APPLE-II  PSEUDO MACHINE   *
*        INTERPRETER        *
*                             *
*     COPYRIGHT (C) 1977      *
*    APPLE COMPUTER,  INC    *
*                             *
*    ALL  RIGHTS RESERVED    *
*                             *
*        S. WOZNIAK          *
*                             *
********************************
*                             *
* TITLE:  SWEET 16 INTERPRETER *
*                             *
********************************

R0L     EQU  $0
R0H     EQU  $1
R14H   EQU  $1D
R15L   EQU  $1E
R15H   EQU  $1F
SAVE   EQU  $FF4A
RESTORE EQU  $FF3F

        ORG  $F689

        AST  32

        JSR  SAVE         ;PRESERVE 6502 REG CONTENTS
        PLA
        STA  R15L         ;INIT SWEET16 PC
        PLA               ;FROM RETURN
        STA  R15H         ;ADDRESS
SW16B JSR  SW16C         ;INTERPRET AND EXECUTE
        JMP  SW16B          ;ONE SWEET16 INSTR.
SW16C INC  R15L
        BNE  SW16D          ;INCR SWEET16 PC FOR FETCH
        INC  R15H
SW16D LDA  >SET           ;COMMON HIGH BYTE FOR ALL ROUTINES
        PHA               ;PUSH ON STACK FOR RTS
        LDY  $0
        LDA  (R15L),Y       ;FETCH INSTR
        AND  $F             ;MASK REG SPECIFICATION
        ASL               ;DOUBLE FOR TWO BYTE REGISTERS
        TAX               ;TO X REG FOR INDEXING
        LSR
        EOR  (R15L),Y       ;NOW HAVE OPCODE
        BEQ  TOBR         ;IF ZERO THEN NON-REG OP
        STX  R14H         ;INDICATE "PRIOR RESULT REG"
        LSR
        LSR               ;OPCODE*2 TO LSB'S
        LSR
        TAY               ;TO Y REG FOR INDEXING
        LDA  OPTBL-2,Y     ;LOW ORDER ADR BYTE
        PHA               ;ONTO STACK
        RTS               ;GOTO REG-OP ROUTINE
TOBR   INC  R15L
        BNE  TOBR2          ;INCR PC
        INC  R15H
TOBR2 LDA  BRTBL,X        ;LOW ORDER ADR BYTE
        PHA               ;ONTO STACK FOR NON-REG OP
        LDA  R14H         ;"PRIOR RESULT REG" INDEX
        LSR               ;PREPARE CARRY FOR BC, BNC.
        RTS               ;GOTO NON-REG OP ROUTINE
RTNZ   PLA                ;POP RETURN ADDRESS
        PLA
        JSR  RESTORE      ;RESTORE 6502 REG CONTENTS
        JMP  (R15L)        ;RETURN TO 6502 CODE VIA PC
SETZ   LDA  (R15L),Y     ;HIGH ORDER BYTE OF CONSTANT
        STA  R0H,X
        DEY
        LDA  (R15L),Y       ;LOW ORDER BYTE OF CONSTANT
        STA  R0L,X
        TYA               ;Y REG CONTAINS 1
        SEC
        ADC  R15L         ;ADD 2 TO PC
        STA  R15L
        BCC  SET2
        INC  R15H
SET2   RTS
OPTBL DFB  SET-1         ;1X
BRTBL DFB  RTN-1         ;0
        DFB  LD-1         ;2X
        DFB  BR-1         ;1
        DFB  ST-1         ;3X
        DFB  BNC-1          ;2
        DFB  LDAT-1        ;4X
        DFB  BC-1         ;3
        DFB  STAT-1        ;5X
        DFB  BP-1         ;4
        DFB  LDDAT-1      ;6X
        DFB  BM-1         ;5
        DFB  STDAT-1      ;7X
        DFB  BZ-1         ;6
        DFB  POP-1          ;8X
        DFB  BNZ-1          ;7
        DFB  STPAT-1      ;9X
        DFB  BM1-1          ;8
        DFB  ADD-1          ;AX
        DFB  BNM1-1        ;9
        DFB  SUB-1          ;BX
        DFB  BK-1         ;A
        DFB  POPD-1        ;CX
        DFB  RS-1         ;B
        DFB  CPR-1          ;DX
        DFB  BS-1         ;C
        DFB  INR-1          ;EX
        DFB  NUL-1          ;D
        DFB  DCR-1          ;FX
        DFB  NUL-1          ;E
        DFB  NUL-1          ;UNUSED
        DFB  NUL-1          ;F

* FOLLOWING CODE MUST BE
* CONTAINED ON A SINGLE PAGE!

SET     BPL  SETZ         ;ALWAYS TAKEN
LD    LDA  R0L,X
BK    EQU  *-1
        STA  R0L
        LDA  R0H,X          ;MOVE RX TO R0
        STA  R0H
        RTS
ST    LDA  R0L
        STA  R0L,X          ;MOVE R0 TO RX
        LDA  R0H
        STA  R0H,X
        RTS
STAT   LDA  R0L
STAT2 STA  (R0L,X)        ;STORE BYTE INDIRECT
        LDY  $0
STAT3 STY  R14H           ;INDICATE R0 IS RESULT NEG
INR     INC  R0L,X
        BNE  INR2         ;INCR RX
        INC  R0H,X
INR2   RTS
LDAT   LDA  (R0L,X)       ;LOAD INDIRECT (RX)
        STA  R0L           ;TO R0
        LDY  $0
        STY  R0H           ;ZERO HIGH ORDER R0 BYTE
        BEQ  STAT3          ;ALWAYS TAKEN
POP     LDY  $0             ;HIGH ORDER BYTE = 0
        BEQ  POP2         ;ALWAYS TAKEN
POPD   JSR  DCR            ;DECR RX
        LDA  (R0L,X)      ;POP HIGH ORDER BYTE @RX
        TAY               ;SAVE IN Y REG
POP2   JSR  DCR            ;DECR RX
        LDA  (R0L,X)      ;LOW ORDER BYTE
        STA  R0L           ;TO R0
        STY  R0H
POP3   LDY  $0           ;INDICATE R0 AS LAST RESULT REG
        STY  R14H
        RTS
LDDAT JSR  LDAT           ;LOW ORDER BYTE TO R0, INCR RX
        LDA  (R0L,X)      ;HIGH ORDER BYTE TO R0
        STA  R0H
        JMP  INR           ;INCR RX
STDAT JSR  STAT           ;STORE INDIRECT LOW ORDER
        LDA  R0H           ;BYTE AND INCR RX. THEN
        STA  (R0L,X)      ;STORE HIGH ORDER BYTE.
        JMP  INR           ;INCR RX AND RETURN
STPAT JSR  DCR          ;DECR RX
        LDA  R0L
        STA  (R0L,X)      ;STORE R0 LOW BYTE @RX
        JMP  POP3         ;INDICATE R0 AS LAST RESULT REG
DCR     LDA  R0L,X
        BNE  DCR2         ;DECR RX
        DEC  R0H,X
DCR2   DEC  R0L,X
        RTS
SUB     LDY  $0             ;RESULT TO R0
        CPR  SEC           ;NOTE Y REG = 13*2 FOR CPR
        LDA  R0L
        SBC  R0L,X
        STA  R0L,Y          ;R0-RX TO RY
        LDA  R0H
        SBC  R0H,X
SUB2   STA  R0H,Y
        TYA               ;LAST RESULT REG*2
        ADC  $0             ;CARRY TO LSB
        STA  R14H
        RTS
ADD     LDA  R0L
        ADC  R0L,X
        STA  R0L           ;R0+RX TO R0
        LDA  R0H
        ADC  R0H,X
        LDY  $0             ;R0 FOR RESULT
        BEQ  SUB2         ;FINISH ADD
BS    LDA  R15L           ;NOTE X REG IS 12*2!
        JSR  STAT2          ;PUSH LOW PC BYTE VIA R12
        LDA  R15H
        JSR  STAT2          ;PUSH HIGH ORDER PC BYTE
BR    CLC
BNC     BCS  BNC2         ;NO CARRY TEST
BR1     LDA  (R15L),Y       ;DISPLACEMENT BYTE
        BPL  BR2
        DEY
BR2     ADC  R15L         ;ADD TO PC
        STA  R15L
        TYA
        ADC  R15H
        STA  R15H
BNC2   RTS
BC    BCS  BR
        RTS
BP    ASL                 ;DOUBLE RESULT-REG INDEX
        TAX               ;TO X REG FOR INDEXING
        LDA  R0H,X          ;TEST FOR PLUS
        BPL  BR1           ;BRANCH IF SO
        RTS
BM    ASL                 ;DOUBLE RESULT-REG INDEX
        TAX
        LDA  R0H,X          ;TEST FOR MINUS
        BMI  BR1
        RTS
BZ    ASL                 ;DOUBLE RESULT-REG INDEX
        TAX
        LDA  R0L,X          ;TEST FOR ZERO
        ORA  R0H,X          ;(BOTH BYTES)
        BEQ  BR1           ;BRANCH IF SO
        RTS
BNZ     ASL               ;DOUBLE RESULT-REG INDEX
        TAX
        LDA  R0L,X          ;TEST FOR NON-ZERO
        ORA  R0H,X          ;(BOTH BYTES)
        BNE  BR1           ;BRANCH IF SO
        RTS
BM1     ASL               ;DOUBLE RESULT-REG INDEX
        TAX
        LDA  R0L,X          ;CHECK BOTH BYTES
        AND  R0H,X          ;FOR $FF (MINUS 1)
        EOR  $FF
        BEQ  BR1           ;BRANCH IF SO
        RTS
BNM1   ASL                ;DOUBLE RESULT-REG INDEX
        TAX
        LDA  R0L,X
        AND  R0H,X          ;CHECK BOTH BYTES FOR NO $FF
        EOR  $FF
        BNE  BR1           ;BRANCH IF NOT MINUS 1
NUL     RTS
RS    LDX  $18          ;12*2 FOR R12 AS STACK POINTER
        JSR  DCR           ;DECR STACK POINTER
        LDA  (R0L,X)      ;POP HIGH RETURN ADDRESS TO PC
        STA  R15H
        JSR  DCR           ;SAME FOR LOW ORDER BYTE
        LDA  (R0L,X)
        STA  R15L
        RTS
RTN     JMP  RTNZ

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« This page (revision-1) was last changed on 14-Mar-2010 15:00 by Carsten Strotmann