!!!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
}}}