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+====== XMODEM/CRC receive ======
+<code>
+; XMODEM/CRC Receiver for the 65C02
+;
+; By Daryl Rictor & Ross Archer  Aug 2002
+;
+; 21st century code for 20th century CPUs (tm?)
+;
+; A simple file transfer program to allow upload from a console device
+; to the SBC utilizing the x-modem/CRC transfer protocol.  Requires just
+; under 1k of either RAM or ROM, 132 bytes of RAM for the receive buffer,
+; and 8 bytes of zero page RAM for variable storage.
+;
+;**************************************************************************
+; This implementation of XMODEM/CRC does NOT conform strictly to the
+; XMODEM protocol standard in that it (1) does not accurately time character
+; reception or (2) fall back to the Checksum mode.
+; (1) For timing, it uses a crude timing loop to provide approximate
+; delays.  These have been calibrated against a 1MHz CPU clock.  I have
+; found that CPU clock speed of up to 5MHz also work but may not in
+; every case.  Windows HyperTerminal worked quite well at both speeds!
+;
+; (2) Most modern terminal programs support XMODEM/CRC which can detect a
+; wider range of transmission errors so the fallback to the simple checksum
+; calculation was not implemented to save space.
+;**************************************************************************
+;
+; Files uploaded via XMODEM-CRC must be
+; in .o64 format -- the first two bytes are the load address in
+; little-endian format:
+;  FIRST BLOCK
+;     offset(0) = lo(load start address),
+;     offset(1) = hi(load start address)
+;     offset(2) = data byte (0)
+;     offset(n) = data byte (n-2)
+;
+; Subsequent blocks
+;     offset(n) = data byte (n)
+;
+; The TASS assembler and most Commodore 64-based tools generate this
+; data format automatically and you can transfer their .obj/.o64 output
+; file directly.
+;
+; The only time you need to do anything special is if you have
+; a raw memory image file (say you want to load a data
+; table into memory). For XMODEM you'll have to
+; "insert" the start address bytes to the front of the file.
+; Otherwise, XMODEM would have no idea where to start putting
+; the data.
+;-------------------------- The Code ----------------------------
+;
+; zero page variables (adjust these to suit your needs)
+;
+;
+crc		=	$38		; CRC lo byte  (two byte variable)
+crch		=	$39		; CRC hi byte
+ptr		=	$3a		; data pointer (two byte variable)
+ptrh		=	$3b		;   "    "
+blkno		=	$3c		; block number
+retry		=	$3d		; retry counter
+retry2		=	$3e		; 2nd counter
+bflag		=	$3f		; block flag
+;
+;
+; non-zero page variables and buffers
+;
+;
+Rbuff		=	$0300      	; temp 132 byte receive buffer
+					;(place anywhere, page aligned)
+;
+;
+;  tables and constants
+;
+;
+; The crclo & crchi labels are used to point to a lookup table to calculate
+; the CRC for the 128 byte data blocks.  There are two implementations of these
+; tables.  One is to use the tables included (defined towards the end of this
+; file) and the other is to build them at run-time.  If building at run-time,
+; then these two labels will need to be un-commented and declared in RAM.
+;
+;crclo		=	$7D00      	; Two 256-byte tables for quick lookup
+;crchi		= 	$7E00      	; (should be page-aligned for speed)
+;
+;
+;
+; XMODEM Control Character Constants
+SOH		=	$01		; start block
+EOT		=	$04		; end of text marker
+ACK		=	$06		; good block acknowledged
+NAK		=	$15		; bad block acknowledged
+CAN		=	$18		; cancel (not standard, not supported)
+CR		=	$0d		; carriage return
+LF		=	$0a		; line feed
+ESC		=	$1b		; ESC to exit
+;
+;^^^^^^^^^^^^^^^^^^^^^^ Start of Program ^^^^^^^^^^^^^^^^^^^^^^
+;
+; Xmodem/CRC upload routine
+; By Daryl Rictor, July 31, 2002
+;
+; v0.3  tested good minus CRC
+; v0.4  CRC fixed!!! init to $0000 rather than $FFFF as stated
+; v0.5  added CRC tables vs. generation at run time
+; v 1.0 recode for use with SBC2
+; v 1.1 added block 1 masking (block 257 would be corrupted)
+		*= 	$7B00		; Start of program (adjust to your needs)
+;
+XModem		jsr	PrintMsg	; send prompt and info
+		lda	#$01
+		sta	blkno		; set block # to 1
+		sta	bflag		; set flag to get address from block 1
+StartCrc	lda	#"C"		; "C" start with CRC mode
+		jsr	Put_Chr		; send it
+		lda	#$FF
+		sta	retry2		; set loop counter for ~3 sec delay
+		lda	#$00
+               	sta	crc
+		sta	crch		; init CRC value
+		jsr	GetByte		; wait for input
+               	bcs	GotByte		; byte received, process it
+		bcc	StartCrc	; resend "C"
+StartBlk	lda	#$FF		;
+		sta	retry2		; set loop counter for ~3 sec delay
+		lda	#$00		;
+		sta	crc		;
+		sta	crch		; init CRC value
+		jsr	GetByte		; get first byte of block
+		bcc	StartBlk	; timed out, keep waiting...
+GotByte		cmp	#ESC		; quitting?
+                bne	GotByte1	; no
+;		lda	#$FE		; Error code in "A" of desired
+                brk			; YES - do BRK or change to RTS if desired
+GotByte1        cmp	#SOH		; start of block?
+		beq	BegBlk		; yes
+		cmp	#EOT		;
+		bne	BadCrc		; Not SOH or EOT, so flush buffer & send NAK
+		jmp	Done		; EOT - all done!
+BegBlk		ldx	#$00
+GetBlk		lda	#$ff		; 3 sec window to receive characters
+		sta 	retry2		;
+GetBlk1		jsr	GetByte		; get next character
+		bcc	BadCrc		; chr rcv error, flush and send NAK
+GetBlk2		sta	Rbuff,x		; good char, save it in the rcv buffer
+		inx			; inc buffer pointer
+		cpx	#$84		; <01> <FE> <128 bytes> <CRCH> <CRCL>
+		bne	GetBlk		; get 132 characters
+		ldx	#$00		;
+		lda	Rbuff,x		; get block # from buffer
+		cmp	blkno		; compare to expected block #
+		beq	GoodBlk1	; matched!
+		jsr	Print_Err	; Unexpected block number - abort
+		jsr	Flush		; mismatched - flush buffer and then do BRK
+;		lda	#$FD		; put error code in "A" if desired
+		brk			; unexpected block # - fatal error - BRK or RTS
+GoodBlk1	eor	#$ff		; 1's comp of block #
+		inx			;
+		cmp	Rbuff,x		; compare with expected 1's comp of block #
+		beq	GoodBlk2 	; matched!
+		jsr	Print_Err	; Unexpected block number - abort
+		jsr 	Flush		; mismatched - flush buffer and then do BRK
+;		lda	#$FC		; put error code in "A" if desired
+		brk			; bad 1's comp of block#
+GoodBlk2	ldy	#$02		;
+CalcCrc		lda	Rbuff,y		; calculate the CRC for the 128 bytes of data
+		jsr	UpdCrc		; could inline sub here for speed
+		iny			;
+		cpy	#$82		; 128 bytes
+		bne	CalcCrc		;
+		lda	Rbuff,y		; get hi CRC from buffer
+		cmp	crch		; compare to calculated hi CRC
+		bne	BadCrc		; bad crc, send NAK
+		iny			;
+		lda	Rbuff,y		; get lo CRC from buffer
+		cmp	crc		; compare to calculated lo CRC
+		beq	GoodCrc		; good CRC
+BadCrc		jsr	Flush		; flush the input port
+		lda	#NAK		;
+		jsr	Put_Chr		; send NAK to resend block
+		jmp	StartBlk	; start over, get the block again
+GoodCrc		ldx	#$02		;
+		lda	blkno		; get the block number
+		cmp	#$01		; 1st block?
+		bne	CopyBlk		; no, copy all 128 bytes
+		lda	bflag		; is it really block 1, not block 257, 513 etc.
+		beq	CopyBlk		; no, copy all 128 bytes
+		lda	Rbuff,x		; get target address from 1st 2 bytes of blk 1
+		sta	ptr		; save lo address
+		inx			;
+		lda	Rbuff,x		; get hi address
+		sta	ptr+1		; save it
+		inx			; point to first byte of data
+		dec	bflag		; set the flag so we won't get another address
+CopyBlk		ldy	#$00		; set offset to zero
+CopyBlk3	lda	Rbuff,x		; get data byte from buffer
+		sta	(ptr),y		; save to target
+		inc	ptr		; point to next address
+		bne	CopyBlk4	; did it step over page boundary?
+		inc	ptr+1		; adjust high address for page crossing
+CopyBlk4	inx			; point to next data byte
+		cpx	#$82		; is it the last byte
+		bne	CopyBlk3	; no, get the next one
+IncBlk		inc	blkno		; done.  Inc the block #
+		lda	#ACK		; send ACK
+		jsr	Put_Chr		;
+		jmp	StartBlk	; get next block
+Done		lda	#ACK		; last block, send ACK and exit.
+		jsr	Put_Chr		;
+		jsr	Flush		; get leftover characters, if any
+		jsr	Print_Good	;
+		rts			;
+;
+;^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+;
+; subroutines
+;
+;					;
+GetByte		lda	#$00		; wait for chr input and cycle timing loop
+		sta	retry		; set low value of timing loop
+StartCrcLp	jsr	Get_chr		; get chr from serial port, don't wait
+		bcs	GetByte1	; got one, so exit
+		dec	retry		; no character received, so dec counter
+		bne	StartCrcLp	;
+		dec	retry2		; dec hi byte of counter
+		bne	StartCrcLp	; look for character again
+		clc			; if loop times out, CLC, else SEC and return
+GetByte1	rts			; with character in "A"
+;
+Flush		lda	#$70		; flush receive buffer
+		sta	retry2		; flush until empty for ~1 sec.
+Flush1		jsr	GetByte		; read the port
+		bcs	Flush		; if chr recvd, wait for another
+		rts			; else done
+;
+PrintMsg	ldx	#$00		; PRINT starting message
+PrtMsg1		lda   	Msg,x
+		beq	PrtMsg2
+		jsr	Put_Chr
+		inx
+		bne	PrtMsg1
+PrtMsg2		rts
+Msg		.byte	"Begin XMODEM/CRC transfer.  Press <Esc> to abort..."
+		.BYTE  	CR, LF
+               	.byte   0
+;
+Print_Err	ldx	#$00		; PRINT Error message
+PrtErr1		lda   	ErrMsg,x
+		beq	PrtErr2
+		jsr	Put_Chr
+		inx
+		bne	PrtErr1
+PrtErr2		rts
+ErrMsg		.byte 	"Upload Error!"
+		.BYTE  	CR, LF
+                .byte   0
+;
+Print_Good	ldx	#$00		; PRINT Good Transfer message
+Prtgood1	lda   	GoodMsg,x
+		beq	Prtgood2
+		jsr	Put_Chr
+		inx
+		bne	Prtgood1
+Prtgood2	rts
+GoodMsg		.byte 	"Upload Successful!"
+		.BYTE  	CR, LF
+                .byte   0
+;
+;
+;======================================================================
+;  I/O Device Specific Routines
+;
+;  Two routines are used to communicate with the I/O device.
+;
+; "Get_Chr" routine will scan the input port for a character.  It will
+; return without waiting with the Carry flag CLEAR if no character is
+; present or return with the Carry flag SET and the character in the "A"
+; register if one was present.
+;
+; "Put_Chr" routine will write one byte to the output port.  Its alright
+; if this routine waits for the port to be ready.  its assumed that the
+; character was send upon return from this routine.
+;
+; Here is an example of the routines used for a standard 6551 ACIA.
+; You would call the ACIA_Init prior to running the xmodem transfer
+; routine.
+;
+ACIA_Data	=	$7F70		; Adjust these addresses to point
+ACIA_Status	=	$7F71		; to YOUR 6551!
+ACIA_Command	=	$7F72		;
+ACIA_Control	=	$7F73		;
+ACIA_Init      	lda	#$1F           	; 19.2K/8/1
+               	sta	ACIA_Control   	; control reg
+               	lda	#$0B           	; N parity/echo off/rx int off/ dtr active low
+               	sta	ACIA_Command   	; command reg
+               	rts                  	; done
+;
+; input chr from ACIA (no waiting)
+;
+Get_Chr		clc			; no chr present
+               	lda	ACIA_Status     ; get Serial port status
+               	and	#$08            ; mask rcvr full bit
+              	beq	Get_Chr2	; if not chr, done
+               	Lda	ACIA_Data       ; else get chr
+	       	sec			; and set the Carry Flag
+Get_Chr2    	rts			; done
+;
+; output to OutPut Port
+;
+Put_Chr	   	PHA                     ; save registers
+Put_Chr1     	lda	ACIA_Status     ; serial port status
+              	and	#$10            ; is tx buffer empty
+               	beq	Put_Chr1        ; no, go back and test it again
+               	PLA                     ; yes, get chr to send
+               	sta	ACIA_Data       ; put character to Port
+               	RTS                     ; done
+;=========================================================================
+;
+;
+;  CRC subroutines
+;
+;
+UpdCrc		eor 	crc+1 		; Quick CRC computation with lookup tables
+       		tax		 	; updates the two bytes at crc & crc+1
+       		lda 	crc		; with the byte send in the "A" register
+       		eor 	CRCHI,X
+       		sta 	crc+1
+      	 	lda 	CRCLO,X
+       		sta 	crc
+       		rts
+;
+; Alternate solution is to build the two lookup tables at run-time.  This might
+; be desirable if the program is running from ram to reduce binary upload time.
+; The following code generates the data for the lookup tables.  You would need to
+; un-comment the variable declarations for crclo & crchi in the Tables and Constants
+; section above and call this routine to build the tables before calling the
+; "xmodem" routine.
+;
+;MAKECRCTABLE
+;		ldx 	#$00
+;		LDA	#$00
+;zeroloop	sta 	crclo,x
+;		sta 	crchi,x
+;		inx
+;		bne	zeroloop
+;		ldx	#$00
+;fetch		txa
+;		eor	crchi,x
+;		sta	crchi,x
+;		ldy	#$08
+;fetch1		asl	crclo,x
+;		rol	crchi,x
+;		bcc	fetch2
+;		lda	crchi,x
+;		eor	#$10
+;		sta	crchi,x
+;		lda	crclo,x
+;		eor	#$21
+;		sta	crclo,x
+;fetch2		dey
+;		bne	fetch1
+;		inx
+;		bne	fetch
+;		rts
+;
+; The following tables are used to calculate the CRC for the 128 bytes
+; in the xmodem data blocks.  You can use these tables if you plan to
+; store this program in ROM.  If you choose to build them at run-time,
+; then just delete them and define the two labels: crclo & crchi.
+;
+; low byte CRC lookup table (should be page aligned)
+		*= $7D00
+crclo
+ .byte $00,$21,$42,$63,$84,$A5,$C6,$E7,$08,$29,$4A,$6B,$8C,$AD,$CE,$EF
+ .byte $31,$10,$73,$52,$B5,$94,$F7,$D6,$39,$18,$7B,$5A,$BD,$9C,$FF,$DE
+ .byte $62,$43,$20,$01,$E6,$C7,$A4,$85,$6A,$4B,$28,$09,$EE,$CF,$AC,$8D
+ .byte $53,$72,$11,$30,$D7,$F6,$95,$B4,$5B,$7A,$19,$38,$DF,$FE,$9D,$BC
+ .byte $C4,$E5,$86,$A7,$40,$61,$02,$23,$CC,$ED,$8E,$AF,$48,$69,$0A,$2B
+ .byte $F5,$D4,$B7,$96,$71,$50,$33,$12,$FD,$DC,$BF,$9E,$79,$58,$3B,$1A
+ .byte $A6,$87,$E4,$C5,$22,$03,$60,$41,$AE,$8F,$EC,$CD,$2A,$0B,$68,$49
+ .byte $97,$B6,$D5,$F4,$13,$32,$51,$70,$9F,$BE,$DD,$FC,$1B,$3A,$59,$78
+ .byte $88,$A9,$CA,$EB,$0C,$2D,$4E,$6F,$80,$A1,$C2,$E3,$04,$25,$46,$67
+ .byte $B9,$98,$FB,$DA,$3D,$1C,$7F,$5E,$B1,$90,$F3,$D2,$35,$14,$77,$56
+ .byte $EA,$CB,$A8,$89,$6E,$4F,$2C,$0D,$E2,$C3,$A0,$81,$66,$47,$24,$05
+ .byte $DB,$FA,$99,$B8,$5F,$7E,$1D,$3C,$D3,$F2,$91,$B0,$57,$76,$15,$34
+ .byte $4C,$6D,$0E,$2F,$C8,$E9,$8A,$AB,$44,$65,$06,$27,$C0,$E1,$82,$A3
+ .byte $7D,$5C,$3F,$1E,$F9,$D8,$BB,$9A,$75,$54,$37,$16,$F1,$D0,$B3,$92
+ .byte $2E,$0F,$6C,$4D,$AA,$8B,$E8,$C9,$26,$07,$64,$45,$A2,$83,$E0,$C1
+ .byte $1F,$3E,$5D,$7C,$9B,$BA,$D9,$F8,$17,$36,$55,$74,$93,$B2,$D1,$F0
+; hi byte CRC lookup table (should be page aligned)
+		*= $7E00
+crchi
+ .byte $00,$10,$20,$30,$40,$50,$60,$70,$81,$91,$A1,$B1,$C1,$D1,$E1,$F1
+ .byte $12,$02,$32,$22,$52,$42,$72,$62,$93,$83,$B3,$A3,$D3,$C3,$F3,$E3
+ .byte $24,$34,$04,$14,$64,$74,$44,$54,$A5,$B5,$85,$95,$E5,$F5,$C5,$D5
+ .byte $36,$26,$16,$06,$76,$66,$56,$46,$B7,$A7,$97,$87,$F7,$E7,$D7,$C7
+ .byte $48,$58,$68,$78,$08,$18,$28,$38,$C9,$D9,$E9,$F9,$89,$99,$A9,$B9
+ .byte $5A,$4A,$7A,$6A,$1A,$0A,$3A,$2A,$DB,$CB,$FB,$EB,$9B,$8B,$BB,$AB
+ .byte $6C,$7C,$4C,$5C,$2C,$3C,$0C,$1C,$ED,$FD,$CD,$DD,$AD,$BD,$8D,$9D
+ .byte $7E,$6E,$5E,$4E,$3E,$2E,$1E,$0E,$FF,$EF,$DF,$CF,$BF,$AF,$9F,$8F
+ .byte $91,$81,$B1,$A1,$D1,$C1,$F1,$E1,$10,$00,$30,$20,$50,$40,$70,$60
+ .byte $83,$93,$A3,$B3,$C3,$D3,$E3,$F3,$02,$12,$22,$32,$42,$52,$62,$72
+ .byte $B5,$A5,$95,$85,$F5,$E5,$D5,$C5,$34,$24,$14,$04,$74,$64,$54,$44
+ .byte $A7,$B7,$87,$97,$E7,$F7,$C7,$D7,$26,$36,$06,$16,$66,$76,$46,$56
+ .byte $D9,$C9,$F9,$E9,$99,$89,$B9,$A9,$58,$48,$78,$68,$18,$08,$38,$28
+ .byte $CB,$DB,$EB,$FB,$8B,$9B,$AB,$BB,$4A,$5A,$6A,$7A,$0A,$1A,$2A,$3A
+ .byte $FD,$ED,$DD,$CD,$BD,$AD,$9D,$8D,$7C,$6C,$5C,$4C,$3C,$2C,$1C,$0C
+ .byte $EF,$FF,$CF,$DF,$AF,$BF,$8F,$9F,$6E,$7E,$4E,$5E,$2E,$3E,$0E,$1E
+;
+;
+; End of File
+;
+</code>