; 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
;