DataMuseum.dk

top - download

    Length: 19313 (0x4b71)
    Types: TextFile
    Names: »serial.z«

└─⟦a0efdde77⟧ Bits:30001252 EUUGD11 Tape, 1987 Spring Conference Helsinki
    └─ ⟦this⟧ »EUUGD11/euug-87hel/sec1/zmac/serial.z« 

TextFile

; SCCS flags:		serial.z	1.8	9/21/82


;
; This program is a quick and dirty controller program
; for the simple Z80 serial interface card for the Red
; display controller.
;
; It uses two 256 byte buffers to buffer data to and from the
; host. This helps make up for the obnoxiously slow rs232.
;
; History:
; jrp	3-18-82	v1.0 Initial version by John Providenza.
;
; jrp	3-22-82	v1.1 Added code to send a Xon (Cntrlq) at reset
;		if the dip switch is set to Xon/Xoff mode.
;
; jrp	4-20-82	v1.2 Added SCCS flags as comment in header and
;		as a "ascii" block after a reset jmp.
;
; jrp	4-20-82	v1.3 Changed crt modem flags to RLSD = Out Buf Full,
;		RI = In Buf Full.
;
; jrp	4-21-82	v1.4 Added diagnostic code to test ram, switches, and
;		uart.
;
; jrp	4-30-82	v1.5 Cleaned up some code, added some more comments.
;
; jrp	5-27-82	v1.6 Fixed bug that caused output buffer to overflow
;		in Hex mode.
;
; jrp	6-22-82	v1.7 Added 'end of message' command in hex mode.
;		This is active only in hex mode and only if a
;		non 0 byte count is specified (0 is default)
;		'l' is used to specify byte count, 'm' specifies
;		the eom char. Both expect 2 hex digits following
;		to specify the apropriate parameter.
;
; jrp	8-23-82	v1.8 Added code to allow send/recv in different modes.
;		Thus the host can send in raw mode and receive in hex
;		mode, allowing CntrlS/Q flow control.
;		Also added 's' command in 'hex' mode to reset the SWTCH
;		settings.
;		Also added break detect to reset the mode/baud to the
;		switch settings.
;			switch	dIN	dOUT	Flow Control.
;			7 6 5
;			0 0 0	raw	raw	No flow control.
;			0 0 1	raw	hex	Xon/Xoff sent to host.
;			0 1 0	hex	raw	Xon/Xoff received from host.
;			0 1 1	hex	hex	Full Xon/Xoff.
;			1 0 0	raw	raw	Full modem flow control.
;			1 0 1	raw	hex	Full modem flow control.
;			1 1 0	hex	raw	Full modem flow control.
;			1 1 1	hex	hex	Full modem flow control.
;
; SCCS flags:	serial.z 1.8 9/21/82
	eject	1
; Serial port equates.
Serial	equ	00H	; base address of 8250 controller.
Ier	equ	01H	; Interrupt Enable Reg
Iir	equ	02H	; Interrupt Ident Reg
Lcr	equ	03H	; Line Control Reg
Mcr	equ	04H	; Modem Control Reg
Lsr	equ	05H	; Line Status Reg
Msr	equ	06H	; Modem Status Reg

; These equates define bits in the Msr.
DsrIn	equ	05	; Data Set Ready input
CtsIn	equ	04	; Clear to Send input
InMt	equ	06	; No data from display controller = 1 (Ring In)
OutMt	equ	07	; Crt ready for next byte = 1 (Rcvd Line Signal Detct)

; These equates define bits in the Lsr
DataRdy	equ	00	; Input data ready.
Break	equ	04	; Break condition.
XmitMt	equ	05	; Xmitter buffer empty.

; These equates define bits in the Mcr
DtrOut	equ	00	; Data terminal ready output.
RtsOut	equ	01	; Request to send output.

; Misc definitions.
Crt	equ	80H	; Parallel port to display controller.
Baud	equ	40H	; Switches port.
Stack	equ	0FFFFH

; Mailbox equates.
Head	equ	0
Tail	equ	1
Count	equ	2
Base	equ	3
Status	equ	4
UnChar	equ	5	; Should be used only for CntrlS and CntrlQ

; Equates for the Queue status byte
XmitOff	equ	00		; xmitter is disabled.

; Baud/Switch equates.
Bmask	equ	0FH
Rawout	equ	020H
RObit	equ	5
Rawin	equ	040H
RIbit	equ	6
Xon	equ	080H
Xonbit	equ	7

; Some ASCII character equates.
CntrlS	equ	19		; Xoff
CntrlQ	equ	17		; Xon
Cr	equ	13		; Carriage return.
	eject	1
	ORG	0FC00H
RAM_START:
; Variable declarations
; Ram is in the top 1K of memory.

; Queues.
; These are the actual data buffers. The only routine that should use
; these labels re INIT_V to set the mailbox data pointers up.
; All I/O is via GETQ and PUTQ routines.
INBUF:	block	256		; input buffer q.
OUTBUF:	block	256		; output buffer q.

UNUSED:	block	256		; unused ram

; Now the ram for variables and stack.

SWTCH:	block	1		; Current baud/switches

; Variable for the H_to_Q routine
; It holds the upper nibble of hex until the lower one arrives.
; Bit 0 = 1 for empty, 0 for upper nibble full.
H_to_QV:	block	1

; End of message variables.
MESS_LEN:	block	1	; How long messages are.
MESS_CNT:	block	1	; Number of chars in current message.
EOM_CHAR:	block	1	; The end of message char.


; In and Out queues variables.
INBOX:	block	6
OUTBOX:	block	6
	eject	1
; Mainline loop.
	ORG	0
	JP	RESET		; Jmp to the code
				; Put in ID string
	ascii	'serial.z 1.8'
RESET:
	LD	SP, Stack
	CALL	CHECK		; Check the hardware out.
	CALL	INIT_HW		; Init the hardware devices.
	CALL	INIT_V		; Init the variables.

	LD	IX,OUTBOX	; Point to the outbox.
	LD	A,(SWTCH)	; Check if we're in Xon/Xoff mode.
	AND	Xon
	LD	A,Cntrlq	; Send a Xon to host if we're in that mode
	CALL	NZ,PUTQ

; Now loop checking for data available from host or display controller.
; Also check if we can send data to them.
LOOP:
	IN	A,(Lsr)		; Get the line status.
	LD	C,A
	IN	A,(Msr)		; Get the modem status.
	LD	B,A		; and save it
				; B = Msr, C = Lsr.
; Check for break condition.
	BIT	Break,C		; test the bit in the Lsr
	JR	Z,LOOP1
	CALL	SETBAUD		; reset the SWTCH variable.
	CALL	INIT_V		; reset all the variables
	JR	LOOP
LOOP1:
	CALL	HOST_IN
	CALL	DISP_IN
	CALL	DISP_OUT
	CALL	HOST_OUT
	JR	LOOP
	eject	1
; Check if data is ready from host.
HOST_IN:
	BIT	DataRdy,C	; Data ready?
	RET	Z		; Ret if no.
				; Handle a byte from the Host.
	LD	IX,INBOX	; data will go into the Input Q.
	LD	A,(SWTCH)	; check for Raw or encoded mode.
	LD	H,A
	AND	Rawin		; NZ for Raw mode
	IN	A,(Serial)	; get the data byte.
	JR	Z,HEX_IN	; Jmp if hex data in.
RAW_IN:				; Process Raw data
	CALL	PUTQ
	JP	STOP_IN		; stop the input if needed.
HEX_IN:
	AND	7FH		; Kill any parity bit.
	CP	' '		; Printable ASCII?
	JR	NC,PRINT	; Jmp if yes
; Control character.
	CP	Cr		; Carriage Ret?
	JR	NZ,IN_FLOW	; Jp if no.
	LD	A,1		; Set the H_to_Q variable to empty.
	LD	(H_to_QV),A	; This flushes any partially assembled byte.
	RET			; Done
; Test for Xon/Xoff commands.
IN_FLOW:
	BIT	Xonbit,H	; Are we sensitive to them?
	RET	Z		; Ret if no.
	LD	HL,OUTBOX+Status	; Get a pointer to our outbox status.
	CP	CntrlS		; Xoff our transmitter?
	JR	NZ,NOT_XOFF	; Jmp if no.
OFF:
	SET	XmitOff,(HL)
	RET
NOT_XOFF:
	CP	CntrlQ		; Xon our xmitter?
	RET	NZ		; ret if no.
ON:
	RES	XmitOff,(HL)
	RET
; Printable char received from host.
PRINT:				; Printable character received in hex mode.
	SET	5,A		; Convert to lower case.
	CP	'l'		; Message length command?
	JR	NZ,PRINT1	; Jmp if no.
	CALL	GET_HEX		; Get byte from UART
	LD	(MESS_LEN),A	; Set the message length.
	LD	(MESS_CNT),A	; Reset the number of chars sent so far.
	RET
PRINT1:
	CP	'm'		; EOM char set command?
	JR	NZ,PRINT2
	CALL	GET_HEX		; Get byte from UART
	LD	(EOM_CHAR),A
	RET
PRINT2:
	CP	's'		; change SWTCH command?
	JR	NZ,PRINT3
	CALL	GET_HEX		; Get byte from UART
	CPL			; Toggle them.
	LD	(SWTCH),A
	RET
PRINT3:
	CALL	H_to_Q		; Pack the encoded data into bytes.
	JP	STOP_IN		; stop the input if needed.
	eject	1
; Data ready from controller?
DISP_IN:
	BIT	InMt,B		; data from controller?
	RET	NZ		; ret if no.
	LD	IX,OUTBOX
	LD	A,(IX+Count)	; Get the Q count.
	ADD	A,3		; Check if Q has room for 3 more bytes.
	RET	C		; ret if no
	LD	A,(SWTCH)	; check if we need to encode the data.
	AND	Rawout
	IN	A,(Crt)		; get the data from the crt.
	JP	NZ,PUTQ		; send the raw data and return.
; hex data out to host.
	CALL	B_to_H		; convert byte to hex format and stick in Q.
				; Check if we need to stick an EOM char in.
	LD	A,(MESS_LEN)	; Get the length.
	AND	A
	RET	Z		; Zero means no EOM character to be sent.
	LD	HL,MESS_CNT	; Point to the counter.
	DEC	(HL)		; Time to send a EOM char?
	RET	NZ		; Ret if no.
	LD	(HL),A		; reset the length.
	LD	A,(EOM_CHAR)	; Get the char and stick it in the Q.
	JP	PUTQ		; and return when done
	eject	1
; Controller ready for data?
DISP_OUT:
	BIT	OutMt,B		; controller ready for data?
	RET	Z		; Jmp if no
	LD	IX,INBOX
	CALL	GETQ		; get a byte for controller.
	RET	C		; ret if no byte available.
	OUT	(Crt),A		; send char to display.
	JP	STRT_IN		; re-enable host xmitter if needed.
	eject	1
; Host ready for data?
HOST_OUT:
	BIT	XmitMt,C	; Uart xmitter empty?
	RET	Z		; ret if no.
	LD	IX,OUTBOX	; Get OutBox pointer.
	LD	A,(SWTCH)	; Check for Xon mode
	AND	Xon
	JR	NZ,H_O_Xon	; Jp if Xon mode.
	BIT	CtsIn,B		; Clear to send?
	RET	Z		; ret if no.
	JR	H_O_Send	; We are clear to send.
H_O_Xon:
	LD	A,(OUTBOX+UnChar)
	AND	A
	JR	NZ,H_O_Send	; Always send an 'UnChar'
	BIT	XmitOff,(IX+Status)
	RET	NZ		; ret if xmitter is disabled.
H_O_Send:
	CALL	GETQ
	RET	C		; Ret if no character available.
	OUT	(Serial),A
	RET
	eject	1
; Check the hardware out.
; Call this routine only after a external reset!!!!

CHECK:
; Check the baud switch (really crude).
	IN	A,(BAUD)	; Get the baud switch.
	LD	B,A		; Save it.
	IN	A,(BAUD)
	CP	B		; Same as last time?
BAD_B:	; Switch ERROR - Can't read switches twice in a row.
	JR	NZ,BAD_B	; Loop if no.
; Check the ram.
; Write the complement of the low byte of address out to all ram,
; then check if it stayed the same.
; Note that this destroys all ram contents.
	POP	DE		; Save the return address in a register.
	LD	HL,RAM_START	; Get the first address of ram.
	LD	B, NOT [RAM_START & 0FFH]
	LD	C,B		; Get complement of low address byte.
				; Load the ram with the pattern.
RAM1:
	LD	(HL),B
	DEC	B
	INC	HL
	LD	A,H		; Test for done.
	OR	L
	JR	NZ,RAM1		; Loop till all locations written.
	LD	HL,RAM_START	; Get the first address of ram.
; Check if ram agrees with what should be there.
RAM2:
	LD	A,(HL)		; Get the byte.
	XOR	C		; Same as its low address byte?
	JR	Z,RAM6		; Jmp if yes.
; Ram error. We have three loops: low bad, high bad, both bad.
	LD	B,A		; Save the symptom.
	AND	0FH		; Low nibble bad?
RAM3:	; Ram ERROR - bad high nibble.
	JR	Z,RAM3		; Jmp if no.
	LD	A,B		; get the symptom back.
	AND	0F0H		; High nibble bad too?
RAM4:	; Ram ERROR - bad low nibble.
	JR	Z,RAM4		; Loop if error.
RAM5:	; Ram ERROR - both nibbles bad.
	JR	RAM5
RAM6:
	DEC	C
	INC	HL
	LD	A,H		; Done?
	OR	L
	JR	NZ,RAM2		; Jmp if no.
	PUSH	DE		; Fix the stack back up.
; Check out the National Semi INS8250 Uart.
; Since we were reset, Lcr should be zero.
	IN	A,(Lcr)		; Get the Line Control reg
	AND	A
U0:	; Uart ERROR - Lcr not reset properly.
	JR	NZ,U0		; Loop if error.
	LD	A,80H
	OUT	(Lcr),A		; And set the Divisor access bit.
	IN	A,(Lcr)		; Check that it got set.
	CP	80H		; Still set?
U1:	; Uart ERROR - Lcr won't hold divisor access bit.
	JR	NZ,U1		; Loop if error.
	LD	A,3		; Try to set 38.4K baud
	OUT	(Serial),A	; Ld the divisor.
	IN	A,(Serial)	; Test that it loaded OK.
	SUB	3		; Check if same (also set A to zero)
U2:	; Uart ERROR - unexpected low divisor.
	JR	NZ,U2		; Loop if error.
	OUT	(Ier),A		; Set high byte to zero
	IN	A,(Ier)
	AND	A		; Still zero?
U3:	; Uart ERROR - unexpected high divisor.
	JR	NZ,U3		; Loop if no (ie, error).
	IN	A,(Lcr)		; Get the Line reg back.
	SUB	80H		; Is it the same as before?
U4:	; Uart ERROR - unexpected Lcr value after setting divisor.
	JR	NZ,U4		; loop if error.
	OUT	(Lcr),A		; Turn off divisor access bit.
	IN	A,(Lcr)		; Check it.
	AND	A
U5:	; Uart ERROR - Lcr won't reset after setting divisor.
	JR	NZ,U5
	LD	A,7
	OUT	(Lcr),A		; 8 bits, no parity, 2 stop bits
	IN	A,(Lcr)
	SUB	7		; Test if the same (also set A to zero)
U6:	; Uart ERROR - Can't set proper operating Lcr.
	JR	NZ,U6		; If we succeed, assume Lcr is Ok.
	OUT	(Ier),A		; Disable all 8250 interrupt conditions (set to 0).
	IN	A,(Ier)
	AND	A
U7:	; Uart ERROR - Can't reset Ier.
	JR	NZ,U7
	RET
	eject	1
; Init the hardware.
INIT_HW:
	CALL	SETBAUD		; Set the Uart baud
	LD	A,7
	OUT	(Lcr),A		; 8 bits, no parity, 2 stop bits
	XOR	A		; Disable all 8250 interrupt conditions.
	OUT	(Ier),A
	LD	A,3		; Dtr, Rts on.
	OUT	(Mcr),A

; Perform the I/O diagnostic with the controller.
; Wait for data from controller, then echo it back.
INITH1:
	IN	A,(Msr)		; Check if controller data ready.
	BIT	InMt,A		; Ready?
	JR	NZ,INITH1	; Jmp if no.
	IN	A,(Crt)		; Get the data.
	OUT	(Crt),A		; And send it back to controller.
	RET


; Init the variables.
INIT_V:
	XOR	A		; zero A

; Init the Q's
	LD	IX,INBOX	; Init the inbox.
	LD	(IX + Head),A
	LD	(IX + Tail),A
	LD	(IX + Count),A
	LD	HL,INBUF
	LD	(IX + Base),H
	LD	(IX + Status),A
	LD	(IX + UnChar),A

	LD	IX,OUTBOX	; Init the outbox.
	LD	(IX + Head),A
	LD	(IX + Tail),A
	LD	(IX + Count),A
	LD	HL,OUTBUF
	LD	(IX + Base),H
	LD	(IX + Status),A
	LD	(IX + UnChar),A

; Init the H_to_Q variable.
	LD	A,1
	LD	(H_to_QV),A

; init the 'end of message' stuff
	LD	A,Cr		; default r is a carriage return.
	LD	(EOM_CHAR),A
	XOR	A
	LD	(MESS_LEN),A
	LD	(MESS_CNT),A
	RET
	eject	1
; These routines handle the input and output queues.
; The Q pointer is passed in IX, result/source in A.
; Queues must be 256 bytes long. We use only 8 bit
; arithmetic for Q manipulation.
; A Q is defined as 6 bytes of status:
;	Tail	Offset for getting next char
;	Head	Offset for putting next char
;	Count	Number of chars in q
;	Base	High byte of the q origin
;	Status	Status of Q
;	UnChar	The 'un_get' char if non-zero
; and 256 bytes of storage.
;

GETQ:
; Get an element from the Q.
; entry	ix = Q pointer
; exit	a  = result
;	ca = set for empty Q, cleared for full Q.
;	bc & de are unchanged.
;	hl = garbage
;
	LD	A,(IX + UnChar) ; Get the unget char
	LD	(IX + UnChar),0	; Set the byte to 0 (empty).
	AND	A
	RET	NZ		; Ret if we got an unget char.
				; A == 0 here.
	CP	(IX + Count)	; Get the q count
	SCF
	RET	Z		; empty Q return (Count == 0).
	DEC	(IX + Count)	; one less item in the Q.
	LD	L,(IX + Tail)	; get a pointer to the element in the Q.
	LD	H,(IX + Base)
	INC	(IX + Tail)	; bump the pointer to the next char.
	OR	(HL)		; Get the element, and clear the carry.
	RET


PUTQ:
; Routine to put a char in a Q.
;entry	ix = pointer to Q structure.
;	a = char to put.
;exit	hl = garbage
;	a, bc & de unchanged.
;	Ca = 1 for Q full, character discarded.
;
	INC	(IX + Count)	; Bump the Q count.
QPUT_ERR:
	JR	Z,QPUT_ERR
QPUT1:
	LD	H,(IX + Base)
	LD	L,(IX + Head)
	LD	(HL),A		; Put the char in the Q
	INC	(IX + Head)
	AND	A		; Clear the carry bit
	RET
	eject	1
; These routines pack and unpack bytes into Hex
; suitable for sending as ASCII over a serial line.
; H_to_Q takes Hex characters
; and packs them into 8 bit bytes to send to the display.
; B_to_H takes bytes from the display and converts them into
; the Hex character stream.
;
; Both routines use Q calls. IX must be set up with the proper
; Q address.
;
;



H_to_Q:
;
; entry	A	= Ascii Hex char (0-9, a-f)
;	IX	= Q pointer
; exit	A, Hl	= Garbage
;	bc, de	= unchanged.
;	Ca	= 1 if Q too full.
;
	CALL	H_to_B		; convert the character to binary.
	LD	HL,H_to_QV	; Point hl to our variable
	BIT	0,(HL)		; check if the upper nibble is full.
	JR	Z,H_SEND	; Jmp if yes.
	ADD	A,A		; Move the nibble to the high 4 bits.
	ADD	A,A
	ADD	A,A
	ADD	A,A
	LD	(HL),A		; Save away the high nibble with low nibble = 0.
	RET
H_SEND:
	OR	(HL)		; Merge in the upper nibble from ram.
	LD	(HL),1		; Set the variable to empty.
	JP	PUTQ		; Send the byte and return.
	eject	1



B_to_H:
; B_to_H takes the byte in A and splits it into two hex characters
; to be sent to the Q specified in IX.
;
; Entry	A	= byte of data to convert to Hex.
;	IX	= Q address.
; Exit	A E Hl	= garbage
;	D Bc Ix	= unchanged.
;

	LD	E,A		; Save the byte
	RRA			; Move the upper nibble to low nibble.
	RRA
	RRA
	RRA
	AND	0Fh		; Get only the upper nibble.
	CP	10		; 0 thru 9?
	JR	C,B_to_H1	; Jmp if yes.
	ADD	A,'A'-'0'-10
B_to_H1:
	ADD	A,'0'
	CALL	PUTQ
	LD	A,E		; Get the byte back
	AND	0Fh		; Mask for only low nibble.
	CP	10		; 0 thru 9?
	JR	C,B_to_H2	; Jmp if yes.
	ADD	A,'A'-'0'-10
B_to_H2:
	ADD	A,'0'
	JP	PUTQ		; Send and return.
	eject	1
GET_HEX:
; This routine gets two hex characters from the UART and
; munches them into a byte in A.
; Entry:	No Params.
; Exit: 	A=byte	H = trash
;		all others unchanged (except for flags)
	IN	A,(Lsr)		; Get the line status
	BIT	DataRdy,A	; Data ready from host?
	JR	Z,GET_HEX	; Jmp if no.
	IN	A,(Serial)	; get the data.
	CALL	H_to_B		; convert to binary.
	ADD	A,A		; Shift up 4 bits
	ADD	A,A
	ADD	A,A
	ADD	A,A
	LD	H,A		; Save in B
GET_HX1:
	IN	A,(Lsr)		; Get the line status
	BIT	DataRdy,A	; Data ready from host?
	JR	Z,GET_HX1	; Jmp if no.
	IN	A,(Serial)	; get the data.
	CALL	H_to_B		; convert to binary.
	OR	H
	RET			; A = 2 input chars munched together.

; Convert hex char to binary.
H_to_B:
	SET	5,A		; convert to lower case.
	SUB	'0'		; less than 0?
	JR	C,HB_ERR	; Jmp if out of bounds.
	CP	10		; bigger than 9?
	RET	C		; Ret if no (0..9)
	SUB	'a'-'0'-10	; try to make it range 10-15
	CP	10
	JR	C,HB_ERR	; Jmp if out of bounds.
	CP	16
	RET	C		; Ret if hex.
HB_ERR:
	XOR	A		; Set to zero.
	RET

	eject	1
SETBAUD:
; This routine reads the BAUD switches and looks the code
; up in the BTABLE to set the baudrate of the 8250 serial chip.
;
; Entry	No parameters
; exit	A Hl De	= garbage.
	IN	A,(Lcr)	; Set the divisor access bit on
	OR	80H
	OUT	(Lcr),A
	IN	A,(Baud)	; Get the baud rate code
	LD	(SWTCH),A
	AND	Bmask		; Get only the baud specifier bits.
	ADD	A,A		; Double it to index into table.
	LD	HL,BTABLE	; Index into table to get the divisor
	LD	E,A
	LD	D,0
	ADD	HL,DE
	LD	A,(HL)		; Get the low order divisor byte
	OUT	(Serial),A
	INC	HL
	LD	A,(HL)		; Get the high divisor byte
	OUT	(Serial+1),A
	IN	A,(Lcr)	; Set the divisor access bit off
	AND	7FH
	OUT	(Lcr),A
	RET

; Baud rate look up table
; Only allow 16 entries.
BTABLE:
	WORD	5	; 38.4 Kbaud
	WORD	10	; 19.2
	WORD	20	; 9600
	WORD	27	; 7200
	WORD	40	; 4800
	WORD	53	; 3600
	WORD	80	; 2400
	WORD	107	; 1800
	WORD	160	; 1200
	WORD	320	; 600
	WORD	640	; 300
	WORD	1280	; 150
	WORD	1428	; 134.5
	WORD	1745	; 110
	WORD	2560	; 75
	WORD	3840	; 50
	eject	1
; STRT_IN and STOP_IN are called when the Input Q is may be too full/empty.
; They check and enable/disable the host xmitter apropriately.
;

STRT_IN:
; Entry	No registers set.
; Exit	A Ix Hl	= garbage.
;	Bc De	= unchanged.
;
	LD	IX,INBOX	; Point to the Q.
	BIT	XmitOff,(IX + Status)	; Is it off?
	RET	Z		; ret if no.
	LD	A,40		; Check if we've gone below low water mark.
	CP	(IX + Count)
	RET	C		; Ret if no, Q still too full.
	LD	A,(SWTCH)	; get the switch settings.
	BIT	Xonbit,A
	JR	Z,STRT_DTR	; Jmp if rs232 modem mode flow control.
; Try to use Xon/Xoff control flow methods.
	BIT	RObit,A		; Raw Output mode?
	RET	NZ		; No way to start/stop host xmitter.
	LD	HL,OUTBOX+UnChar
	LD	A,(HL)		; Anything in unget spot?
	AND	A
	RET	NZ		; Ret if yes.
	LD	(HL),CntrlQ	; 'unget' a control Q.
	JR	STRT_END
; Set DTR bit on.
STRT_DTR:
	IN	A,(Mcr)		; get the modem controls.
	SET	DtrOut,A
	OUT	(Mcr),A
STRT_END:
	RES	XmitOff,(IX + Status)	; Mark as enabled.
	RET


STOP_IN:
; Entry	No registers set.
; Exit	A Ix Hl	= garbage.
;	Bc De	= unchanged.
;
	LD	IX,INBOX	; Point to the Q.
	BIT	XmitOff,(IX + Status)	; Already disabled?
	RET	NZ		; ret if yes.
	LD	A,256-40	; Check if we've gone above high water mark.
	CP	(IX + Count)
	RET	NC		; Ret if no, Q still too empty.
	LD	A,(SWTCH)
	BIT	Xonbit,A	; test for Xon/Xoff vs. modem flow cntrl.
	JR	Z,STP_DTR	; jmp if rs232 modem mode
; try to send an Xoff to the host.
	BIT	RObit,A		; Are we in raw out?
	RET	NZ		; Can't control the host xmitter.
	LD	HL,OUTBOX+UnChar
	LD	A,(HL)		; Anything in unget spot?
	AND	A
	RET	NZ		; Ret if yes.
	LD	(HL),CntrlS	; 'unget' a control S.
	JR	STP_END
; Modem mode flow control, set DTR bit off.
STP_DTR:
	IN	A,(Mcr)		; get the modem controls.
	RES	DtrOut,A
	OUT	(Mcr),A
STP_END:
	SET	XmitOff,(IX + Status)	; Mark as disabled.
	RET

	END