C64 ROM: Routine at EED7

EED7: do RS232 parity bit

Used by the routine at EEBB. enters with RS232 bit count = 0
EED7	A9 20	LDA #%00100000	mask 00x0 0000, parity enable bit
EED9	2C 94 02	BIT $0294	test the pseudo 6551 command register
EEDC	F0 14	BEQ $EEF2	if parity disabled go ??
EEDE	30 1C	BMI $EEFC	if fixed mark or space parity go ??
EEE0	70 14	BVS $EEF6	if even parity go ??
else odd parity
EEE2	A5 BD	LDA $BD	get RS232 parity byte
EEE4	D0 01	BNE $EEE7	if parity not zero leave parity bit = 0
EEE6	CA	DEX	make parity bit = 1
EEE7	C6 B4	DEC $B4	decrement RS232 bit count, 1 stop bit
EEE9	AD 93 02	LDA $0293	get pseudo 6551 control register
EEEC	10 E3	BPL $EED1	if 1 stop bit save parity bit and exit
else two stop bits ..
EEEE	C6 B4	DEC $B4	decrement RS232 bit count, 2 stop bits
EEF0	D0 DF	BNE $EED1	save bit and exit, branch always
parity is disabled so the parity bit becomes the first, and possibly only, stop bit. to do this increment the bit count which effectively decrements the stop bit count.
EEF2	E6 B4	INC $B4	increment RS232 bit count, = -1 stop bit
EEF4	D0 F0	BNE $EEE6	set stop bit = 1 and exit
do even parity
EEF6	A5 BD	LDA $BD	get RS232 parity byte
EEF8	F0 ED	BEQ $EEE7	if parity zero leave parity bit = 0
EEFA	D0 EA	BNE $EEE6	else make parity bit = 1, branch always
fixed mark or space parity
EEFC	70 E9	BVS $EEE7	if fixed space parity leave parity bit = 0
EEFE	50 E6	BVC $EEE6	else fixed mark parity make parity bit = 1, branch always
This entry point is used by the routine at EEBB. decrement stop bit count, set stop bit = 1 and exit. $FF is one stop bit, $FE is two stop bits
EF00	E6 B4	INC $B4	decrement RS232 bit count
EF02	A2 FF	LDX #$FF	set stop bit = 1
EF04	D0 CB	BNE $EED1	save stop bit and exit, branch always