C64 ROM: Routine at F92C

On Commodore computers, the streams consist of four kinds of symbols that denote different kinds of low-to-high-to-low transitions on the read or write signals of the Commodore cassette interface.

A break in the communications, or a pulse with very long cycle time.
A short pulse, whose cycle time typically ranges from 296 to 424 microseconds, depending on the computer model.
A medium-length pulse, whose cycle time typically ranges from 440 to 576 microseconds, depending on the computer model.
A long pulse, whose cycle time typically ranges from 600 to 744 microseconds, depending on the computer model.

The actual interpretation of the serial data takes a little more work to explain. The typical ROM tape loader (and the turbo loaders) will initialize a timer with a specified value and start it counting down. If either the tape data changes or the timer runs out, an IRQ will occur. The loader will determine which condition caused the IRQ. If the tape data changed before the timer ran out, we have a short pulse, or a "0" bit. If the timer ran out first, we have a long pulse, or a "1" bit. Doing this continuously and we decode the entire file.

On Commodore computers, the streams consist of four kinds of symbols that denote different kinds of low-to-high-to-low transitions on the read or write signals of the Commodore cassette interface. A break in the communications, or a pulse with very long cycle time. A short pulse, whose cycle time typically ranges from 296 to 424 microseconds, depending on the computer model. A medium-length pulse, whose cycle time typically ranges from 440 to 576 microseconds, depending on the computer model. A long pulse, whose cycle time typically ranges from 600 to 744 microseconds, depending on the computer model. The actual interpretation of the serial data takes a little more work to explain. The typical ROM tape loader (and the turbo loaders) will initialize a timer with a specified value and start it counting down. If either the tape data changes or the timer runs out, an IRQ will occur. The loader will determine which condition caused the IRQ. If the tape data changed before the timer ran out, we have a short pulse, or a "0" bit. If the timer ran out first, we have a long pulse, or a "1" bit. Doing this continuously and we decode the entire file.
read T2C which has been counting down from $FFFF. subtract this from $FFFF
F92C	AE 07 DC	LDX $DC07	read VIA 1 timer B high byte
F92F	A0 FF	LDY #$FF	set $FF
F931	98	TYA	A = $FF
F932	ED 06 DC	SBC $DC06	subtract VIA 1 timer B low byte
F935	EC 07 DC	CPX $DC07	compare it with VIA 1 timer B high byte
F938	D0 F2	BNE $F92C	if timer low byte rolled over loop
F93A	86 B1	STX $B1	save tape timing constant max byte
F93C	AA	TAX	copy $FF - T2C_l
F93D	8C 06 DC	STY $DC06	save VIA 1 timer B low byte
F940	8C 07 DC	STY $DC07	save VIA 1 timer B high byte
F943	A9 19	LDA #$19	load timer B, timer B single shot, start timer B
F945	8D 0F DC	STA $DC0F	save VIA 1 CRB
F948	AD 0D DC	LDA $DC0D	read VIA 1 ICR
F94B	8D A3 02	STA $02A3	save VIA 1 ICR shadow copy
F94E	98	TYA	Y = $FF
F94F	E5 B1	SBC $B1	subtract tape timing constant max byte
A = $FF - T2C_h
F951	86 B1	STX $B1	save tape timing constant max byte
$B1 = $FF - T2C_l
F953	4A	LSR A	A = $FF - T2C_h >> 1
F954	66 B1	ROR $B1	shift tape timing constant max byte
$B1 = $FF - T2C_l >> 1
F956	4A	LSR A	A = $FF - T2C_h >> 1
F957	66 B1	ROR $B1	shift tape timing constant max byte
$B1 = $FF - T2C_l >> 1
F959	A5 B0	LDA $B0	get tape timing constant min byte
F95B	18	CLC	clear carry for add
F95C	69 3C	ADC #$3C
F95E	C5 B1	CMP $B1	compare with tape timing constant max byte
compare with ($FFFF - T2C) >> 2
F960	B0 4A	BCS $F9AC	branch if min + $3C >= ($FFFF - T2C) >> 2
min + $3C < ($FFFF - T2C) >> 2
F962	A6 9C	LDX $9C	get byte received flag
F964	F0 03	BEQ $F969	if not byte received ??
F966	4C 60 FA	JMP $FA60	store the tape character
F969	A6 A3	LDX $A3	get EOI flag byte
F96B	30 1B	BMI $F988
F96D	A2 00	LDX #$00
F96F	69 30	ADC #$30
F971	65 B0	ADC $B0	add tape timing constant min byte
F973	C5 B1	CMP $B1	compare with tape timing constant max byte
F975	B0 1C	BCS $F993
F977	E8	INX
F978	69 26	ADC #$26
F97A	65 B0	ADC $B0	add tape timing constant min byte
F97C	C5 B1	CMP $B1	compare with tape timing constant max byte
F97E	B0 17	BCS $F997
F980	69 2C	ADC #$2C
F982	65 B0	ADC $B0	add tape timing constant min byte
F984	C5 B1	CMP $B1	compare with tape timing constant max byte
F986	90 03	BCC $F98B
F988	4C 10 FA	JMP $FA10
F98B	A5 B4	LDA $B4	get the bit count
F98D	F0 1D	BEQ $F9AC	if all done go ??
F98F	85 A8	STA $A8	save receiver bit count in
F991	D0 19	BNE $F9AC	branch always
F993	E6 A9	INC $A9	increment ?? start bit check flag
F995	B0 02	BCS $F999
F997	C6 A9	DEC $A9	decrement ?? start bit check flag
F999	38	SEC
F99A	E9 13	SBC #$13
F99C	E5 B1	SBC $B1	subtract tape timing constant max byte
F99E	65 92	ADC $92	add timing constant for tape
F9A0	85 92	STA $92	save timing constant for tape
F9A2	A5 A4	LDA $A4	get tape bit cycle phase
F9A4	49 01	EOR #%00000001
F9A6	85 A4	STA $A4	save tape bit cycle phase
F9A8	F0 2B	BEQ $F9D5
F9AA	86 D7	STX $D7
F9AC	A5 B4	LDA $B4	get the bit count
F9AE	F0 22	BEQ $F9D2	if all done go ??
F9B0	AD A3 02	LDA $02A3	read VIA 1 ICR shadow copy
F9B3	29 01	AND #%00000001	mask 0000 000x, timer A interrupt enabled
F9B5	D0 05	BNE $F9BC	if timer A is enabled go ??
F9B7	AD A4 02	LDA $02A4	read VIA 1 CRA shadow copy
F9BA	D0 16	BNE $F9D2	if ?? just exit
F9BC	A9 00	LDA #$00	clear A
F9BE	85 A4	STA $A4	clear the tape bit cycle phase
F9C0	8D A4 02	STA $02A4	save VIA 1 CRA shadow copy
F9C3	A5 A3	LDA $A3	get EOI flag byte
F9C5	10 30	BPL $F9F7
F9C7	30 BF	BMI $F988
F9C9	A2 A6	LDX #$A6	set timimg max byte
F9CB	20 E2 F8	JSR $F8E2	set timing
F9CE	A5 9B	LDA $9B
F9D0	D0 B9	BNE $F98B
F9D2	4C BC FE	JMP $FEBC	restore registers and exit interrupt
F9D5	A5 92	LDA $92	get timing constant for tape
F9D7	F0 07	BEQ $F9E0
F9D9	30 03	BMI $F9DE
F9DB	C6 B0	DEC $B0	decrement tape timing constant min byte
F9DD		.BYTE $2C	makes next line BIT $B0E6
F9DE	E6 B0	INC $B0	increment tape timing constant min byte
F9E0	A9 00	LDA #$00
F9E2	85 92	STA $92	clear timing constant for tape
F9E4	E4 D7	CPX $D7
F9E6	D0 0F	BNE $F9F7
F9E8	8A	TXA
F9E9	D0 A0	BNE $F98B
F9EB	A5 A9	LDA $A9	get start bit check flag
F9ED	30 BD	BMI $F9AC
F9EF	C9 10	CMP #$10
F9F1	90 B9	BCC $F9AC
F9F3	85 96	STA $96	save cassette block synchronization number
F9F5	B0 B5	BCS $F9AC
F9F7	8A	TXA
F9F8	45 9B	EOR $9B
F9FA	85 9B	STA $9B
F9FC	A5 B4	LDA $B4
F9FE	F0 D2	BEQ $F9D2
FA00	C6 A3	DEC $A3	decrement EOI flag byte
FA02	30 C5	BMI $F9C9
FA04	46 D7	LSR $D7
FA06	66 BF	ROR $BF	parity count
FA08	A2 DA	LDX #$DA	set timimg max byte
FA0A	20 E2 F8	JSR $F8E2	set timing
FA0D	4C BC FE	JMP $FEBC	restore registers and exit interrupt
FA10	A5 96	LDA $96	get cassette block synchronization number
FA12	F0 04	BEQ $FA18
FA14	A5 B4	LDA $B4
FA16	F0 07	BEQ $FA1F
FA18	A5 A3	LDA $A3	get EOI flag byte
FA1A	30 03	BMI $FA1F
FA1C	4C 97 F9	JMP $F997
FA1F	46 B1	LSR $B1	shift tape timing constant max byte
FA21	A9 93	LDA #$93
FA23	38	SEC
FA24	E5 B1	SBC $B1	subtract tape timing constant max byte
FA26	65 B0	ADC $B0	add tape timing constant min byte
FA28	0A	ASL A
FA29	AA	TAX	copy timimg high byte
FA2A	20 E2 F8	JSR $F8E2	set timing
FA2D	E6 9C	INC $9C
FA2F	A5 B4	LDA $B4
FA31	D0 11	BNE $FA44
FA33	A5 96	LDA $96	get cassette block synchronization number
FA35	F0 26	BEQ $FA5D
FA37	85 A8	STA $A8	save receiver bit count in
FA39	A9 00	LDA #$00	clear A
FA3B	85 96	STA $96	clear cassette block synchronization number
FA3D	A9 81	LDA #$81	enable timer A interrupt
FA3F	8D 0D DC	STA $DC0D	save VIA 1 ICR
FA42	85 B4	STA $B4
FA44	A5 96	LDA $96	get cassette block synchronization number
FA46	85 B5	STA $B5
FA48	F0 09	BEQ $FA53
FA4A	A9 00	LDA #$00
FA4C	85 B4	STA $B4
FA4E	A9 01	LDA #$01	disable timer A interrupt
FA50	8D 0D DC	STA $DC0D	save VIA 1 ICR
FA53	A5 BF	LDA $BF	parity count
FA55	85 BD	STA $BD	save RS232 parity byte
FA57	A5 A8	LDA $A8	get receiver bit count in
FA59	05 A9	ORA $A9	OR with start bit check flag
FA5B	85 B6	STA $B6
FA5D	4C BC FE	JMP $FEBC	restore registers and exit interrupt