Commodore 64: Supertape loader
Can be found on 'Input 64' tapes (German disk-mag) also on some games by "Magic Bytes" such as Blue Angel 69, Beam, Crazy Cars, Mini Golf, USS John Young, Cyberworld, Domination.
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3 different pulses are used...
- (S)hort : 261 cycles (TAP byte= $21)
- (M)edium : 396 cycles (TAP byte= $32)
- (L)ong : 532 cycles (TAP byte= $43)
Endianess = LSbF
Each pulse type (S,M or L) has meaning depending on the current ' Status '...
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Status 1..
Pulse | Bit sequence
S 0
M 1,0 (+ switch to other status)
L 1,1 (+ force status 1)
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Status 2..
Pulse | Bit sequence
S 0
M 1 (+ switch to other status)
L 1,1 (+ force status 1)
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The loader uses an internal BIT BUFFER, and when it has (at least) 8 bits available it will decode them as a byte and store at the current destination address.
It is important to note that after a byte is decoded, the BIT BUFFER is not flushed, any bits remaining will be used as part of the next byte.
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ie..
V (bit pointer)
BIT BUFFER 15 14 13 12 11 10 09 08 : 07 06 05 04 03 02 01 00
0 0 0 0 0 0 0 1 : 1 1 1 1 1 1 1 1 (byte = $FF)
As each pulse is "unpacked" to its bit sequence, those bits are pushed into the buffer and the pointer is incremented by the number of bits pushed in, when the pointer reaches 8 (or higher), the lower 8 bits in the buffer (00 ->07) are recorded as the finished byte and the whole buffer contents shifts right by 8 places.
V
BIT BUFFER 15 14 13 12 11 10 09 08 : 07 06 05 04 03 02 01 00
0 0 0 0 0 0 0 0 : 0 0 0 0 0 0 0 1 (one bit remains after last decode)
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Pilot byte : $16 (*63 or 64 is common)
Sync byte : $2A (for header blocks). $C5 (for data blocks).
Note :
Header Sync byte *can* be $3A, the loader AND's them with $EF and just looks for $2A.
Each file consists of two blocks, the header and the data. Each block follows a pilot, which is usually around 64 bytes of $16.
The Supertape header file is 27 bytes long, its structure is...
ofst size
00-15: 16 bytes : Filename
16 : 1 byte : $00 (not used). always seems to be #$01 on first 2 header files for no apparent reason.
17-18: 2 bytes : Start address (LSBF).
19-20: 2 bytes : Length of file (LSBF).
21-24: 4 bytes : $00 (not used).
25-26: 2 bytes : Parity-checksum, should match the amount of Bit 1's in header bytes 0 to 24.
The structure of the data-block is even more simple...
n bytes : data
followed by two bytes parity-checksum, that should match the amount of Bit 1's in data modulo $10000.
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Notes:
The formatting of the pilot sequence in terms of pulses is as follows...
1st pilot byte... : 2nd pilot byte...
S, L, S, M, S, S... S, M, M, M, S, S, S... ; repeats til sync byte is found.
0 11 0 10 0 0 : 0 1 10 1 0 0 0
("Status 1" $16) : ("Status 2" $16)
The pilot consists of a number of bytes with value $16 when decoded, the above shows the alternating encoding of each byte, the first pilot byte contains 1 ' M ' pulse, this switches the loader to status 2, the second pilot byte has 3 ' M ' pulses and this leaves the loader in status 1 ready to correctly decode the next "Status 1" pilot byte.
The ' L ' pulse in the " Status 1 " pilot byte will of course force the loader to status 1 if it isnt already there and guarantees the correct pilot synchronization within a few pulses time.
So, the ' status ' of the loader is ' 1 ' at the start of pilot but it can actually be in either state when the 1st proper data byte is reached (and there MAY be bits left in the buffer too), it really depends on the quantity of pilot bytes read through and the value of the sync byte (differs for header and data blocks).
The important point here is that to correctly decode the data, you must first read through pilot and sync bytes to get the loader (or simulated loader) into the correct state, this includes both the STATUS and the BIT BUFFER .
Note:
TAPs I (SJW) have examined frequently contain 6 " stray " pulses before the 1st (Status 1) pilot byte, the stray pulses appear to contain a ' broken ' " Status 2 " pilot byte.
I would recommend this be removed by anyone writing a cleaning tool but it does suggest that originally the pilot tone began in " Status 2 " (for no particularly good reason).
