Commodore 64: C64 ROM Tape loader
Can be found... EVERYWHERE!...The only tape loader available on a freshly booted C64 is this one, In order for cassette software to load into the C64 there *must* be files of this format on the tape, if only at the beginning (typically containing an alternative 'turbotape' loader).
The loader is unpopular because it is PAINFULLY slow due to the fact that all files are written out twice and each encoded byte requires 20 pulses!. However, there DO exist a few commercial games (not many) written out using ONLY this format, they include...
Laser Zone, Burger Time, Gridrunner, Matrix, Caverns of Khafka and H.E.R.O.
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3 different pulse types are used...
(S)hort : $30
(M)edium : $42
(L)ong : $56
Pulses are always interpreted as a pair...
(L,M) = new-data marker
(L,S) = end-of-data marker
(S,M) = 0 bit
(M,S) = 1 bit
Pilot stream: 'S' pulses (various quantities).
Pilot terminates when a new byte begins,
Each byte is organized as follows...
(L,M) (?,?) (?,?) (?,?) (?,?) (?,?) (?,?) (?,?) (?,?) (?,?)
| | | | | | | | | |
| bit0 bit1 bit2 bit3 bit4 bit5 bit6 bit7 |
| |
new-data marker checkbit Each byte is encoded as a sequence of 20 pulses (10 pairs), beginning with a new-data marker (L,M), then the 8 bits of the byte in LSbF format and always terminating in a checkbit which is computed with...
1 xor Bit0 xor Bit1 xor Bit2 xor Bit3 xor Bit4 xor Bit5 xor Bit6 xor Bit7.
The data is finished when you read an end-of-data marker (L,S) instead of (L,M) at the start of a new byte.
Note: Each block has a final checkbyte which is always the last byte of the block, ie. the one before the end-of-data marker.
The checkbyte is computed as.. 0 xor all other bytes.
Following the end-of-data marker is always some trailing tone ('S' pulses).
Structure :
When the C64 saves a program to tape it creates 4 files...
HEADER.
HEADER REPEATED.
DATA.
DATA REPEATED.
The correct structure on tape is...
> pilot tone (27136 ($6A00) 'S' pulses)
> HEADER (9+192+1 bytes)
> pilot tone (79 'S' pulses)
> HEADER REPEATED (9+192+1 bytes)
> pilot tone (78 'S' pulses)
> pause (roughly 0.3 sec)
> pilot tone (5376 ($1500) 'S' pulses)
> DATA (9+DataSize+1 bytes)
> pilot tone (79 'S' pulses)
> DATA REPEATED (9+DataSize+1 bytes)
> pilot tone (78 'S' pulses)
Both HEADER and DATA blocks have the following sequence in the first 9 bytes:
$89 $88 $87 $86 $85 $84 $83 $82 $81
Both HEADER and DATA *repeated* blocks have the same sequence with bit 7 clear:
$09 $08 $07 $06 $05 $04 $03 $02 $01
For the Header the following information is sent after the sync sequence:
1 Byte : File type...
$01= BASIC program
$02= Data block for SEQ file
$03= PRG file
$04= SEQ file
$05= End-of-tape marker
2 Bytes : Start Address (LSBF).
2 Bytes : End Address (LSBF).
16 Bytes : File Name.
After the Name there is some additional data, making the WHOLE length of the Header block 202 bytes long....
- 9 byte sync sequence
- 21 bytes of file information (see above)
- 171 bytes unused
- 1 byte checksum
The "unused" 171 bytes in the header block are often used by commercial loaders to store executable loader code or pretty much any additional data or code the loader or program may require.
Notes :
Header blocks always load into the Tape Buffer at $033C.
If the File Type is 'BASIC Program' the start address for loading will be $0801 regardless of what may be written in the 'Start Address' Fields
