+3DOS File Header

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Tape files have headers which contain some system information. +3DOS files may, or may not, have headers. All files created by BASIC's SAVE command will have headers.

The +3DOS header mechanism provides a dedicated 8 byte area in each headed file reserved for BASIC's use. The remainder of the header is reserved for +3DOS. This 8 byte header is utilised in files created by BASIC commands (see DOS OPEN description).

+3DOS files may have a single header in the first 128 bytes of the file - the header record. These headers are detected by a 'signature' and checksum. If the signature and checksum are as expected, then a header is present; if not, there is no header. Thus, it is possible, but unlikely, that a file without a header could be mistaken for one with a header.

BASIC File Type

Type	File Description			Param 1			Param 2
0	BASIC Program			Line number to start			Start of variable area relative to program start
1	Number Array			Variable name, e.g. if DIM a() then this is 0x80 a=0x81, y=0x99
2	Character Array			Variable name, e.g. if DIM a$() then this is 0x80 a=0xC1, y=0xD9
3	CODE			Start when saved			32768

Notes

• The file header is the first 128 bytes of the file on disk.
• The Length at bytes 11 is a 32-bit number, the max file size is 8Mb. This includes the 128 byte header.
• The BASIC header at byte 15 is what IX points to after a call to DOS_READ_HEAD so (IX+0) is the File Type.
• The length in the BASIC header is the actual length of the file, without the 128 byte header.
• For File Type 0 - BASIC, Param 1 is either the line number to start execution or 0x8000 for none. Param 2 is the offset of variables. For most purposes this can be the length of the program so no variables.
• For File Type 3 - CODE, Param 1 is the load address. Param 2 is unused
• The checksum byte 127 in the header is the sum of the rest of the header, not XOR like tape files.