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Atari 8bit Technical Documentation
Documentation Technique Atari 8bit

These kind of file can be sometimes found on DOS disk. They are like DOS file as they contain the information bytes for the sector chaining (Byte per Sector and Next Sector), but they don't have data block or segment and they don't have the standard $FF $FF header, but they have a BOOT program header.

1. First sector:

In the fisrt sector we can find the BOOT sector structure with the chaining sector information bytes.

BYT#>00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F	CHAR>	0123456789ABCDEF
00 : 00 54 00 3E 70 3F 00 00 00 00 00 00 00 00 00 00 10 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 30 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 40 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 60 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 70 : 00 00 00 00 00 00 00 00 00 00 00 00 00 04 93 7D		................ ................ ................ ................ ................ ................ ................ ...............}
Checksum = $22, Next sector = $093, Bytes per sector = $7D

The 6 first bytes of the sector give the following information:

00 54 : these 2 bytes clearly show that this is not a standard Atari binary DOS file, because this header is different than $FF $FF. This is then some boot data code converted into data file. The loading procedure is as following.

All the data will be loaded and stored in memory from the $3E00 Atari memory address (given by the byte #2 and byte #3), as for a boot program. The last 3 bytes of the sector are not included in the data to be stored in memory, because they are used for the sector chaining: the next sector to load and the byte of data number per sector. But the 6 first bytes are data to be stored, and they will be stored in the memory at the loading address.

2. Following sectors:

Identical to the sector of the standard Atari DOS binary file sector.

The loading of the boot file is complete when the next sector value is null and the number of bytes to read in the last sector is reached.

3. Binary BOOT file execution (running):

The 2 first bytes are not used for the boot file loading (byte #0 and #1).

The byte #4 and #5, which give the $3F70 address in our example, set the DOSINI (and CASINI) Atari vectors.

Once loaded into memory, the loading file utility program runs the instruction JSR (storing address value + 6) i.e. JSR $3E06 in our exemple.

If a RTS instruction is executed, then the loading file utility program runs the JSR (DOSINI) instruction, i.e. JSR $3F70 in our exemple.

If a RTS instruction is executed again, then the loading file utility program runs the JMP (DOSVEC) instruction. The DOSVEC Atari vector must then be set in the executed file code (this is usually done in the DOSINI subroutine, in the instruction code following the $3E06 address generally).