curzon@kaoa01.dec.com (10/23/86)
Can anyone give the lowdown on the boot sectors of an Atari disk?
ie: how the "boot" ROM routines work,
what parameters are picked up from the disk,
where are they located,
what functions served by the boot sector code vs
the DOS itself
differences between the boot sectors on a DOS disk
and a "boot" disk
and anything else that is interesting or relevant!
Thanks
Dick Curzon
Digital Equipment of Canada
PO Box 13000
Kanata Ontario K2K 2A6
Canada.
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(ARPA) curzon%kaoa01.DEC@decwrl.ARPAhans@umd5 (Hans Breitenlohner) (11/04/86)
In article <6074@decwrl.DEC.COM> curzon@kaoa01.dec.com writes: >Can anyone give the lowdown on the boot sectors of an Atari disk? > First boot sector: Byte 0: A flag byte. As far as I know nobody uses it for anything. It ends up at location $240 (more on that later). This is zero for most disks I have looked at, but not all. Byte 1: Number of sectors to be loaded by the ROM boot code. Bytes 2,3: Memory address to load first boot sector (including the first 6 bytes) in low byte, high byte format. Bytes 4,5: Address (low byte, high byte) of startup code. Byte 6 and up: Initialization code (note: this is the code itself, not the address of the code). Boot procedure: (this applies primarily to an XL system, I can only assume that 400/800's are similar). After machine initialization is complete, the disk boot routine is called. If disk unit 1 is present, it reads sector 1 to address $400. The first four bytes are copied to $240. The next two bytes are copied to $0C and $0D. The whole sector is then copied to the address given in bytes 2 and 3. Sectors are then read in sequence and copied to memory behind the first. The count at $241 is decremented as sectors are read, and will be zero when the boot is done. Next the initialization code will be called, using a JSR instruction. If it returns with the carry flag set, the BOOT ERROR message is displayed, and the boot process will be repeated. Next the startup code will be called through the vector at $0c, $0d, again via JSR. Upon returning location $9 will be set to 1, indicating successful disk boot. Next a subroutine to load a relocatable peripheral handler is executed. I have no idea what it is meant to do, except that it probably won't do anything unless you have a strange peripheral on your serial bus. (If anyone can shed light on this, please post!) After that $244 is set to zero, indicating that a warm start can be done for the next reset. Finally, either a JMP ($BFFA) or a JMP ($000A) is performed, depending on whether a cartridge (or built in basic) exists, and the cartridge's option byte. Notes: 1. Either the initialization or the startup code must put an appropriate address at location $0A and $0B. This vector will be used when initialization is complete, and on subsequent warm starts. 2. The ROM boot code can only deal with 128 byte sectors. To work with double density disks, only the first three sectors of a disk may be used in the boot, anything else must be read by user supplied routines. This, of course, is the reason for the peculiar sector lengths for the first three sectors of double density disks. 3. For DOS disks, the three boot sectors contain mainly pointers, the disk i/o routines, and the initialization code. The initialization code loads DOS.SYS (i.e. FMS and the resident portion of DOS). The startup code sets the DOS vector (at $0A, $0B) to a routine which loads DUP.SYS, initializes buffers and disk drive tables, loads and runs AUTORUN.SYS if there is one, and then returns. After boot completion (or when calling DOS from BASIC) the non-resident part of DOS will be loaded and executed. Most of this is extracted from: Mapping the Atari by Ian Chadwick Inside Atari Dos by Bill Wilkinson and disassemblies of code (i.e. DOS and OS ROMs). I hope this is what you were looking for!