BIOS INT 13H with AH=2 can only read 72 sectors each time. Why?

问题

I am using Bochs 2.4.5 to write a boot sector code. I use the INT 13H to read sectors from floppy. But I found that if the sector count to read > 72, the INT13 will fail. And the return code is AH=1. Below's the code and here is the INT13. The return code is AH=1.

Why can't the INT 13H read more than 72 sectors?

   xorb %ah, %ah
   xorb %dl, %dl
   int $0x13      # reset the floppy

   movw $0x8000, %ax
   movw %ax,%es        
   movw $0, %bx  # ES:BX is the buffer  
   movb $0x02, %ah
   movb $73, %al # how many sectors to read. 72 is ok, but >=73 is wrong.
   movb $0, %ch
   movb $1, %cl
   movb $0, %dh
   movb $0, %dl

   int $0x13

Thanks for any help.

Update

Following Matthew Slattery's instruction, I found the corresponding code. I listed it here for others as confused as me. The full code is located here.

7220       if ((drive > 1) || (head > 1) || (sector == 0) ||
7221           (num_sectors == 0) || (num_sectors > 72)) {
7222         BX_INFO("int13_diskette: read/write/verify: parameter out of range\n");
7223         SET_AH(1);
7224         set_diskette_ret_status(1);
7225         SET_AL(0); // no sectors read
7226         SET_CF(); // error occurred
7227         return;
7228       }

回答1:

You're using Bochs, so the answer can be found in the Bochs BIOS source: the BIOS is performing an explicit range check on the number of sectors, and rejecting it if it is greater than 72 (or equal to 0).

回答2:

The simple answer as pointed out by Matthew Slattery is that this is merely a range check performed by Bochs. This is based on a 2.88Mb floppy multi track floppy disk controller read which would be limited to 72 sectors.

The happy answer is that current BIOS due to backwards compatibility only support a single side read which for a 1.44Mb floppy is a maximum of 18 sectors. This depends on the starting sector so that the actual maximum would be up to the total remaining on the track. Booting and running from a USB flash drive is a simple alternative to the obsolete floppy disk for os dev. The same BIOS disk/diskette functions can be used depending on the boot drive identifier (dl register).

回答3:

According to Standard Floppy Disks Supported by MS-DOS, the number of sectors per track on a 1.44 MB floppy disk is 18. Since 18*4 = 72, that might be a clue. It's possible that the BIOS doesn't want to read more than a certain number of tracks at once.

来源：https://stackoverflow.com/questions/3645803/bios-int-13h-with-ah-2-can-only-read-72-sectors-each-time-why

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operating-system

dos

bios