New subject: [Bochs-developers] [Bug #2956217] INT13 AH=8 returns wrong valueswhen cylinders=1

4 Mar 2010

      Sebastian Herbszt wrote:
...
The following disk description is used to trigger this bug:
ata0-master: type=disk, path=fdtest.img, mode=flat, cylinders=1, heads=150, spt=63
The current BIOS returns CX=0xFFFF. This value is wrong.
rombios.c got
 nlc = nlc - 2; /* 0 based, last sector not used */
 SET_AL(0);
 SET_CH(nlc & 0xff);
 SET_CL(((nlc >> 2) & 0xc0) | (nlspt & 0x3f));
 SET_DH(nlh - 1);

with BX_USE_ATADRV and without it's
 max_cylinder = hd_cylinders - 2; /* 0 based */
 SET_AL(0);
 SET_CH(max_cylinder & 0xff);
 SET_CL(((max_cylinder >> 2) & 0xc0) | (hd_sectors & 0x3f));
 SET_DH(hd_heads - 1);

nlc and max_cylinder are Bit16u so substracting 2 from 1 gets us the wrong value.
Section "INT 13 - DISK - GET DRIVE PARAMETERS (PC,XT286,CONV,PS,ESDI,SCSI)"
from Ralf Browns "Interrupt List, part 2 of 18" got
"the maximum cylinder number reported in CX is usually two less than
  the total cylinder count reported in the fixed disk parameter table
  (see INT 41h,INT 46h) because early hard disks used the last cylinder
  for testing purposes; however, on some Zenith machines, the maximum
  cylinder number reportedly is three less than the count in the fixed
  disk parameter table.
for BIOSes which reserve the last cylinder for testing purposes, the
  cylinder count is automatically decremented"
I don't think Bochs BIOS uses "the last cylinder for testing purpose" so i would suggest
decrementing by 1 not 2. This should also fix the reported problem.
Thoughts?

Sebastian

SeaBIOS has the same problem and similar wrong comment about last sector when
decrementing the cylinder count:
} else if (regs->dl < EXTSTART_CD) {
        // Hard drive
        count = GET_BDA(hdcount);
        nlc--;  // last sector reserved
    } else {
- Sebastian

Re: [SeaBIOS] [Bochs-developers] [Bug #2956217] INT13 AH=8 returns wrong valueswhen cylinders=1