This 16-byte structure can be found at the vector address of INT 41H (the
4-byte address at 0:0104).

The parameters for a second hard disk (if any) are found at the vector for
INT 46H.  These tables specify a variety of critical variables for hard
disk drive operations.

XT The switches on the controller board select one of four drive
types for each hard disk drive.  At boot time, the BIOS sets
INT 41H and INT 46H as indicated on these switches.

AT The INT 41H and INT 46H vectors are set according to the
AT Drive Type value which is a CMOS Memory variable stored by
the "Setup" program.

HardDiskParmRec
Offset Size Contents
                                                                          
+0      2  wMaxCyls     maximum number of cylinders
+2      1  bMaxHds      maximum number of heads
+3      2  wRWCyl       starting reduced-write current cylinder
+5      2  wWPCyl       starting write precompensation cylinder
+7      1  bECCLen      maximum ECC data burst length
+8      1  rOptFlags    drive step options:
bits 2-0: drive option
bit    7: disable retries
bit    6: disable ECC
+9      1  bTimeOutStd  standard timeout value
+0aH     1  bTimeOutFmt  timeout value for format drive
+0bH     1  bTimeOutChk  timeout value for check drive
+0cH     4  res          (reserved)
16               length of a HardDiskParmRec

See Also: INT 13H (disk I/O)
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Hard Disk Parameter Table