Specifications

AMD Geode™ GeodeROM Functional Specification 67
Non-Removable Media
32087C
6.5 INT 41h/46h Vectors
INT 41h and INT 46h are not interrupt vectors in the normal sense. Attempting to execute an INT 41h or INT 46h instruction
is likely to crash the system immediately. These locations hold 32-bit segment:offset pointers to physical parameter tables
for fixed disks 0 and 1, respectively. If the fixed disk count in location 40:75h is one, only the pointer in the INT 41h location
(0:104h) is valid. If the value in 40:75h is two, both pointers are valid.
The INT 41h and INT 46h vectors are “well-behaved” low memory locations, meaning they should contain valid information
on any PC/AT or compatible system. Each of these pointers references the same type of parameter table structure, shown
in Section 6.5.1 in MASM-compatible form.
6.5.1 Standard Disk Drive Parameter Table
Below is the standard IBM PC/AT style fixed disk parameter table.
;STRUCT FIXED_PARMS – THE ORIGINAL PC/AT FIXED DISK PARAMETER TABLE
FIXED_PARMS STRUCT 1
FP_CYLS WORD ? ; Maximum cylinder
FP_HEADS BYTE ? ; Maximum head
FP_RES1 WORD ? ; Unused
FP_WR_PRECMP WORD ? ; Write pre-comp start cylinder
FP_RES2 BYTE ? ; Unused
FP_CTL BYTE ? ; Extended head count flag
FP_RES3 BYTE ? ; Unused
FP_RES4 BYTE ? ; Unused
FP_RES5 BYTE ? ; Unused
FP_LANDING WORD ? ; Head parking cylinder
FP_SPT BYTE ? ; Sectors per track
FP_RES6 BYTE ?
FIXED_PARMS ENDS
The following clarifies some of the fields in the Fixed Disk Parameter table above:
• FP_WR_PRECMP, Write Pre-Comp:
— Early drives were formatted with the same number of sectors per cylinder and required the BIOS to compensate for
the fact that the magnetic media near the edge of a disk is farther apart than the media located closer to the center of
the disk. The Write Pre-Comp field tells the BIOS the exact cylinder at which to start compensating during write oper-
ations on older drives. Modern drives vary the number of sectors per track and automatically convert the caller’s CHS
parameters to the correct physical byte on the disk and therefore do not require the information in this field.
• FP_LANDING, Head Parking Cylinder:
— Early drives required BIOS or application support to seek to the landing-zone cylinder, a cylinder designated as a
safe place to leave the read/write heads if the drive was to be physically moved. Modern drives automatically park
their read/write heads when power is removed and the information in this field is no longer needed.
• FP_CTL, Extended Head Count Flag
— On old machines, this flag signifies a drive that contains eight or more read/write heads. Although this bit is still
supported, applications and utilities pay little or no attention to it anymore.