Specifications

68 AMD Geode™ GeodeROM Functional Specification
Non-Removable Media
32087C
6.5.2 Enhanced Disk Drive Parameter Table
The Enhanced Disk Drive (or EDD) table is backwardly compatible with the standard parameter table (described in Section
6.5.1 "Standard Disk Drive Parameter Table" on page 67). The preceding discussion (Section 6.5 "INT 41h/46h Vectors" on
page 67) applies to this type of table as well as the standard IBM drive parameter table. Modern systems support the EDD
table, which is a superset of IBM’s original table.
The EDD table format was introduced by Phoenix Technologies and is valuable for a few reasons. First, because it is check-
summed, an EDD table allows applications to verify that the data being examined is really a disk parameter table and not
just random data in the system’s ROM or RAM. Second, the EDD table provides the BIOS scratchpad space to record
geometry translation information. Lastly, the format of the EDD table allows system software to determine if and what kind
of translation is currently active based on the contents of its physical vs. logical cylinder, head, and sector counts.
The format of the EDD table appears here in MASM-compatible format:
; STRUCT EDD_PARMS – ENHANCED DISK DRIVE TABLE
EDD_PARMS STRUCT 1
LOGCYL DW ? ; Maximum Logical Cylinder
LOGHEAD DB ? ; Maximum Logical Head
SIGNATURE DB ? ; Translation Active Signature
PHYSPT DB ? ; Physical Sector/Track Count
WPRECOMP DW ? ; Write Pre-Comp Cylinder
RSV2 DB ? ; Unused/Reserved
CTRLBYTE DB ? ; Extended Head Count Flag
PHYCYLN DW ? ; Maximum Physical Cylinder
PHYHEAD DB ? ; Maximum Physical Head
LZONE DW ? ; Head Park Cylinder
LOGSPT DB ? ; Logical Sector/Track Count
CHKSUM DB ? ; Checksum Byte
EDD_PARMS ENDS
The difference between “physical” and “logical” parameters is a source of confusion when learning about and interfacing
with IDE devices.
The physical parameters that a drive reports are a reference point shared by the drive and the system software. It is hard to
imagine a 2.5” portable fixed disk actually having fifteen individual read/write heads inside a housing that stands about a
half inch tall, however, this is the number of “physical” heads that many such drives report.
Logical parameters are those parameters that the system BIOS reports to the operating system. They may or may not be
the same as the physical parameters reported by the drive itself. If the logical parameters differ from the physical parame-
ters, the BIOS has some kind of translation active. To understand why translation is important and often necessary, refer to
Section 6.3.2.1 "Large IDE Drive Support API Subset" on page 56, which describes the methods used by system firmware
and drivers to support “Large” IDE drives.