The IDEATA Interface

1
The IDE/ATA Interface

• How can our mini-Operating System, executing in
  protected mode, access the hard disk?

2
The EDD services

• So far we have relied upon the Extended Disk
  Drive service-functions (available in real-mode
  via int 0x13) to perform any transfers of data
  between main memory and our computer systems
  hard disk
• But these service-functions in ROM-BIOS are
  designed to be executed in real-mode
• Can we achieve their effects when PE1?

3
Persistent data storage

• Any operating system we might design, no matter
  how minimal, would need to offer a way to write
  and to read persistent data, organized into
  some kind of file system that is maintained on
  a secondary storage device, such as a hard disk
  or a diskette

4
Hardware interfacing

• Modern computer platforms are expected to provide
  a specially designed peripheral processor, the
  hard disk controller, which can be programmed by
  system software to carry out data-transfers
  between a disk and the computers primary memory
• The programming interface for hard disk
  controllers continues to evolve, but a stable set
  of standard capabilities exists

5
A few cautions

• Our classroom and laboratory computers are shared
  by many users who are taking various computer
  sciences courses
• Writing to the hard disk in a careless way can do
  damage to the operating systems (making a machine
  completely unusable)
• In your early experiments you will need to use
  great caution to avoid corrupting vital areas of
  these shared hard disks!

6
Fixed-Size blocks

• All data-transfers to and from the hard disk are
  comprised of fixed-size blocks called sectors
  (whose size equals 512 bytes)
• On modern hard disks, these sectors are
  identified by sector-numbers starting at 0
• This scheme for addressing disk sectors is known
  as Logical Block Addressing (LBA)
• So the hard disk is just an array of sectors

7
Platform-specific parameters

• Although older PCs used some standard I/O
  port-addresses for communicating with their Disk
  Controllers, newer PCs like ours allow these
  port-addresses to be assigned dynamically during
  the systems startup
• To keep our demonstration-code as short and
  uncluttered as possible, we will hard-code the
  port-numbers our machines use

8
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9
The ATA/IDE Interface

• All communication between our driver and the Hard
  Disk Controller is performed with in and out
  instructions that refer to ports
• Older PCs had standard i/o port-numbers for
  communicating with the Disk Controller
• But newer PCs assign these dynamically

10
Command Block registers

• When reading
• Data
• Error
• Sector Count
• LBA Low
• LBA Mid
• LBA High
• Device
• Status

• When writing
• Data
• Features
• Sector Count
• LBA Low
• LBA Mid
• LBA High
• Device
• Command

11
Control Block Registers

• When reading
• Alternate Status

• When writing
• Device Control

INCRITS InterNational Committee on Information
Technology Standards Committee T-13
12
Bus Master DMA

• When reading
• Primary DMA Control
• Primary DMA Status
• Primary PRD Pointer

• When writing
• Primary DMA Control
• Primary DMA Status
• Primary PRD Pointer

INTEL ICH6 I/O Controller Hub Datasheet SATA
Controller Registers
13
Two I/O design-paradigms

• PIO Programmed I/O for reading
• The cpu outputs parameters to the controller
• The cpu waits till the data becomes available
• The cpu transfers the data into main memory
• DMA Direct Memory Access for reading
• The cpu outputs parameters to the controller
• The cpu activates the DMA engine to begin
• The cpu deactivates the DMA engine to end

14
PIO algorithm overview

• First select the device to read from
• Wait until the controller is not busy and does
  not have any data that it wants to transfer
• Write to Command Blocks Device register to
  select the disk to send the command to
• Wait until the controller indicates that it is
  ready to receive your new command

15
PIO overview (continued)

• Place the commands parameters into the
  appropriate Command Block registers
• Put command-code in Command register
• Then wait until the controller indicates that it
  has read the requested sectors data and is ready
  for you to transfer it into memory
• Use a loop to input 256 words (one sector) from
  the Command Blocks Data register

16
PIO overview (conclusion)

• After you have transferred a sector, check the
  Controller Status to see if there were any errors
  (if so read the Error register)
• To implement this algorithm, we need to look at
  the meaning of some individual bits in the Status
  register (and Error register)

17
Status register (cmd7)
7 6 5 4 3
2 1 0
BSY
DRDY
DF
DRQ
ERR
Legend BSY (Device still Busy with prior
command) 1yes, 0no DRDY (Device is Ready for
a new command) 1yes, 0no DF (Device Fault
command cannot finish) 1yes, 0no DRQ
(Data-transfer is currently Requested) 1yes,
0no ERR (Error information is in Error
Register) 1 yes, 0no
18
Device register (cmd6)
7 6 5 4 3
2 1 0
1
LBA (1)
1
DEV (0/1)
Sector-ID 27..24
Legend LBA (Logical Block Addressing) 1yes,
0no DEV (Device selection) 1slave, 0master
Sector-ID Most significant 4-bits of 28-bit
Sector-Address
19
Error register (cmd1)
7 6 5 4 3
2 1 0
UNC
MC
IDNF
MCR
ABRT
NM
Legend UNC (Data error was UnCorrectable)
1yes, 0no MC (Media was Changed) 1yes,
0no IDNF (ID Not Found) 1yes, 0no MCR
(Media Change was Requested) 1yes, 0no ABRT
(Command was Aborted) 1 yes, 0no NM (No
Media was present) 1yes, 0no
20
Device Control register (ctl2)
7 6 5 4 3
2 1 0
HOB
0
0
0
0
SRST
nIEN
0
Legend HOB (High-Order Byte) 1yes, 0no
SRST (Software Reset requested) 1yes, 0no
nIEN (negate Interrupt Enabled) 1yes,
0no NOTE The HOB-bit is unimplemented on our
machines it is for large-capacity disks that
require 44-bit sector-addresses
21
PIO demo ideload1.s

• We have created a substitute boot-loader which
  implements the same functionality as our previous
  quickload.s program
• Its purpose is two-fold
• It shows how to read 16 disk sectors with PIO
• It could easily be rewritten to be executed in
  protected-mode by an operating system

22
Advantage of DMA

• For a multiprogramming operating system that
  employs timesharing to concurrently execute
  multiple tasks, there is an obvious advantage in
  offloading the data-transfer step to a
  peripheral processor
• It frees the CPU to do work on other tasks during
  the time-interval when the data is actually being
  transferred

23
DMA Command register
7 6 5 4 3
2 1 0
0
0
0
0
Read/ Write
0
0
Start /Stop
Legend Start/Stop 1 Start DMA transfer 0
Stop DMA transfer Read/Write 1 Read to
memory 0 Write from memory
24
DMA Status register
7 6 5 4 3
2 1 0
PRDIS
-
-
0
0
INT
ERR
ACT
Legend ACT (DMA engine is currectly Active)
1 yes 0 no ERR (The controller encountered
an Error) 1yes 0no INT (The controller
generated an Interrupt 1yes 0no PRDIS (PRD
Interrupt Status) 1active, 0inactive
Software clears these labeled bits by writing 1s
to them
25
PRD Pointer register
31
0
Physical memory-address of the PRD Table (must be
quadword aligned)
Each PRD (Physical Region Descriptor) consists of
these three fields
Base-address of the physical region
bytes 3..0
Size of the region
Reserved (0)
E O T
bytes 7..4
NOTE The total size of the PRD Table cannot
exceed 64KB
26
DMA algorithm overview

• First select the device to read from
• Wait until the controller is not busy and does
  not have any data that it wants to transfer
• Write to Command Blocks Device register to
  select the disk to send the command to
• Wait until the controller indicates that it is
  ready to receive your new command
• NOTE This step is the same as for PIO

27
DMA overview (continued)

• Engage the DMA engine for writing to memory by
  outputting 0x08 to the DMA Command Port
• Clear the labeled bits in the DMA Status register
• Place the commands parameters into the
  appropriate IDE Command Block registers
• Put command-code in IDE Command register
• Activate the DMA data-transfer by outputting 0x09
  to the DMA Command register
• Then wait until the DMA Status register indicates
  that the DMA data-transfer has been completed

28
DMA algorithm (concluded)

• Turn off the DMA engine by writing 0x08 to the
  DMA Command register (to clear ACT)
• Clear the labeled bits in the DMA Status register
  (by writing 1s to those bits)
• Read the IDE Status register (to clear any
  interrupt from the IDE Controller), and if bit 0
  is set (indicating some error-information is
  available), then read IDE Error register

29
DMA demo ideload2.s

• We also created a substitute boot-loader which
  implements the same functionality as our previous
  quickload.s program, but uses DMA instead of
  PIO
• Its purpose is two-fold
• It shows how to read 16 disk sectors via DMA
• It could easily be rewritten to be executed in
  protected-mode by an operating system

30
In-class exercise

• Choose one of these demo-programs and use the
  ideas it contains to perform a disk read
  operation while in protected-mode
• For example, write a program that reads the hard
  disks Master Boot Record (i.e., sector 0) and
  display its Partition Table