Session 8,9 PCI Express

Soft Polynomials (I) Pvt. Ltd.
PCI EXPRESS
SYSTEM
ARCHITECTURE
1Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

• Architectural Perspective
• Architecture Overview
• Address Spaces & Transaction Routing
• Packet Based Transaction
• ACK / NAK Protocol
• QoS / TCs / VCs and Arbitration
• Flow Control
• Interrupts
• Error Detection and Handling
• Physical Layer Logic
• Electrical Physical Layer
Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd. 2
Contents

ARCHITECTURAL PERSPECTIVE

Introduction to PCI Express
 PCI Express is the third generation high performance I/O bus
used to interconnect peripheral devices in applications such as
computing and communication platforms.
 It has the application in the mobile, desktop, workstation,
server, embedded computing and communication platforms.

The Role of the Original PCI Solution
Don’t Throw Away What is Good !!! Keep It !!!
 Same usage model and load-store communication model.
 Support familiar transactions.
 Memory read/write, IO read/write and configuration read/write transactions.
 The memory, IO and configuration address space model is the same as PCI and PCI-X
address spaces.
 By maintaining the address space model, existing OSs and driver software will run in
a PCI Express system without any modifications.
 In other words, PCI Express is software backwards compatible with PCI and PCI-X
systems.
 It supports chip-to-chip interconnect and board-to-board interconnect.

Make Improvements for the Future
 It Implements a serial, point-to-point type interconnect for
communication between two devices.
 Multiple PCI Express devices are interconnected via the use of
switches.
 PCI Express transmission and reception data rate is 2.5
Gbits/sec.
 A packet-based communication protocol.
 Hot Plug/Hot Swap

Looking into the Future
 In the future, PCI Express communication frequencies are
expected to double and quadruple to 5 Gbits/sec.
Taking advantage of these frequencies will require Physical
Layer re-design of a device with no changes necessary to the
higher layers of the device design.

Bus Performances and Number of Slots Compared
Bus Type Clock Frequency Peak Bandwidth* Number of Card
Slots per Bus
PCI 32 – bit 33 MHz 133 Mbytes / sec 4 – 5
PCI 32 – bit 66 MHz 266 Mbytes / sec 1 – 2
PCI – X 32 – bit 66 MHz 266 Mbytes / sec 4
PCI – X 32 – bit 133 MHz 533 Mbytes / sec 1 – 2
PCI – X 32 – bit 266 MHz effective 1066 Mbytes / sec 1
PCI – X 32 – bit 533 MHz effective 2131 Mbytes / sec 1
* Double all these bandwidth numbers for 64 – bit bus implementations

PCI Express Aggregate Throughput
 A PCI Express interconnect that connects two devices together is
referred to as a Link.
 A Link consists of either x1, x2, x3, x4, x8, x12, x16 or x32 signal pairs in
each direction.
 These signals are referred to as Lanes.
 To support a greater degree of robustness during data transmission
and reception, each byte of data transmitted is converted into a 10 –
bit code (via an 8b /10b encoder in the transmission device).
The result is 25% additional overhead to transmit a byte of data.

PCI Express Aggregate Throughput
 To obtain the aggregate bandwidth numbers in table multiply 2.5
Gbits/sec by 2 (for each direction), then multiply by number of Lanes, and
finally divide by 10 – bits per Byte (to account for the 8 – to – 10 bit
encoding)
PCI Express
Link Width
x1 x2 x4 x8 x12 x16 x32
Aggregate
Band – Width
(Gbytes/sec)
0.5 1 2 4 6 8 16
Table : PCI Express Aggregate Throughput for Various Link Widths

Performance Per Pin Compared
11
Comparison of Performance Per Pin for Various Buses
 In Figure, the first 7 bars are
associated with PCI and PCI – X
buses where we assume 84 pins per
device.
 This includes 46 signal pins,
interrupt and power management
pins, error pins and the remainder
are power and ground pins.
The last bar associated with a x8
PCI Express Link assumes 40 pins
per device which include 32 signal
lines (8 differential pains per
direction) and the rest are power
and ground pins.
Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

I/O Bus Architecture Perspective
33 MHz PCI Bus Based System
 The PCI bus clock is 33 MHz.
 The address bus width is 32 bits (4GB memory address space), although
PCI optionally supports 64-bit address bus.
 The data bus width is implemented as either 32-bits or 64-bits depending
on bus performance requirement.
 The address and data bus signals are multiplexed on thye same pins (AD
bus) to reduce pin count.
 PCI supports 12 transaction types.
 A PCI master device implements a minimum of 49 signals.

33 MHz PCI Bus Based Platform

33 MHz PCI Based System Showing Implementation of a PCI-to-PCI Bridge
14

15
Typical PCI Burst Memory Read Bus Cycle
Bus / Machine Cycles

PCI Transaction Model - Programmed IO
PCI Transaction Model

17
 CPU communicates with a PCI
peripheral such as an Ethernet device.
 Software commands the CPU to
initiate a memory.
 The North bridge arbitrates for use of
the PCI bus and when it wins
ownership of the bus generates a PCI
memory or IO read/write bus cycle.
 First clock of this bus cycle (known as
the address phase), all target devices
decode the address.

One target decodes the address and
claims the transaction.
The master communicates with the
claiming target.
 Data is transferred between master and
target in subsequent clocks after the address
phase of the bus cycle.
 Either 4 bytes or 8 bytes of data are
transferred per clock tick depending on the
PCI bus width.
 The bus cycle is referred to as a burst bus
cycle if data
is transferred back-to-back between master
and target during multiple data phases of
that bus cycle.

PCI EXPRESS
ARCHITECTURE OVERVIEW

20
Architecture Overview
PCI Cards - Slots

21
Slot Pin Allotment

22
Slot Pin Allotment

PCI Express Transactions
 Communication involves the transmission and reception of
packets called Transaction Layer packets (TLPs).
 PCI Express transactions can be grouped into four categories:
1) Memory
2) IO
3) Configuration
4) Message Transactions
 Transactions are defined as a series of one or more packet
transmissions required to complete an information transfer
between a requester and a completer.

Transaction Type Non-Posted or Posted
Memory Read Non-Posted
Memory Write Posted
Memory Read Lock Non-Posted
IO Read Non-Posted
IO Write Non-Posted
Configuration Read (Type 0 and Type 1) Non-Posted
Configuration Write (Type 0 and Type 1) Non-Posted
Message Posted
PCI Express Non-Posted and Posted Transactions

 For Non-posted transactions, a requester transmits a TLP
request packet to a completer.
 At a later time, the completer returns a TLP completion
packet back to the requester.
 The purpose of the completion TLP is to confirm to the
requester that the completer has received the request TLP.
Non – posted Transactions

 For Posted transactions, a requester transmits a TLP request
packet to a completer.
 The completer however does NOT return a completion TLP
back to the requester.
 Posted transactions are optimized for best performance in
completing the transaction at the expense of the requester not
having knowledge of successful reception of the request by the
completer.
Posted Transactions

PCI Express Transaction Protocol
PCI Express TLP Packet Types 28Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Non – Posted Read Transactions
 Requesters may be root complex or endpoint devices
(endpoints do not initiate configuration read/write requests
however).
 The request TLP is routed through the fabric of switches using
information in the header portion of the TLP.
 The packet makes its way to a targeted completer.
 The completer can be a root complex, switches, bridges or
endpoints.

 The completer can return up to 4 KBytes of data per CplD
packet.
 The completion packet contains routing information necessary
to route the packet back to the requester.
 If a completer is unable to obtain requested data as a result of
an error, it returns a completion packet without data (Cpl) and
an error status indication.
The requester determines how to handle the error at the
software layer.

Non-Posted Read Transaction Protocol

Non – Posted Read Transactions for
Locked Requests
 The requester can only be a root complex which initiates a
locked request on the behalf of the CPU.
 Endpoints are not allowed to initiate locked requests.
 The completer can only be a legacy endpoint.
 The entire path from root complex to the endpoint is locked
including the ingress and egress port of switches in the
pathway.

Locked Requests
Non-Posted Locked Read Transaction Protocol

Locked Requests
 The completer creates one or more locked completion TLP with
data (CplDLk) along with a completion status.
 Requesters uses a tag field in the completion to associate it with a
request TLP of the same tag value it transmitted earlier.
 Use of a tag in the request and completion TLPs allows a
requester to manage multiple outstanding transactions.
 If the completer is unable to obtain the requested data as a
result of an error, it returns a completion packet without data
(CplLk) and an error status indication within the packet.

Locked Requests
 The requester who receives the error notification via the CplLk
TLP must assume that atomicity of the lock is no longer
guaranteed and thus determine how to handle the error at the
software layer.
 The path from requester to completer remains locked until the
requester at a later time transmits an unlock message to the
completer.
 The path and ingress/egress ports of a switch that the unlock
message passes through are unlocked.

Non – Posted Write Transactions
 Non-posted write request TLPs include IO write request (IOWr),
configuration write request type 0 or type 1 (CfgWr0, CfgWr1)
TLPs.
 Memory write request and message requests are posted
requests.
Requesters may be a root complex or endpoint device (though
not for configuration write requests).
 The completer creates a single completion packet without data
(Cpl) to confirm reception of the write request.

Non-Posted Write Transaction Protocol

 The requester gets confirmation notification that the write
request did make its way successfully to the completer.
 If the completer is unable to successfully write the data in the
request to the final destination or if the write request packet
reaches the completer in error, then it returns a completion packet
without data (Cpl) but with an error status indication.
 The requester who receives the error notification via the Cpl
TLP determines how to handle the error at the software layer.

Posted Memory Write Transactions
 If the write request is received by the completer in error, or is
unable to write the posted write data to the final destination due
to an internal error, the requester is not informed via the
hardware protocol.
 The completer could log an error and generate an error
message notification to the root complex. Error handling software
manages the error.

Posted Memory Write Transactions
Posted Memory Write Transaction Protocol

Posted Message Transactions
 There are two categories of message request TLPs, Msg and MsgD.
 Some message requests propagate from requester to completer, some are
broadcast requests from the root complex to all endpoints, some are
transmitted by an endpoint to the root complex.
 The completer accepts any data that may be contained in the packet (if the
packet is MsgD) and/or performs the task specified by the message.
 Message request support eliminates the need for side-band signals in a PCI
Express system.
 They are used for PCI style legacy interrupt signaling, power management
protocol, error signaling, unlocking a path in the PCI Express fabric, slot power
support, hot plug protocol, and vender defined purposes.

Posted Message Transactions
Posted Message Transaction Protocol

Memory Read Originated by CPU,
Targeting an Endpoint
Non-Posted Memory Read Originated by CPU and Targeting an Endpoint

ADDRESS SPACES & TRANSACTIONS
ROUTING

Introduction
 Unlike shared-bus architectures such as PCI and PCI-X, where
traffic is visible to each device and routing is mainly a concern of
bridges, PCI Express devices are dependent on each other to accept
traffic or forward it in the direction of the ultimate recipient.
 As traffic arrives at the inbound side of a link interface (called the
ingress port), the device checks for errors, then makes one of three
decisions:
1) Accept the traffic and use it internally.
2) Forward the traffic to the appropriate outbound (egress) port.
3) Reject the traffic because it is neither the intended target nor an interface to it
(note that there are also other reasons why traffic may be rejected)

Introduction
Multi-Port PCI Express Devices Have Routing Responsibilities

Receivers Check For Three
Types of Link Traffic
 Assuming a link is fully operational, the physical layer receiver interface of each device is
prepared to monitor the logical idle condition and detect the arrival of the three types of
link traffic:
1) Ordered Sets
2) DLLPs
3) TLPs
 Using control (K) symbols which accompany the traffic to determine framing boundaries
and traffic type, PCI Express devices then make a distinction between traffic which is local
to the link vs. traffic which may require routing to other links (e.g. TLPs).
 Local link traffic, which includes Ordered Sets and Data Link Layer Packets (DLLPs),
isn’t forwarded and carries no routing information.
 Transaction Layer Packets (TLPs) can and do move from link to link, using routing
information contained in the packet headers.

Multi-port Devices Assume
the Routing Burden
 It should be apparent in Figure that devices with multiple PCI
Express ports are responsible for handling their own traffic as well as
forwarding other traffic between ingress ports and any enabled
egress ports.
Also note that while peer-peer transaction support is required of
switches, it is optional for a multi-port Root Complex.
It is up to the system designer to account for peer-to-peer traffic
when selecting devices and laying out a motherboard.

Endpoints Have Limited
Routing Responsibilities
 It should also be apparent in Figure that endpoint devices have
a single link interface and lack the ability to route inbound traffic
to other links.
For this reason, and because they don’t reside on shared busses,
endpoints never expect to see ingress port traffic which is not
intended for them (this is different than shared-bus PCI(X), where
devices commonly decode addresses and commands not targeting
them).
Endpoint routing is limited to accepting or rejecting transactions
presented to them.

System Routing Strategy Is
Programmed
 Before transactions can be generated by a requester, accepted by the completer,
and forwarded by any devices in the path between the two, all devices must be
configured to enforce the system transaction routing scheme.
Routing is based on traffic type, system memory and IO address assignments, etc.
In keeping with PCI plug-and-play configuration methods, each PCI express
device is discovered, memory and IO address resources are assigned to them, and
switch/bridge devices are programmed to forward transactions on their behalf.
Once routing is programmed, bus mastering and target address decoding are
enabled.
Thereafter, devices are prepared to generate, accept, forward, or reject
transactions as necessary.

Two Types of Local Link
Traffic
 Local traffic occurs between the transmit interface of one
device and the receive interface of its neighbour for the purpose
of managing the link itself.
 This traffic is never forwarded or flow controlled; when sent, it
must be accepted.
 Local traffic is further classified as Ordered Sets exchanged
between the Physical Layers of two devices on a link or Data
Link Layer packets (DLLPs) exchanged between the Data Link
Layers of the two devices.

Ordered Sets
 These are sent by each physical layer transmitter to the
physical layer of the corresponding receiver to initiate link
training, compensate for clock tolerance, or transition a link to
and from the Electrical Idle state.
As indicated in Table, there are five types of Ordered Sets.
Each ordered set is constructed of 10-bit control (K) symbols
that are created within the physical layer.

Ordered Sets
Ordered Set Types

Ordered Sets
PCI Express Link Local Traffic: Ordered Sets

P C I :
Please
Calm down
Immediately
We are taking a
break

PACKET – BASED TRANSACTIONS

Introduction to the Packet-
Based Protocol
 With the exception of the logical idle indication and physical layer Ordered
Sets, all information moves across an active PCI Express link in fundamental
chunks called packets which are comprised of 10 bit control (K) and data (D)
symbols.
The two major classes of packets exchanged between two PCI Express devices
are high level Transaction Layer Packets (TLPs), and low-level link maintenance
packets called Data Link Layer Packets (DLLPs).
Collectively, the various TLPs and DLLPs allow two devices to perform
memory, IO, and Configuration Space transactions reliably and use messages to
initiate power management events, generate interrupts, report errors, etc.

Figure depicts TLPs and DLLPs on a PCI Express link.

Why Use A Packet-Based
Transaction Protocol
TLP And DLLP Packets

Why Use A Packet-Based
Transaction Protocol
Packet Formats Are Well Defined
 Each PCI Express packet has a known size and format, and the packet header-
-positioned at the beginning of each DLLP and TLP packet-- indicates the packet
type and presence of any optional fields.
The size of each packet field is either fixed or defined by the packet type.
The size of any data payload is conveyed in the TLP header Length field.
Once a transfer commences, there are no early transaction terminations by the
recipient.
This structured packet format makes it possible to insert additional information
into the packet into prescribed locations, including framing symbols, CRC, and a
packet sequence number (TLPs only).

Framing Symbols Indicate
Packet Boundaries
 Each TLP and DLLP packet sent is framed with a Start and End
control symbol, clearly defining the packet boundaries to the
receiver.
Note that the Start and End control (K) symbols appended to
packets by the transmitting device are 10 bits each.
A PCI Express receiver must properly decode a complete 10 bit
symbol before concluding link activity is beginning or ending.
Unexpected or unrecognized control symbols are handled as
errors.

Transaction Layer Packets
 High-level transactions originate at the device core of the
transmitting device and terminate at the core of the receiving
device.
The Transaction Layer is the starting point in the assembly of
outbound Transaction Layer Packets (TLPs), and the end point for
disassembly of inbound TLPs at the receiver.
Along the way, the Data Link Layer and Physical Layer of each
device contribute to the packet assembly and disassembly.

Transaction Layer Packets
PCI Express Layered Protocol And TLP Assembly/Disassembly

CRC Protects Entire
Packet
 Unlike the side-band parity signals used by PCI devices during the address and
each data phase of a transaction, the in-band 16-bit or 32-bit PCI Express CRC
value “protects” the entire packet (other than framing symbols).
In addition to CRC, TLP packets also have a packet sequence number
appended to them by the transmitter so that if an error is detected at the
receiver, the specific packet(s) which were received in error may be resent.
The transmitter maintains a copy of each TLP sent in a Retry Buffer until it is
checked and acknowledged by the receiver.
This TLP acknowledgement mechanism (sometimes referred to as the Ack/Nak
protocol) forms the basis of link-level TLP error correction and is very important
in deep topologies where devices may be many links away from the host in the
event an error occurs and CPU intervention would otherwise be needed.

ACK / NAK PROTOCOL

Introduction
 ‘Reliable’ transport of TLPs from one device to another device
across the Link.
The use of ACK DLLPs to confirm reception of TLPs and the use
of NAK DLLPs to indicate error reception of TLPs is explained.

Reliable Transport of
TLPs Across Each Link
 The function of the Data Link Layer (shown in Figure 5-1 on
page 210) is two fold:
• ‘Reliable’ transport of TLPs from one device to another device across the
Link.
• The receiver’s Transaction Layer should receive TLPs in the same order that
the transmitter sent them. The Data Link Layer must preserve this order
despite any occurrence of errors that require TLPs to be replayed (retried).

Data Link Layer

Overview of the ACK/NAK Protocol

Elements of the ACK/NAK
Protocol
Packet order is maintained by the transmitter’s and
receiver’s Transaction Layer.
Elements of the ACK/NAK Protocol 14Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

QOS / TCS / VCS AND
ARBITRATION

Quality of Service
 Quality of Service (QoS) is a generic term that normally refers to
the ability of a network or other entity (in our case, PCI Express) to
provide predictable latency and bandwidth.
 Note that QoS can only be supported when the system and
device-specific software is PCI Express aware.
 QoS can involve many elements of performance including:
1) Transmission rate
2) Effective Bandwidth
3) Latency
4) Error rate
5) Other parameters that affect performance

Quality of Service
 Several features of PCI Express architecture provide the
mechanisms that make QoS achievable.
The PCI Express features that support QoS include:
• Traffic Classes (TCs)
• Virtual Channels (VCs)
• Port Arbitration
• Virtual Channel Arbitration
• Link Flow Control
 PCI Express uses these features to support two general classes of
transactions that can benefit from the PCI Express implementation
of QoS. 17Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Quality of Service
Isochronous Transactions
 These transactions require a constant bus bandwidth at
regular intervals along with guaranteed latency.
 Isochronous transactions are most often used when a
synchronous connection is required between two devices.
Iso (same) + chronous (time)

Quality of Service
Asynchronous Transactions
 This class of transactions involves a wide variety of applications that
have widely varying requirements for bandwidth and latency.
QoS can provide the more demanding applications (those
requiring higher bandwidth and shorter latencies) with higher priority
than the less demanding applications.
In this way, software can establish a hierarchy of traffic classes for
transactions that permits differentiation of transaction priority based
on their requirements.
The specification refers to this capability as differentiated services.

Isochronous
Transaction Support
Synchronous Versus Isochronous Transactions
Example Application of Isochronous Transaction

Isochronous Transaction
Support
Isochronous Transaction Management
 Management of an isochronous communications channel is based
on a Traffic Class (TC) value and an associated Virtual Channel
(VC) number that software assigns during initialization.
Hardware components including the Requester of a transaction
and all devices in the path between the requester and completer
are configured to transport the isochronous transactions from link
to link via a hi-priority virtual channel.
The requester initiates isochronous transactions that include a TC
value representing the desired QoS.

Isochronous Transaction Management
 The Requester injects isochronous packets into the fabric at the required rate
(service interval), and all devices in the path between the Requester and
Completer must be configured to support the transport of the isochronous
transactions at the specified interval.
Any intermediate device along the path must convert the TC to the associated
VC used to control transaction arbitration.
This arbitration results in the desired bandwidth and latency for transactions
with the assigned TC.
Note that the TC value remains constant for a given transaction while the VC
number may change from link to link.

Isochronous Transaction
Support
Differentiated Services
 Various types of asynchronous traffic (all traffic other than
isochronous) have different priority from the system perspective.
 For example, ethernet traffic requires higher priority (smaller
latencies) than mass storage transactions.
 PCI Express software can establish different TC values and
associated virtual channels and can set up the communications paths
to ensure different delivery policies are established as required.
 Note that the specification does not define specific methods for
identifying delivery requirements or the policies to be used when
setting up differentiated services. 23Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Perspective on QOS/TC/VC
and Arbitration
Traffic Classes and Virtual Channels
 During initialization a PCI Express device-driver
communicates the levels of QoS that it desires for its
transactions, and the operating system returns TC values that
correspond to the QoS requested.
 The TC value ultimately determines the relative priority of a
given transaction as it traverses the PCI Express fabric.

FLOW CONTROL

FLOW CONTROL
Introduction
 Flow control guarantees that transmitters will never send
Transaction Layer Packets (TLPs) that the receiver can’t
accept.
 This prevents receive buffer over-runs and eliminates the
need for inefficient disconnects, retries, and wait-states on the
link.

Flow Control Concept
 Before a transaction packet can be sent across a link to the receiving port, the
transmitting port must verify that the receiving port has sufficient buffer space to
accept the transaction to be sent.
 If the transaction is rejected due to insufficient buffer space, the transaction is
resent (retried) until the transaction completes.
 This procedure can severely reduce the efficiency of a bus, by wasting bus
bandwidth when other transactions are ready to be sent.
 The Flow Control mechanism would be ineffective, if only one transaction
stream was pending transmission across a link.
 PCI Express improves link efficiency by implementing multiple flow-control
buffers for separate transaction streams.

 If the Flow Control buffer for one VC is full, the transmitter can advance to
another VC buffer and send transactions associated with it.
 The link Flow Control mechanism uses a credit-based mechanism that allows
the transmitting port to check buffer space availability at the receiving port.
 During initialization each receiver reports the size of its receive buffers (in
Flow Control credits) to the port at the opposite end of the link.
 The receiving port continues to update the transmitting port regularly by
transmitting the number of credits that have been freed up.
 This is accomplished via Flow Control DLLPs.

Location of Flow Control Logic

INTERRUPTS

Two Methods of Interrupt
Delivery
 When a native PCI Express function does depend upon delivering
interrupts to call its device driver, Message Signaled Interrupts
(MSI) must be used.
 However, in the event that a device connecting to a PCI Express
link cannot use MSIs (i.e., legacy devices), an alternate mechanism
is defined.

Two Methods of Interrupt
Delivery
Native PCI Express Interrupt Delivery
 A Message Signalled Interrupt is not a PCI Express Message,
instead it is simply a Memory Write transaction.
 A memory write associated with an MSI can only be
distinguished from other memory writes by the address locations
they target, which are reserved by the system for Interrupt
delivery.

Two Methods of
Interrupt Delivery
Legacy PCI Interrupt Delivery
 Legacy functions use one of the interrupt lines to signal an
interrupt.
 An INTx# signal is asserted to request interrupt service and
deasserted when the interrupt service accesses a device-specific
register, thereby indicating the interrupt is being serviced.
 PCI Express defines in-band messages that act as virtual INTx#
wires, which target the interrupt controller located typically
within the Root Complex.

Two Methods of
Interrupt Delivery
Native PCI Express and Legacy PCI Interrupt Delivery

Message Signaled
Interrupts
 Message Signaled Interrupts (MSIs) are delivered to the Root
Complex via memory write transactions.
The MSI Capability register provides all the information that the
device requires to signal MSIs.
This register is set up by configuration software and includes the
following information:
• Target memory address
• Data Value to be written to the specified address location
• The number of messages that can be encoded into the data

Message Signaled
Interrupts
64-bit MSI Capability Register Format

Message Signaled
Interrupts
32-bit MSI Capability Register Set Format

ERROR DETECTION AND
HANDLING

ERROR DETECTION AND
HANDLING
Background
 The original PCI bus implementation provides for basic parity
checks on each transaction as it passes between two devices
residing on the same bus.
 The PCI architecture provides a method for reporting the
following types of errors:
• data parity errors
• data parity errors during multicast transactions (special cycles)
• address and command parity errors
• other types of errors

Background
 Errors reported via PERR# are considered potentially recoverable.
 How the errors reported via PERR# are handled is left up to the
implementer.
 Error handling may involve only hardware, device-specific software, or
system software.
 Errors signaled via SERR# are reported to the system and handled by
system software.
 PCI-X uses the same error reporting signals as PCI, but defines specific error
handling requirements depending on whether device-specific error handling
software is present.

Introduction to PCI
Express Error Management
PCI Express defines a variety of mechanisms used for
checking errors, reporting those errors and identifying the
appropriate hardware and software elements for handling
these errors.

Introduction to PCI Express
Error Management
PCI Express Error Checking Mechanisms
Each layer of the PCI Express interface includes error checking capability as
described in the following sections.

PCI Express Error Checking
Mechanisms
Transaction Layer Errors
 The transaction layer checks are performed only by the Requestor and
Completer.
Packets traversing switches do not perform any transaction layer checks.
Checks performed at the transaction layer include:
• ECRC check failure (optional check based on end-to-end CRC)
• Malformed TLP (error in packet format)
• Completion Time-outs during split transactions
• Flow Control Protocol errors (optional)
• Unsupported Requests
• Data Corruption (reported as a poisoned packet)
• Completer Abort (optional)
• Unexpected Completion (completion does not match any Request pending completion)
• Receiver Overflow (optional check) 43Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Mechanisms
Data Link Layer Errors
 Link layer error checks occur within a device involved in delivering
the transaction between the requester and completer functions.
This includes the requesting device, intermediate switches, and the
completing device.
Checks performed at the link layer include:
• LCRC check failure for TLPs
• Sequence Number check for TLP s
• LCRC check failure for DLLPs
• Replay Time-out
• Replay Number Rollover
• Data Link Layer Protocol errors

Mechanisms
Physical Layer Errors
 Physical layer error checks are also performed by all devices
involved in delivering the transaction, including the requesting
device, intermediate switches, and the completing device.
Checks performed at the physical layer include:
• Receiver errors (optional)
• Training errors (optional)

Error Reporting
Mechanisms
 PCI Express provides three mechanisms for establishing the error
reporting policy.
 These mechanisms are controlled and reported through
configuration registers mapped into three distinct regions of
configuration space.
• PCI-compatible Registers (required) —
1) This error reporting mechanism provides backward compatibility with existing
PCI compatible software and is enabled via the PCI configuration Command
Register.
2) This approach requires that PCI Express errors be mapped to PCI compatible
error registers. 46Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Error Reporting
Mechanisms
• PCI Express Capability Registers (required) —
1) This mechanism is available only to software that has knowledge of PCI
Express.
2) This required error reporting is enabled via the PCI Express Device Control
Register mapped within PCI-compatible configuration space.
• PCI Express Advanced Error Reporting Registers (optional) —
1) This mechanism involves registers mapped into the extended configuration
address space.
2) PCI Express compatible software enables error reporting for individual errors
via the Error Mask Register.

Error Reporting
Mechanisms
Location of PCI Express Error-Related Configuration Registers

Error Handling
Mechanisms
• Correctable errors — handled by hardware
• Uncorrectable errors-nonfatal — handled by device-specific
software
• Uncorrectable errors-fatal — handled by system software

PHYSICAL LAYER LOGIC

PHYSICAL LAYER LOGIC
 Byte Striping and Un-Striping logic, Scrambler and De-
Scrambler, 8b/10b Encoder and Decoder, Elastic Buffers and
more.
The transmit logic of the Physical Layer essentially processes
packets arriving from the Data Link Layer, then converts them
into a serial bit stream.

Physical Layer Overview
Physical Layer 52Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Physical Layer Overview
Logical and Electrical Sub-Blocks of the Physical Layer 53Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Transmit Logic Overview
Physical Layer Details 54Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

 Figure shows the elements that make up the transmit logic:
• a multiplexer (mux),
• byte striping logic (only necessary if the link implements more than one data
lane),
• scramblers,
• 8b/10b encoders,
• and parallel-to-serial converters.
 TLPs and DLLPs from the Data Link layer are clocked into a Tx
(transmit) Buffer.
 With the aid of a multiplexer, the Physical Layer frames the TLPs
or DLLPs with Start and End characters. 55Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

 These characters are framing symbols which the receiver device
uses to detect start and end of packet.
The framed packet is sent to the Byte Striping logic which
multiplexes the bytes of the packet onto the Lanes.
One byte of the packet is transferred on one Lane, the next byte
on the next Lane and so on for the available Lanes.
The Scrambler uses an algorithm to pseudo-randomly scramble
each byte of the packet.
The Start and End framing bytes are not scrambled. 56Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

 Scrambling eliminates repetitive patterns in the bit stream.
Repetitive patterns result in large amounts of energy concentrated in
discrete frequencies which leads to significant EMI noise generation.
Scrambling spreads energy over a frequency range, hence
minimizing average EMI noise generated.
The scrambled 8-bit characters (8b characters) are encoded into 10-
bit symbols (10b symbols) by the 8b/10b Encoder logic.
And yes, there is a 25% loss in transmission performance due to the
expansion of each byte into a 10-bit character. 57Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

A Character is defined as the 8-bit un-encoded byte of a packet.
A Symbol is defined as the 10-bit encoded equivalent of the 8-bit
character.
The 10b symbols are converted to a serial bit stream by the Parallel-
to-Serial converter.
This logic uses a 2.5 GHz clock to serially clock the packets out on
each Lane.
The serial bit stream is sent to the electrical sub-block which
differentially transmits the packet onto each Lane of the Link. 58Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Receive Logic Overview
 Figure shows the elements that make up the receiver logic:
• receive PLL,
• serial-to-parallel converter,
• elastic buffer,
• 8b/10b decoder,
• de-scrambler,
• byte un-striping logic (only necessary if the link implements more than one
data lane),
• control character removal circuit,
• and a packet receive buffer.
As the data bit stream is received, the receiver PLL is synchronized
to the clock frequency with which the packet was clocked out of the
remote transmitter device. 59Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

 The transitions in the incoming serial bit stream are used to re-
synchronize the PPL circuitry and maintain bit and symbol lock while
generating a clock recovered from the data bit stream.
 The serial-to-parallel converter is clocked by the recovered clock
and outputs 10b symbols.
 The 10b symbols are clocked into the Elastic Buffer using the
recovered clock associated with the receiver PLL.
 The 10b symbols are converted back to 8b characters by the 8b/10b
Decoder.
 The Start and End characters that frame a packet are eliminated.60Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

 The 8b/10b Decoder also looks for errors in the incoming 10b symbols.
For example, error detection logic can check for invalid 10b symbols
or detect a missing Start or End character.
The De-Scrambler reproduces the de-scrambled packet stream from
the incoming scrambled packet stream.
The De-Scrambler implements the inverse of the algorithm
implemented in the transmitter Scrambler.
The bytes from each Lane are un-striped to form a serial byte stream
that is loaded into the receive buffer to feed to the Data Link layer.61Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

ELECTRICAL PHYSICAL LAYER

Electrical Physical Layer
Overview
 This sub-block contains differential drivers (transmitters) and
differential receivers (receivers).
 The transmitter serializes outbound symbols on each Lane and
converts the bit stream to electrical signals that have an embedded
clock.
 The receiver detects electrical signaling on each Lane and generates
a serial bit stream that it de-serializes into symbols, and supplies the
symbol stream to the logical Physical Layer along with the clock
recovered from the inbound serial bit stream.
 The drivers and receivers are short-circuit tolerant, making them
ideally suited for hot insertion and removal events. 63Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd.

Electrical Physical Layer
Overview
Electrical Sub-Block of the Physical Layer

ESD and Short Circuit
Requirements
 All signals and power pins must withstand (without damage) a
2000V Electro-Static Discharge (ESD) using the human body
model and 500V using the charged device model.
The ESD requirement not only protects against electro-static
damage, but facilitates support of surprise hot insertion and
removal events.
Transmitters and receivers are also required to be short-circuit
tolerant.

Khatam!!!
Next session is
how to use all
this stuff we
were trying to
learn_____

Subhash Iyer, Program Head, Soft Polynomials (I) Pvt. Ltd
Getting to PCI Express
But first some Background

Interfacing Processor and Peripherals
 Overview
Main
memory
I/O
controller
I/O
controller
I/O
controller
Disk Graphics
output
Network
MemoryĞI/O bus
Processor
Cache
Interrupts
Disk

Introduction
 I/O often viewed as second class to
processor design
– Processor research is cleaner
– System performance given in terms of processor
– Courses often ignore peripherals
– Writing device drivers is not fun
 This is crazy - a computer with no I/O is
pointless

Peripheral design
 As with processors, characteristics of I/O
driven by technology advances
– E.g. properties of disk drives affect how they
should be connected to the processor
– PCs and super computers now share the same
architectures, so I/O can make all the difference
 Different requirements from processors
– Performance
– Expandability
– Resilience

Peripheral performance
 Harder to measure than for the processor
– Device characteristics
• Latency / Throughput
– Connection between system and device
– Memory hierarchy
– Operating System
 Assume 100 secs to execute a benchmark
– 90 secs CPU and 10 secs I/O
– If processors get 50% faster per year for the next
5 years, what is the impact?

Relative performance
 CPU time + IO time = total time (% of IO time)
 Year 0: 90 + 10 = 100 (10%)
 Year 1: 60 + 10 = 70 (14%)
 :
 Year 5: 12 + 10 = 22 (45%)
 !

IO bandwidth
 Measured in 2 ways depending on application
– How much data can we move through the system
in a given time
• Important for supercomputers with large amounts of
data for, say, weather prediction
– How many IO operations can we do in a given time
• ATM is small amount of data but need to be handled
rapidly
 So comparison is hard. Generally
– Response time lowered by handling early
– Throughput increased by handling multiple
requests together

I/O Performance Measures
 I/O bandwidth (throughput) – amount of
information that can be input (output) and
communicated across an interconnect (e.g., a
bus) to the processor/memory (I/O device) per
unit time
1. How much data can we move through the system in a
certain time?
2. How many I/O operations can we do per unit time?
 I/O response time (latency) – the total elapsed
time to accomplish an input or output operation
– An especially important metric in real-time systems
 Many applications require both high throughput
and short response times

I/O System Performance
 Designing an I/O system to meet a set of bandwidth
and/or latency constraints means
Finding the weakest link in the I/O system – the
component that constrains the design
– The processor and memory system
– The underlying interconnection (e.g., bus)
– The I/O controllers
– The I/O devices themselves
(Re)configuring the weakest link to meet the
bandwidth and/or latency requirements
Determining requirements for the rest of the
components and (re)configuring them to support
this latency and/or bandwidth

I/O System Performance Example
 A disk workload consisting of 64KB reads and writes where
the user program executes 200,000 instructions per disk I/O
operation and
– a processor that sustains 3 billion instr/s and averages
100,000 OS instructions to handle an I/O operation
– a memory-I/O bus that sustains a transfer rate of
1000 MB/s
The maximum I/O rate of the processor is
-------------------------- = ------------------------ = 10,000 I/Os/sec
Instr execution rate 3 x 109
Instr per I/O (200 + 100) x 103
Each I/O reads/writes 64 KB so the maximum I/O rate of the bus
is
---------------------- = ----------------- = 15,625 I/O’s/sec
Bus bandwidth 1000 x 106
Bytes per I/O 64 x 103

Input and Output Devices
 I/O devices are incredibly diverse with respect to
– Behavior – input, output or storage
– Partner – human or machine
– Data rate – the peak rate at which data can be transferred
between the I/O device and the main memory or processor
Device Behavior Partner Data rate
(Mb/sec)
Keyboard input human 0.0001
Mouse input human 0.0038
Laser printer output human 3.2000
Graphics display output human 800.0000-
8000.0000
Network input or
output
machine 100.0000-
1000.0000
Magnetic disk storage machine 240.0000-
2560.0000
8ordersofmagnitude
range

Mouse
 Communicates with
– Pulses from LED
– Increment / decrement counters
 Mice have at least 1 button
– Need click and hold
 Movement is smooth, slower than processor
– Polling
– No submarining
– Software configuration
Initial
position
of mouse
+20 in XĞ20 in X
+20 in Y
+20 in Y
+20 in X
+20 in Y
Ğ20 in X
Ğ20 in Y
Ğ20 in Y
+20 in X
Ğ20 in Y
Ğ20 in X

Mouse guts
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF(Uncompressed) decompressor are needed tosee this picture.

Hard disk
 Rotating rigid platters with magnetic
surfaces
 Data read/written via head on armature
– Think record player
 Storage is non-volatile
 Surface divided into tracks
– Several thousand concentric circles
 Track divided in sectors
– 128 or so sectors per track

Diagram
Platter
Track
Platters
Sectors
Tracks

Access time
 Three parts
1. Perform a seek to position arm over correct
track
2. Wait until desired sector passes under head.
Called rotational latency or delay
3. Transfer time to read information off disk
– Usually a sector at a time at 2~4 Mb / sec
– Control is handled by a disk controller,
which can add its own delays.

Calculating time
 Seek time:
– Measure max and divide by two
– More formally: (sum of all possible seeks)/number
of possible seeks
 Latency time:
– Average of complete spin
– 0.5 rotations / spin speed (3600~5400 rpm)
– 0.5/ 3600 / 60
– 0.00083 secs
– 8.3 ms

Comparison
 Currently, 7.25 Gb (7,424,000) per inch
squared
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

More faking
 Disk drive hides
internal
optimisations
from external
world QuickTime™ anda TIFF (Uncompressed) decompressor are needed to see this picture.

Disk Latency & Bandwidth Milestones
Disk latency is one average seek time plus the rotational
latency. Disk bandwidth is the peak transfer time of formatted
data from the media (not from the cache).
In the time that the disk bandwidth doubles the latency
improves by a factor of only 1.2 to 1.4
CDC Wren SG ST41 SG ST15 SG ST39 SG ST37
RSpeed (RPM) 3600 5400 7200 10000 15000
Year 1983 1990 1994 1998 2003
Capacity
(Gbytes)
0.03 1.4 4.3 9.1 73.4
Diameter
(inches)
5.25 5.25 3.5 3.0 2.5
Interface ST-412 SCSI SCSI SCSI SCSI
Bandwidth
(MB/s)
0.6 4 9 24 86
Latency (msec) 48.3 17.1 12.7 8.8 5.7

Media Bandwidth/Latency Demands
 Bandwidth requirements
– High quality video
• Digital data = (30 frames/s) × (640 x 480 pixels) × (24-b
color/pixel) = 221 Mb/s (27.625 MB/s)
– High quality audio
• Digital data = (44,100 audio samples/s) × (16-b audio
samples) × (2 audio channels for stereo) = 1.4 Mb/s (0.175
MB/s)
– Compression reduces the bandwidth requirements
considerably
 Latency issues
– How sensitive is your eye (ear) to variations in video
(audio) rates?
– How can you ensure a constant rate of delivery?
– How important is synchronizing the audio and video
streams?
• 15 to 20 ms early to 30 to 40 ms late tolerable

Magnetic Disk Examples (www.seagate.com)
Characteristic Seagate ST37 Seagate ST32 Seagate ST94
Disk diameter (inches) 3.5 3.5 2.5
Capacity (GB) 73.4 200 40
# of surfaces (heads) 8 4 2
Rotation speed (RPM) 15,000 7,200 5,400
Transfer rate (MB/sec) 57-86 32-58 34
Minimum seek (ms) 0.2r-0.4w 1.0r-1.2w 1.5r-2.0w
Average seek (ms) 3.6r-3.9w 8.5r-9.5w 12r-14w
MTTF (hours@25o
C) 1,200,000 600,000 330,000
Dimensions (inches) 1”x4”x5.8” 1”x4”x5.8” 0.4”x2.7”x3.9”
GB/cu.inch 3 9 10
Power: op/idle/sb (watts) 20?/12/- 12/8/1 2.4/1/0.4
GB/watt 4 16 17
Weight (pounds) 1.9 1.4 0.2

Buses: Connecting I/O devices
 Interfacing subsystems in a computer system
is commonly done with a bus: “a shared
communication link, which uses one set of
wires to connect multiple sub-systems”

Why a bus?
 Main benefits:
– Versatility: new devices easily added
– Low cost: reusing a single set of wires many ways
 Problems:
– Creates a bottleneck
– Tries to be all things to all subsystems
 Comprised of
– Control lines: signal requests, acknowledgements
and to show what type of information is on the
– Data lines:data, destination / source address

Controlling a bus
 As the bus is shared, need a protocol to
manage usage
 Bus transaction consists of
– Sending the address
– Sending / receiving the data
 Note than in buses, we talk about what the
bus does to memory
– During a read, a bus will ‘receive’ data

Bus transaction 1 - disk write
Memory Processor
Control lines
Data lines
Disks
Memory Processor
Control lines
Data lines
Disks
Processor
Control lines
Data lines
Disks
a.
b.
c.
Memory

Bus transaction 2 - disk read
Memory Processor
Control lines
Data lines
Disks
Processor
Control lines
Data lines
Disks
a.
b.
Memory

Types of Bus
 Processor-memory bus
– Short and high speed
– Matched to memory system (usually Proprietary)
 I/O buses
– Lengthy,
– Connected to a wide range of devices
– Usually connected to the processor using 1 or 3
 Backplane bus
– Processors, memory and devices on single bus
– Has to balance proc-memory with I/O-memory
– Usually requires extra logic to do this

Bus type diagram
Processor Memory
Backplane bus
a. I/O devices
Processor Memory
Processor-memory bus
b.
Bus
adapter
Bus
adapter
I/O
bus
I/O
bus
Bus
adapter
I/O
bus
Processor Memory
Processor-memory bus
c.
Bus
adapter
Backplane
bus
Bus
adapter
I/O bus
Bus
adapter
I/O bus

Synchronous and Asynchronous buses
 Synchronous bus has a clock attached to the
control lines and a fixed protocol for
communicating that is relative to the pulse
 Advantages
– Easy to implement (CC1 read, CC5 return value)
– Requires little logic (FSM to specify)
 Disadvantages
– All devices must run at same rate
– If fast, cannot be long due to clock skew
 Most proc-mem buses are clocked

Asynchronous buses
 No clock, so it can accommodate a variety of
devices (no clock = no skew)
 Needs a handshaking protocol to coordinate
different devices
– Agreed steps to progress through by sender and
receiver
– Harder to implement - needs more control lines

Example handshake - device wants a
word from memory
DataRdy
Ack
Data
ReadReq 1
3
4
5
7
6
42 2

FSM control
1
Record from
data lines
and assert
Ack
ReadReq
ReadReq
________
ReadReq
ReadReq
3, 4
Drop Ack;
put memory
data on data
lines; assert
DataRdy
Ack
Ack
6
Release data
lines and
DataRdy
________
___
Memory
2
Release data
lines; deassert
ReadReq
Ack
DataRdy
DataRdy
5
Read memory
data from data
lines;
assert Ack
DataRdy
DataRdy
7
Deassert Ack
I/O device
Put address
on data
lines; assert
ReadReq
________
Ack
___
________
New I/O request
New I/O request

Increasing bus bandwidth
 Key factors
– Data bus width: Wider = fewer cycles for transfer
– Separate vs Multiplexed, data and address lines
• Separating allows transfer in one bus cycle
– Block transfer: Transfer multiple blocks of data in
consecutive cycles without resending addresses
and control signals etc.

Obtaining bus access
 Need one, or more, bus masters to prevent
chaos
 Processor is always a bus master as it needs
to access memory
– Memory is always a slave
 Simplest system as a single master (CPU)
 Problems
– Every transfer needs CPU time
– As peripherals become smarter, this is a waste of
time
 But, multiple masters can cause problems

Bus Arbitration
 Deciding which master gets to go next
– Master issues ‘bus request’ and awaits ‘granted’
 Two key properties
– Bus priority (highest first)
– Bus fairness (even the lowest get a go, eventually)
 Arbitration is an overhead, so good to reduce
it
– Dedicated lines, grant lines, release lines etc.

Different arbitration schemes
 Daisy chain: Bus grant line runs through
devices from highest to lowest
 Very simple, but cannot guarantee fairness
Device n
Lowest priority
Device 2Device 1
Highest priority
Bus
arbiter
Grant
Grant Grant
Release
Request

Centralised Arbitration
 Centralised, parallel: All devices have
separate connections to the bus arbiter
– This is how the PCI backplane bus works (found in
most PCs)
– Can guarantee fairness
– Arbiter can become congested

Distributed
 Distributed arbitration by self selection:
 Each device contains information about
relative importance
 A device places its ID on the bus when it
wants access
 If there is a conflict, the lower priority
devices back down
 Requires separate lines and complex devices
 Used on the Macintosh II series (NuBus)

Collision detection
 Distributed arbitration by collision detection:
 Basically ethernet
 Everyone tries to grab the bus at once
 If there is a ‘collision’ everyone backs off a
random amount of time

Bus standards
 To ensure machine expansion and peripheral
re-use, there are various standard buses
– IBM PC-AT bus (de-facto standard)
– SCSI (needs controller)
– PCI (Started as Intel, now IEEE)
– Ethernet
 Bus bandwidth depends on size of transfer
and memory speed

PCI
Type Backplane
Data width 32-64
Address/data Multiplexed
Bus masters Multiple
Arbitration Central parallel
Clocking Synch. 33-66 Mhz
Theoretical Peak 133-512 MB/sec
Achievable peak 80 MB/sec
Max devices 1024
Max length 50 cm
Bananas none

My Old Macintosh
Main
memory
I/O
controller
I/O
controller
Graphics
output
PCI
CDROM
Disk
Tape
I/O
controller
Stereo
I/O
controller
Serial
ports
I/O
controller
Apple
desktop bus
Processor
PCI
interface/
memory
controller
EthernetSCSI
bus
outputinput

Example: The Pentium 4’s Buses
System Bus (“Front Side Bus”):
64b x 800 MHz (6.4GB/s), 533
MHz, 400 MHz
2 serial ATAs:
150 MB/s
8 USBs: 60 MB/s
2 parallel ATA:
100 MB/s
Hub Bus: 8b x 266 MHz
Memory Controller Hub
(“Northbridge”)
I/O Controller Hub
(“Southbridge”)
Gbit ethernet: 0.266 GB/s
DDR SDRAM
Main
Memory
Graphics output:
2.0 GB/s
PCI: 32b x
33 MHz

Buses in Transition
Companies are transitioning from synchronous,
parallel, wide buses to asynchronous narrow buses
– Reflection on wires and clock skew makes it difficult to
(synchronously) use 16 to 64 parallel wires running at a
high clock rate (e.g., ~400 MHz) so companies are
transitioning to buses with a few one-way, asynchronous
wires running at a very high clock rate (~2 GHz)
PCI PCIexpres
s
ATA Serial ATA
Total # wires 120 36 80 7
# data wires 32 – 64
(2-way)
2 x 4
(1-way)
16
(2-way)
2 x 2
(1-way)
Clock (MHz) 33 – 133 635 50 150
Peak BW (MB/s) 128 – 1064 300 100 150

ATA Cable Sizes
 Serial ATA cables (red) are much thinner
than parallel ATA cables (green)

Giving commands to I/O devices
 Processor must be able to address a device
– Memory mapping: portions of memory are
allocated to a device (Base address on a PC)
• Different addresses in the space mean different things
• Could be a read, write or device status address
– Special instructions: Machine code for specific
devices
• Not a good idea generally

Communicating with the Processor
 Polling
– Process of periodically checking the status bits to
see if it is time for the next I/O operation
– Simplest way for device to communicate (via a
shared status register
– Mouse
– Wasteful of processor time

Interrupts
 Notify processor when a device needs
attention (IRQ lines on a PC)
 Just like exceptions, except for
– Interrupt is asynchronous with program execution
• Control unit only checks I/O interrupt at the start of each
instruction execution
– Need further information, such as the identity of
the device that caused the interrupt and its
priority
• Remember the Cause Register?

Interrupt-Driven I/O
 With I/O interrupts
– Need a way to identify the device generating the interrupt
– Can have different urgencies (so may need to be prioritized)
 Advantages of using interrupts
– Relieves the processor from having to continuously poll for
an I/O event; user program progress is only suspended
during the actual transfer of I/O data to/from user memory
space
 Disadvantage – special hardware is needed to
– Cause an interrupt (I/O device) and detect an interrupt and
save the necessary information to resume normal
processing after servicing the interrupt (processor)

Interrupt-Driven Input
memory
user
program
1. input
interrupt
2.1 save state
Processor
ReceiverMemory
add
sub
and
or
beq
lbu
sb
...
jr
2.2 jump to
interrupt
service routine
2.4 return
to user code
Keyboard
2.3 service
interrupt
input
interrupt
service
routine

Interrupt-Driven Output
Processor
TrnsmttrMemory
Display
add
sub
and
or
beq
lbu
sb
...
jr
memory
user
program
1.output
interrupt
2.1 save state
output
interrupt
service
routine
2.2 jump to
interrupt
service routine
2.4 return
to user code
2.3 service
interrupt

Transferring Data Between Device and
Memory
 We can do this with Interrupts and Polling
– Works best with low bandwidth devices and
keeping cost of controller and interface
– Burden lies with the processor
 For high bandwidth devices, we don’t want
the processor worrying about every single
block
 Need a scheme for high bandwidth
autonomous transfers

Direct Memory Access (DMA)
 Mechanism for offloading the processor and
having the device controller transfer data
directly
 Still uses interrupt mechanism, but only to
communicate completion of transfer or error
 Requires dedicated controller to conduct the
transfer

Doing DMA
 Essentially, DMA controller becomes bus
master and sets up the transfer
 Three steps
– Processor sets up the DMA by supplying
• device identity
• Operation on device
• Memory Address (source or destination)
• Amount to transfer
– DMA operates devices, supplies addresses and
arbitrates bus
– On completion, controller notifies processor

DMA and the memory system
 With DMA, the relationship between memory
and processor is changed
– DMA bypasses address translation and hierarchy
 So, should DMA use virtual or physical
addresses?
– Virtual addresses: DMA must translate
– Physical addresses: Hard to cross page boundary

DMA address translation
 Can provide the DMA with a small address
translation table for pages - provided by OS
at transfer time
 Get the OS to break the transfer into chunks,
each chunk relating to a single page
 Regardless, the OS cannot relocate pages
during transfer

Evolution vs Revolution
 Evolutionary approaches tend to be invisible
to users except for
– Lower cost and better performance
 Revolutionary approaches require new
languages and applications
– Looks good on paper
– Must be worth the effort
– KCM

BUSSSS!!!!!
Now let’s dive into the actual
scenario

Session 8,9 PCI Express

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