Chapter 6
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Chapter 6 System Integration and Performance
Chapter goals • Describe the implementation of the system bus and bus protocol. • Describe how the CPU and bus interact with peripheral devices. • Describe the purpose and function of device controllers.
Chapter goals cont. • Describe how interrupts coordinate actions of the CPU with secondary storage and I/O devices • Describe how buffers, caches, and data compression improve computer system performance
Role of the system bus • Bus is mechanism that allows computer components to work together • Is made up of parallel communication lines connecting computer components • CPU, hard drive, parallel port, modem, etc. • Can connect two or more devices • Information can travel in both directions
System bus (cont.) • Can connect both internal (hard drive) and external (printer) devices • System bus has three parts • Data bus – carries data • Address bus – used if RAM is involved • Control bus – commands and status information • Each bus line carries 1 bit of information
Bus Clock • Like the CPU, the bus has a clock that acts as a timing device • For CPU, each tick is trigger to execute an instruction • For system bus, each tick is an opportunity to transmit data or a control message
Bus clock cont. • Bus clock is MUCH slower than CPU clock • Think of CPU as the highway and the system bus as the local streets
Why slower? • Data on bus must travel a longer physical distance than data in CPU • Even though data is traveling at the speed of light it still needs more time to travel over a greater distance • Need to allows time to factor out noise, interference • Also allows time to operate controller logic in peripheral devices
Bus data transfer rate • Called the bus data capacity • Expresses how much data can travel across bus over time • Is a combination of • Bus clock speed • Data transfer unit (usually a word) • Is used to calculate things like • Time required to load large files (i.e. video)
Bus protocol • Data transportation rules that ensure the smooth transfer of information without error • Dictates the format, content, and timing of data, memory addresses, and messages • Every peripheral device (no matter the manufacturer) must follow the bus protocol rules
Bus protocol cont. • Protocol can impact (reduce) data transfer rates • Protocols often require exchanges of control signals • Control signals consume bus cycles that could otherwise send data
Sample protocol • Example: if a disk drive transfers data to RAM as the result of an explicit CPU instruction, the following steps are followed: • CPU sends command to the drive • Drive send acknowledgement to CPU • Drive carries out transfer • Drive sends confirmation to CPU that transfer is complete
Why use protocols? • Protocol regulates bus access • Stops devices from interfering with each other • I/O data transfer is the largest cause of errors in computers • I/O commands need to be acknowledged and confirmed
What if two devices need the bus? • When two (or more) peripheral devices need access to the bus at the same time that is called a collision • Three solutions are in place to deal with this • Master-slave • Multiple master • Peer to peer
Master-slave • CPU is bus master • Traditional computer architecture • No device can access the bus unless in response to explicit command from CPU • Allows a very simple protocol • No collision is possible as long as CPU waits for response from device before proceeding to the next bus request
Master-slave cont. • Overall system performance is severely degraded • If devices can only communicate through the CPU, then transfers between devices, i.e. memory to disk, must pass through the CPU • Every transfer takes at least 2 bus cycles • CPU cannot execute software while it is managing the bus
A better solution • System performance is improved if storage and I/O devices can transmit data among themselves without explicit CPU involvement • Direct Memory Access (DMA) controller is attached to the bus and main memory • DMA assumes the role of bus master for all transfers between memory and other storage or I/O devices • CPU is free to do whatever
Multiple master bus • Any device can assume control of the bus, or act as bus master for transfer to any other device (not just memory) • Still only a single device can be master at one time • Bus arbitration unit is a simple processor attached to a multiple master bus • It decides which devices must wait when multiple devices want to become a bus master
Logical vs. Physical Access • I/O port is a communication pathway from the CPU to a peripheral device • I/O port is often implemented as a memory address that can be accessed (read or written to) by • The CPU • Or a single peripheral device
I/O Ports • Each peripheral device may have several I/O ports and use them for different purposes • Dedicated bus hardware controls data movement between I/O ports and peripheral devices • CPU reads and writes to I/O ports using ordinary data movement instructions or dedicated I/O instructions
The CPU and I/O Ports • I/O port is more than a memory address, it is a data conduit • It is a logical abstraction used by the CPU and the bus to interact with each peripheral device in a similar way
Logical access • CPU and the bus both interact with each peripheral device as if it was a storage device containing one or more bytes of contiguous memory • CPU and the bus deals with each device the same way, but devices are different • Storage capacity • Internal data coding methods • If storage or I/O device
Linear address space • A read/write operation to/from this hypothetical device is called a logical access • The set of sequentially numbered storage locations is called a linear address space
How logical becomes physical • Logical access assumes device is similar to memory (RAM) • Bus address lines carry the position within the linear address space being read or written • Device controller makes the conversion via a conversion table or a simple algorithm
Device controllers • Storage devices have intermediaries that connect them to the system bus • Translate logical access to physical access • Handles bus protocol (receiving and acknowledging commands) • Permits several devices to share a bus connection
Device controllers cont. • Device controllers monitor the bus control lines for signals to peripheral devices • Translates those signals into appropriate commands for its device
Interrupts • Secondary storage and I/O device transfer rates are much slower than the CPU • Why? • Slower bus clock • Peripheral devices have mechanical elements (access arm, spin mechanism) that are slower than speed of electricity
Interrupts cont. • When the CPU issues a read/write instruction it ALWAYS has to wait • This waiting time can translate into thousands, millions, or even billions of CPU cycles • To allow CPU to be used more efficiently, interrupts are used
How interrupts work • When a program (task, process, thread) needs I/O, CPU makes I/O request over the system bus • Then puts your task aside (asleep) • Does something else for the time being
Interrupts cont. • When I/O is complete, interrupt signal is sent to the CPU • CPU can now restart your task with I/O task being complete
CPU and Interrupts • Portion of the CPU (separate from the fetch execute cycle) continuously monitors the bus for interrupt signals • The signal is an interrupt code that indicates the bus port number of the device sending the interrupt • CPU copies any interrupt signals it encounters into an interrupt register
The CPU and Interrupts cont. • As an extra step in the fetch execute cycle, the CPU checks the interrupt register after completing an instruction but before fetching another one • If interrupt register has a non-zero value CPU must respond to the interrupt
CPU and Interrupts • If CPU is to process an interrupt it does the following: • Puts aside (suspends) current task • Resets interrupt register to 0 (zero) • Processes interrupt by calling interrupt handler • After interrupt processing is complete, resumes suspended program
Interrupt handlers • Interrupts are a mechanism for calling (invoking) system software processes and programs • Operating system (OS) provides low-level processing routines (service calls) • Examples: reading data in from the keyboard • Writing to a file
Interrupt handlers cont. • There is a unique individual interrupt handler (i.e. program) to process each possible interrupt • Each handler is a separate program stored in a separate part of main memory
Interrupt table • A conversion table in main memory that has a list of all interrupt codes • Interrupt code is used as an index into interrupt table • For each interrupt code, interrupt table has the memory address of each interrupt handler
Interrupt handlers • Supervisor (OS) examines the interrupt code, uses it as an index into the interrupt table • Looks up memory location of needed interrupt handler • Loads that memory location into the PC (program counter) • Interrupt handler begins executing
Multiple interrupts • It is possible (even likely) that interrupts will interrupt each other • OS has an algorithm to determine what goes first • Assigns priorities to different interrupts based on • Error conditions • Critical hardware failures
Suspending a process • Whenever a process is suspended or interrupted the system must save whatever information is necessary to allow the process to restart again • Typically that involved saving • PC and IR • Any other specialized or general purpose registers that were in use
Saving a process • The collection of information needed to restart a process is called the “machine state” • It is saved in a special storage location called the stack
The Stack • The stack is a specialize storage location in RAM • It is a data structure where you add and delete information from the same end • Therefore the last process saved by the CPU is the first one it will pick up
Buffers • Buffers are a mechanism that uses RAM to overcome slow data transfer rate to peripheral devices • Small storage area (in RAM) used to hold data in transit from one device to another
Buffers and printing • Printed version of document with formatting information is copied to RAM • When full page is ready it is released from the buffer • Document is written from RAM to printer • Also have input buffers – keyboard, modem, etc.
Cache • Pronounced “cash” • Separate high speed storage area specifically managed to improve overall system performance • Idea is most often needed data is kept in the cache • Must be managed intelligently
