Suppose that a certain program takes 200 seconds of elapsed time to execute. Out of these 200 seconds, 180 seconds is the CPU time and the rest is I/O time. If the CPU performance improves by 40% every year for the next seven years because of developments in technology, but the I/O performance stays the same then

Review

CS501 Advanced Computer Architecture Lecture23 Dr.Noor Muhammad Sheikh

Elapsed time = CPU time + I/O time the I/O time = 200 – 180 = 20 sec which is 10 % of the elapsed time.

Year # CPU Time I/O Elapsed ...I/O Time.. Elapsed Time 180 20 200 10 % 1 129 149 13.42 % 2 92 112 17.85 % 3 66 86 23.25 % 4 47 67 29.85 % 5 34 54 37.03 % 6 24 44 45.45 % 7 17 37 54.05 %

For Pentium processor, the I/O devices are mapped on the I/O space. For SRC, there is no separate I/O space. Some address space out of the memory address space is used to map I/O devices in SRC.

Advantages of Memory Mapped I/O All the instructions which access memory can be used for I/O devices. No need for including separate I/O instructions in the ISA of the processor.

Disadvantages of Memory Mapped I/O The I/O interface will become complex. If partial decoding is used to reduce the complexity of the I/O interface, then a lot of memory addresses will be consumed.

The waveforms correspond to a “read” operation. A similar explanation is applied to a “write” operation. The latching of the incoming data can be done by the master either by using the rising edge of the Enable signal or by using its falling-edge.

Framing Error Parity Error Overrun Error Underrun Error

Framing Error A 0 is received instead of a stop bit (which is always a 1).

Parity Error The parity of the received data is not the same as it should be.

Overrun Error The prior character that was received, was not still read by the CPU and is over written by a new received character.

Underrun Error No character is available at the beginning of an interval.

Block diagram of a modern general purpose digital computer Computer bus or system bus Now we want to focus on what a computer is. The speaker may explain the building blocks in this slide The bus interface unit is usually between the CPU and the system.

PROCESSOR EXTERNAL BUS I/O Sub System HARD DISK CPU PROCESSOR EXTERNAL BUS BUS INTERFACE UNIT CD-ROM MEMORY BUS VIDEO CARD MONITOR MEMORY SYSTEM I/O BUS

Examples of I/O buses : The PCI bus The ISA bus

I/O buses provide an “abstract interface”. By standardizing the I/O buses, as done by several agencies, the third party manufacturers can build add-on sub systems for existing architectures. The location of these I/O buses may be different in different computers.

Earlier generation computers used a single bus to communicate with the memory as well as the I/O devices. This causes the bandwidth of the bus to shared between the memory and I/O devices.

Now-a-days ,the computers have separate memory and I/O buses. This situation gives more flexibility to users wanting to upgrade their existing systems.

Disadvantages of I/O Buses Every bus has a fixed bandwidth. Electrical constraints further reduce the bandwidth .

Problem Statement: What will be the implications? Consider an I/O bus that can transfer 4 bytes of data in one bus cycle. Suppose that a designer is considering to attach the following two components to this bus: Hard drive, with a transfer rate of 40MBytes/sec. Video card, with a transfer rate of 128MBytes/sec. What will be the implications?

The maximum frequency of the bus is 30 MHz. The maximum bandwidth = 30 x 4 of this bus = 120Mbytes/sec. The demand for bandwidth of these = 128 + 40 two components = 168Mbytes/sec

Conclusion: 168MBytes is more than the 120 Mbytes/sec that the bus can provide. One or both of these components will operate at reduced bandwidth.

Bus Arbitration