BCS361: Computer Architecture I/O Devices

2 Input/Output CPU Cache Bus MemoryDiskNetworkUSBDVD …

3 I/O Hierarchy CPU Cache Memory Bus Memory I/O Controller NetworkUSBDVD … I/O Bus Disk

4 Bus Design Review The bus is a shared resource – any device can send data on the bus and all other devices can read this data off the bus The address/control signals on the bus specify the intended receiver of the message The length of the bus determines its speed (hence, a hierarchy makes sense) Buses can be synchronous or asynchronous

5 Memory-Mapped I/O Each I/O device has its own special address range  The CPU issues commands such as these: sw [some-data] [some-address]  Usually, memory services these requests… if the address is in the I/O range, memory ignores it  The data is written into some register in the appropriate I/O device – this serves as the command to the device

6 Polling Vs. Interrupt-Driven When the I/O device is ready to respond, it can send an interrupt to the CPU; the CPU stops what it was doing; the OS examines the interrupt and then reads the data produced by the I/O device (and usually stores into memory) In the polling approach, the CPU (OS) periodically checks the status of the I/O device and if the device is ready with data, the CPU reads it

7 Role of I/O Activities external to the CPU are typically orders of magnitude slower Example: while CPU performance has improved by 50% per year, disk latencies have improved by 10% every year Typical strategy on I/O: switch contexts and work on something else

BCS361: Computer Architecture Multiprocessors

Crossroads: Conventional Wisdom in Comp. Arch Old Conventional Wisdom: Power is free, Transistors expensive New Conventional Wisdom: “Power wall” Power expensive, Xtors free (Can put more on chip than can afford to turn on) Old CW: Sufficiently increasing Instruction Level Parallelism via compilers, innovation (Out-of-order, speculation, …) New CW: “ILP wall” law of diminishing returns on more HW for ILP Old CW: Multiplies are slow, Memory access is fast New CW: “Memory wall” Memory slow, multiplies fast (200 clock cycles to DRAM memory, 4 clocks for multiply) Old CW: Uniprocessor performance 2X / 1.5 yrs New CW: Power Wall + ILP Wall + Memory Wall = Brick Wall – Uniprocessor performance now 2X / 5(?) yrs  Sea change in chip design: multiple “cores” (2X processors per chip / ~ 2 years) More simpler processors are more power efficient