I/O Subsystem Organization and Interfacing Cs 147 Peter Nguyen Chapter 4 I/O Subsystem Organization and Interfacing Cs 147 Peter Nguyen

General Information Computer generally consists of three subsystems Communication between subsystems by buses The processors sends out the address to be accessed in memory or the address of the I/O device via the address bus Data bus carries data between the subsystems Information sent on the control bus coordinates all data transfers The processor goes through an instruction cycle Fetch an instruction from memory, and decode the instruction, and execute the instruction All instructions are fetched and decoded in the same way, but the execute cycle is different for every instruction

Three Sections of the CPU The register section, used for data storage The arithmetic/logic unit, performs computations on data with the CPU The control unit outputs signals to control the rest of the processor

Types of Memory Chips ROM, PROM, EPROM, and EEPROM are all read only memory chips (data does not change) SRAM and DRAM, these are random access memory (data change or lost when the computer is turned off)

Two types of I/O access Memory mapped I/O Treats I/O device as if it were a memory location The same instructions are used to access both memory and I/O devices, but memory and the I/O device cannot both use the same address Isolated I/O Different instructions are used to access I/O devices and memory Requires an additional control line to distinguish between the two, but this allows memory and I/O devices to both use the same address

Presentation Outline : Part I Input Module Output Module Bi-directional Module An overall view within the organization of a simple computer

_________________________ -Definition - _________________________

Memory is defined as homogeneous From the CPU’s perspective, each location is read from and written to in exactly the same way

What’s the correlation ? They are quite different but the comparisons is needed in order to understand the structure and organization of the computer

Memory vs. I/O Every memory has a unique address Each memory location is read from and written to in exactly the same way, performs the same function – that is, its stores a data value or an instruction for use by the CPU Every I/O device has a unique address Perform vastly different functions; even though, they are part of the same subsystem

The major components of the Input Module Input device (keyboard, mouse, …) Tri-state buffers Data bus Enable logic (Address bus and Control Bus)

An input device: (a) with its interface and (b) the enable logic for the tri-state buffers

The tri-state buffers are used in input device interfaces to make sure that no more than one device writes data to the bus at any time.

Enable logic is the key to this design. What is Enable Logic ? Enable logic is the key to this design. Because of the unique address, enable logic must not enable the buffers unless it receives the correct address from the address bus. It must also get the correct control signals from the control bus.

Components of the Output Module Output device (monitor) Register Data bus Enable Logic (Address bus and Control bus)

Output Device The difference is in the REGISTER, the tri-state buffers are replaced by a register. Since the output devices read data from the bus, rather than write data, they do not need the buffers. The data is made available to all output devices; only the device with the correct address will read it in

___ The output devices read data from the bus, rather that write data to it, they do not need the buffers. The data can be made available to all output devices; only the device with the correct address will read it in.

Load Logic In the Output Module – the load logic plays the role of the enable logic in the input device interface. When this logic receives the correct address and control signals, it asserts the LD signal of the register, causing it to read data from the system’s data bus.

__ By doing so, the output device can then read the data from the register at its leisure while the CPU performs other tasks.

Components of the bi-directional module I/O device (hard disk drive) Two interfaces (one input, tri-state and one output, register) Logic elements (gates to check address on the address bus)

Bi-directional module

I/O devices are much slower than CPUs and memory Timing problems exist when interacting with the CPU

How does the CPU work around this processes ?

Ready Normal inputs are high … when the CPU outputs the address of the I/O device and the correct control signals, enabling the tri-state buffers of the I/O device interface, the I/O device sets READY low.

… which cause the buffers to remain enabled. The CPU reads this signal and continues to output the same address and control signals … which cause the buffers to remain enabled.

By having the extra clock cycles generated when READY is set low This is called wait states

Ready is good for small processes Many systems actually use the interrupts

For larger computers Interrupts Direct memory access, DMA

Performs useful work while waiting for the much slower I/O devices Interrupts Performs useful work while waiting for the much slower I/O devices

Direct memory access (DMA) … method used to bypass the CPU in the transferring process

Building a Simple Computer CPU 8K of ROM, starting with address 0 8K of RAM Memory mapped Bidirectional I/O with a port address of 80000H Read and Write control lines

Central Processing Unit (CPU) 16-bit addresses pin Relatively Simple CPU Systems data bus accesses Read control lines Write control lines

The memory subsystem Memory Subsystem 0000 0000 0000 0000 The memory configuration is 8K of ROM starting at address 0 0000 0000 0000 0000 0001 1111 1111 1111

To access the memory chip, the processor must supply an address used by the chip, as well as the proper control signals

Memory ROM RAM Address 001 Address 000 ROM chip will be enabled if 000 RAM chip will be enabled if 001 Address 001

Combinatorial logic is used to realize these functions and set the chip enable signals of the memory chips

ROM chip will output data only when its output enable and chip enable signals are asserted RAM has two control inputs , RD and WR. Both the read and write signals from the control bus can drive these two signals

I/O subsystem Register Tri-state buffers I/O Device

Freehand drawing of a computer organization Address Bus Memory Subsystem CPU Data Bus Control Bus I/O Device I/O Device I/O Subsystem

Historical Perspective Please see attached

Questions ?