Chapters 6-7 Part I – Computer Organisation and Architecture in [B2]

Question 1

Overview of different types of external memory

Answer 1

Optical disks, magnetic discks, tapes, external hard drive etc

Question 2

What is a I/O module?

Answer 2

An I/O module, or Input/Output module, is a device or component in a computer system that facilitates communication between the central processing unit (CPU) and external devices. These external devices can include peripherals such as keyboards, mice, printers, monitors, storage devices, and other hardware components.
The primary function of an I/O module is to manage the flow of data between the CPU and these external devices.

Question 3

What are techniques of I/O operations?

Answer 3

To execute an I/O-related instruction, the processor issues an address, specifying the particular I/O module and external device, and an I/O command. There are four types of I/O commands that an I/O module may receive when it is addressed by a processor:

■ Control: Used to activate a peripheral and tell it what to do. For example, a magnetic-tape unit may be instructed to rewind or to move forward one record. These commands are tailored to the particular type of peripheral device.

■ Test: Used to test various status conditions associated with an I/O module and its peripherals. The processor will want to know that the peripheral of interest is powered on and available for use. It will also want to know if the most recent I/O operation is completed and if any errors occurred.

■ Read: Causes the I/O module to obtain an item of data from the peripheral and place it in an internal buffer (depicted as a data register in Figure 7.3). The processor can then obtain the data item by requesting that the I/O module place it on the data bus.

■ Write: Causes the I/O module to take an item of data (byte or word) from the data bus and subsequently transmit that data item to the peripheral.

Question 4

What is DMA? What is cycle stealing?

Answer 4

DMA(Direct Memory Access) is a module on the system bus.
The DMA module is capable of mimicking the processor and taking over control of the system from the processor.
When the processor wishes to read or write a block of data, it issues a command to the DMA module, by sending to the DMA module the following information:
Whether a read or write is requested
The address of the I/O device involved.
The starting location in memory to read and write to.
The number of words to be read or written.

Cycle stealing is a technique used to manage the transfer of data between a peripheral device and the computer’s memory without using the CPU for each data transfer.
Here’s how cycle stealing works:

1. Request and Acknowledge: A device requests control of the system bus from the CPU, typically using a DMA controller.
2. Transfer Control: Once the CPU grants control, the DMA controller takes charge of the bus.
3. Data Transfer: The device, under the control of the DMA controller, can transfer data directly to or from the system’s memory without the CPU. This process occurs in “bursts” or “steals” of bus cycles, where the DMA controller temporarily takes control of the bus from the CPU during each transfer.
4. Release Control: After the data transfer is complete, the DMA controller releases control of the bus back to the CPU.