Gate cse syllabus based topic Virtual Memory and interfacing Input/Output

Objective of this gate study material for computer science is to understand the concepts of Interface techniques used Input and output and to introduce about Virtual memory. In this gate study material for computer science based article as per gate cse syllabus for GATE 2020 we are going to discuss different techniques used for input/output interface

Following two types of techniques are used for input-output

1. Isolated I/O: In the isolated, I/O configuration the CPU has distinct input and output instructions, and each of these instructions is associated with the address of an interface register. When the CPU fetches and decodes the operation code of an input or output instruction, it places the address associated with the instruction into the common address lines. At the same time, it enables the I/O read or I/O write control line. When the CPU is fetching an instruction or an operand from memory, it places the memory address on the address lines and enables the memory read or memory write control line. The isolated I/O method isolates memory and I/O addresses so that memory address values are not affected by interface address assignment since each has its own address space.
2. Memory Mapped I/O: In computers that employ only one set of read and write signals and do not distinguish between memory and I/O addresses. The configuration is referred to as memory-mapped I/O. In a memory-mapped I/O organization there are no specific input or output instructions. The CPU can manipulate I/O data residing in interface registers with the same instructions that are used to manipulate memory words. The advantage is that the load and store instructions used for reading and writing from memory can be used to input and output data from I/O registers.

Virtual Memory Introduction

Virtual memory is a concept used in some large computer that permits the user to construct programs as through a large memory space were available, equal to the totality of auxiliary memory. Virtual memory is used to give programmes the illusion that they have a very large memory at their disposal, even though the computer actually has a relatively small main memory. A virtual memory system provides a mechanism for translating program generated addresses into correct main memory locations. In a Virtual Memory system, programmes are told that they have the total address space at their disposal. The address field of the instruction code has a sufficient number of bits to specify all virtual address. In our example, the address field of an instruction code will consist of 20 bits but physical memory addresses as per operating system tutorial must be specified with only 15 bits. Thus CPU will reference instructions and data with a 20-bit address, but the information at this address must be taken from physical memory because access to auxiliary storage for individual words will be prohibitively long.

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