Unit - 2
I/O Organization
, Mode of Transfer
The binary information that is received from an external device is usually stored in the memory unit. The information that is
transferred from the CPU to the external device is originated from the memory unit. CPU merely processes the information
but the source and target is always the memory unit. Data transfer between CPU and the I/O devices may be done in different
modes.
Data transfer to and from the peripherals may be done in any of the three possible ways
1. Programmed I/O.
2. Interrupt- initiated I/O.
3. Direct memory access( DMA).
,Programmed I/O
● It is due to the result of the I/O instructions that are written in the computer program. Each data
item transfer is initiated by an instruction in the program. Usually the transfer is from a CPU
register and memory. In this case it requires constant monitoring by the CPU of the peripheral
devices.
● In this case, the I/O device does not have direct access to the memory unit. A transfer from I/O
device to memory requires the execution of several instructions by the CPU, including an input
instruction to transfer the data from device to the CPU and store instruction to transfer the data
from CPU to memory. In programmed I/O, the CPU stays in the program loop until the I/O unit
indicates that it is ready for data transfer. This is a time consuming process since it needlessly
keeps the CPU busy. This situation can be avoided by using an interrupt facility.
,
I/O Organization
, Mode of Transfer
The binary information that is received from an external device is usually stored in the memory unit. The information that is
transferred from the CPU to the external device is originated from the memory unit. CPU merely processes the information
but the source and target is always the memory unit. Data transfer between CPU and the I/O devices may be done in different
modes.
Data transfer to and from the peripherals may be done in any of the three possible ways
1. Programmed I/O.
2. Interrupt- initiated I/O.
3. Direct memory access( DMA).
,Programmed I/O
● It is due to the result of the I/O instructions that are written in the computer program. Each data
item transfer is initiated by an instruction in the program. Usually the transfer is from a CPU
register and memory. In this case it requires constant monitoring by the CPU of the peripheral
devices.
● In this case, the I/O device does not have direct access to the memory unit. A transfer from I/O
device to memory requires the execution of several instructions by the CPU, including an input
instruction to transfer the data from device to the CPU and store instruction to transfer the data
from CPU to memory. In programmed I/O, the CPU stays in the program loop until the I/O unit
indicates that it is ready for data transfer. This is a time consuming process since it needlessly
keeps the CPU busy. This situation can be avoided by using an interrupt facility.
,
