## DMA

 Question 1
The size of the data count register of a DMA controller is 16 bits. The processor needs to transfer a ﬁle of 29,154 kilobytes from disk to main memory. The memory is byte addressable. The minimum number of times the DMA controller needs to get the control of the system bus from the processor to transfer the ﬁle from the disk to main memory is ________.
 A 456 B 457 C 458 D 459
Computer-Organization       DMA       2016 set-01
Question 1 Explanation:
Since nothing is mentioned about the mode of DMA working whether it is cycle stealing mode or burst mode, we consider it as burst mode by default.
As the data count register of the DMA is 16 bits long in burst mode DMA transfers 216 Bytes (= 64KB) once it gets the control.
To transfer 29,154 KB, no. of times DMA needs to take control = (29,154 KB / 64KB) = 29,154/64 = 455.53, means 456 times.
 Question 2

 A 3.4 B 4.4 C 5.1 D 6.7
Computer-Organization       DMA       Gate 2011
Question 2 Explanation:
No. of clock cycles required by using load-store approach = 2 + 500 × 7 = 3502 and that of by
using DMA = 20 + 500 × 2 = 1020
Required speed up = 3502/1020=3.4
 Question 3

A hard disk with a transfer rate of 10 Mbytes/second is constantly transferring data to memory using DMA. The processor runs at 600 MHz, and takes 300 and 900 clock cycles to initiate and complete DMA transfer respectively. If the size of the transfer is 20 Kbytes, what is the percentage of processor time consumed for the transfer operation?

 A 5.0% B 1.0% C 0.5% D 0.1%
Computer-Organization       DMA       Gate-2004
Question 3 Explanation:
Clock cycle time = 1/600 × 10-6 (time = 1/frequency)
For DMA initiation and completion total 1200 cycles is required. So total time will be = 1200 × 1/600 × 10-6 = 2μs
Disk transfer rate = 10MB/s
1B = 1/107 s
So, total transfer 20KB time taken will be,
20 × 103 × 1/(107) s
= 2000μs
∴ Percentage of processor time consumed is,
2/2+2000 × 100 = 0.1%
 Question 4
Which of the following requires a device driver?
 A Register B Cache C Main memory D Disk
Computer-Organization       DMA       Gate-2001
Question 4 Explanation:
Disk driver is software which enables communication between internal hard disk (or drive) and computer.
There are 4 questions to complete.