Question

The processor needs to transfer a file of 32768 kilobytes from disk to main memory. The memory is byte addressable. The size of the data count register of a DMA controller is 16 bits. What is 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 in the following transfer modes

1.) Cycle stealing mode

2.) Burst Transfer mode

Answer 1

1, Cycle stealing mode: Here one block of data is transferred and then control is returned to CPU

Size of data count register of the DMA controller = 16 bits
Data that can be transferred in one go = 2¹⁶ bytes = 64 kilobytes
File size to be transferred = 32768 kilobytes
So, 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 = ceil(32768 /64) = 512

2. Burst Transfer mode: Here number of blocks of data are transferred continuously before returning control to CPU until whole data is transferred.
Data that can be transferred in one go = 32768 kilobytes
File size to be transferred = 32768 kilobytes
So, 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 = ceil(32768 /32768 ) = 1