Describing the different modes of the DMA (Direct Memory Access Controller)

1. NOOR NABI Microprocessors And Microcontrollers 19-TE-28
ASSIGNMENT # 2 Submitted Date: 17-08-2020 Page 1
Q#1. Define the types of mode in DMA?
Types of modes in DMA:
Mainly there are two basic types of modes in DMA which are as follows:
1. Slave Mode 2. Master Mode
1. Slave Mode:
In slave mode, the 82357 monitors the bus and decodes and responds to I/O read and write
commands that addresses its registers.
When DMA is in mode and serving a DMA slave, it works in conjunction with the bus
controller to create bus cycles on the system bus. [3]
2. MasterMode:
In master mode, the DMA controller is either serving a DMA slave’s request for DMA cycles,
generating refresh cycles, or allowing 16-bit ISA master to use the bus via a cascaded signal.
The DMA places addresses and the memory read/write signal on the host CPU bus. [3]
Q#2. Define conceptof Masterand Slave in Microprocessors andexplain when DMA
controllerwill be in Masterand Slave Mode?
Conceptof MasterSlave in DMA:
A system that has more than one processorand in which one of the processorsis designated as
being the master and all other processors are slaves. The master processoris capable of actions
that the slaves cannot perform. [4]
=>
 Communications based on master/slave model occurs in many places. Some examples are
in database replications, devices connected to a bus in a computer.
 Simply, a master is a device or a process that controls other devices or processes. The
direction of the controlalways flows from master to slave.
 Whereas is a device or a process that is controlled by another device or a process. For
example, in database replication, the database considered as the slave will use the updates
recorded in the master database to synchronize its data with the master. [4]

2. NOOR NABI Microprocessors And Microcontrollers 19-TE-28
ASSIGNMENT # 2 Submitted Date: 17-08-2020 Page 2
Q#3. Define the modes of DMA Operations?
Basically there are several modes of operations in DMA; three of them are as follows:
Single Byte Operation mode:
When the DMA becomes the bus master, it will transfer only ONE BYTE and return the bus
back to the microprocessor.As soonas the microprocessorperforms one bus cycle, DMAC will
once again take the bus back from the microprocessor. [1]
Block TransferOperationMode:
In this mode, the DMAC is programmed to transfer all the bytes in one complete DMA
operation. After a byte is transferred, the CAR and CWCR are adjusted accordingly. The
system bus is returned, only after all the bytes are transferred. [1]
CascadeOperationMode:
In this mode more than one DMA are cascaded together. It is used to increase the number of
devices interfaced to the µP. Here we have one Master DMAC, to which one or more Slave
DMACs are connected. [1]
Q#4. Define the types of DMA Transfer?
There are three modes of DMA which can be defined as:
Burst mode:
In Burst mode the entire block of data is transferred in one continuous sequence. Once the
DMA controller is granted access to the system bus by the CPU, it transfers all bytes of data in
the data block before releasing controlof the system buses back to the CPU. [2]
Transparentmode:
Transparent mode takes the most time to transfer a block of data, yet it is also the most efficient
mode in terms of overall system performance. In transparent mode, the DMA controller
transfers data only when the CPU is performing operations that do not use the system buses. [2]
Cycle stealing mode:
The cycle stealing mode is used in systems in which the CPU should not be disabled for the
length of time needed for burst transfer modes. In the cycle stealing mode, the DMA controller
obtains access to the system bus the same way as in burst mode, using the Bus request and Bus
grant signals, which are the two signals controlling the interface between the CPU and the
DMA controller. [2]

3. NOOR NABI Microprocessors And Microcontrollers 19-TE-28
ASSIGNMENT # 2 Submitted Date: 17-08-2020 Page 3
REFERENCES:
1. https://www.ques10.com/p/32917
2. https://en.wikipedia.org/wiki/Direct_memory_access
3. http://members.tripod.com/
4. http://www.difference.com