memory mapped io and io mapped io pdf

Memory mapped io and io mapped io pdf

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Engineering in your pocket

Memory-mapped I/O

Memory layout

Built with from Grav and Hugo. In order to study exceptions and interrupts we will use the Mips 32 simulator Mars.

So, total addressed capacity is memory connected only. Let's take an example of the processor. It has 16 address lines i.

Engineering in your pocket

In some instances, a given register may fit more than one of the above categories, e. The logic circuit that contains these registers is called the device controller , and the software that communicates with the controller is called a device driver. Simple devices such as keyboards and mice may be represented by only a few registers, while more complex ones such as disk drives and graphics adapters may have dozens. Which type of device we access depends only on the address used! The bit MIPS architecture has a bit address, and hence an address space of 4 gigabytes. This address may be fixed for certain devices, and auto-assigned for others. The SPIM simulator provides simple keyboard and display devices.

How do we access these two from the microprocessor? How do we, for example, store some data in the interfaced memory? Or send some output to an interfaced device? We use some form of address and then transfer or output the data at that particular address. The size of this address space, of course, depends on the number of pins that are used to connect. Separate from what? Separate from the memory.

Memory-mapped I/O

Thus, the CPU instructions used to access the memory can also be used for accessing devices. The reservation may be permanent, or temporary as achieved via bank switching. Different CPU-to-device communication methods, such as memory mapping, do not affect the direct memory access DMA for a device, because, by definition, DMA is a memory-to-device communication method that bypasses the CPU. Hardware interrupts are another communication method between the CPU and peripheral devices, however, for a number of reasons, interrupts are always treated separately. An interrupt is device-initiated, as opposed to the methods mentioned above, which are CPU-initiated. It is also unidirectional, as information flows only from device to CPU.

Memory mapped IO uses one address space for memory and input and output devices. In other words, some addresses are assigned to memory while others are assigned to store the addresses of IO devices. There is one set of read and write instruction lines. The same set of instructions work for both memory and IO operations. Therefore, the instructions used to manipulate memory can be used for IO devices too. Hence, it can lessen the addressing capability of memory because some are occupied by the IO.

However, as far as the peripheral is concerned, both methods are really identical. The difference between the two schemes occurs within the microprocessor. Intel has, for the most part, used the port mapped scheme for their microprocessors and Motorola has used the memory mapped scheme. Picture source : IO Devices. To access a hardware device, simply read or write to those 'special' addresses using the normal memory access instructions. The disadvantage to this method is that the entire address bus must be fully decoded for every device. For example, a machine with a bit address bus would require logic gates to resolve the state of all 32 address lines to properly decode the specific address of any device.


Memory Mapped IO. (and the n We want a consistent interface to I/O Block Diagram. Latch. /rd /wr. /wr /rd. RAM. P2. P0. ALE. A[]. A[]. Data.


Memory layout

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