Study notes on the memory mapped I/O scheme and the I/O mapped I/O scheme for microprocessor 8085

hi friend here i discuss on the memory mapped I/O scheme and the I/O mapped I/O scheme for microprocessor 8085. read carefully and download this study material from bellow.

Describe the memory mapped I/O scheme.

In memory mapped I/O scheme we can use only one address space. This particular one address space is allocated to both memory and I/O devices. In total memory address some addresses are assigned to memories and some to I/O devices. But we have to assign the address for I/O devices are different from the addresses which have been assigned to memories. In this scheme remember that I/O device is also treated as a memory location. And one address is assigned to each memory location (unique address) and one address is assigned to each I/O device.

Now if we explain it with some example then it will be very easy to understand so remember all data transfer instructions of the microprocessor can be used for transferring data from and to either memory or I/O devices. Now take a very good example, MOV C, M instruction would transfer one byte of data from a memory location or it can also transfer an input device to the register C, depending on whether the address in the H-L register pair is assigned to a memory location or to an input device. If H-L contains address of a memory location, data will be transferred from that memory location to register C, while if H-L pair contains the address of an input device, data will be transferred from that input device to register C.

Explain the I/O mapped I/O scheme.

We know that Some CPUs provide one or more control lines like IO/ M line for 8085, which indicates the status of operation, is memory or I/O operation. If we get the status of IO/ M line is high, it indicates I/O operation and when we get low, it points to memory operation. But we have to remember main thing that in this case the same address may be assigned to both memory and an I/O device depending on the status of IO/M line.

The above mansion scheme is referred or we can call as I/O mapped I/O scheme. Look in this scheme two separate address spaces exist. One space is meant exclusively for memory operations and the other for I/O operations. 

Now see block represent of memory allocation for above mansion scheme.

The following figure shows, pictorially, both the schemes. Here it is assumed that the system has a 64 KB of memory and 256 I/O space.

See the difference between Memory Mapped I/O and  I/O Mapped I/O

Memory Mapped I/O

I/O Mapped I/O

In Memory Mapped I/O Address width is 16-bit. A0 to A15 are used to generate address of the device.  MEMR and MEMW control signals are used to control read and write I/O operations respectively. Instructions available are STA addr, LDA addr, LDAX rp, STAX rp, ADD M, CMP M, MOV r, M, etc. Data transfer takes place between any register and I/O device. Maximum number of I/O devices that can be addressed is 65536 (theoretically). Execution speed using STA addr, LDA addr is 13 T-state and for MOV M, r, etc.,it is 7-T states. It requires more hardware circuitry because it decodes 16-bit address.

In I/O Mapped I/O Address width is 8-bit. A0 to A15 lines are used to generate address of the device.  IOR and IOW control signals are used to control read and write I/O operations respectively. IN and OUT are the only available instructions. Data transfer takes place between accumulator and I/O device. Maximum number of I/O devices that can be addressed is 256. Execution speed is 10 T-states.  It requires less hardware circuitry because it decodes 8-bit address.

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