2. Memory Organization of 8085
• A microprocessor system requires both RAM (Random
Access memory) and EPROM (Erasable Programmable
Read Only Memory).
• Hence the memory space has to be divided between RAM
and EPROM. (Erasable-Programmable Read Only Memory).
• The 8085 microprocessor has 64 K bytes of addressable
memory space and allocating this address space for RAM
and EPROM depends on the applications.
• Some system may require large memory space. But some
system requires less memory space and in this case the full
memory space will not be utilized.
3. Memory Organization of 8085
• Then the unused memory address can be used for
addressing I/O devices such I/O devices are called memory
mapped I/O devices.
• In EPROM is used as a program memory and RAM is used
as a data memory when EPROM and RAM are used, the
total 64 Kbytes address space is shared by them.
• For an example the address map of a 64 Kbytes memory
space which can be used by the microprocessor as shown
in figure.3.1
• The entire 64 K bytes of address space is divided into four
subsets.
4. Memory Organization of 8085
• The first subset of 8 Kbytes of addresses are assigned to
the EPROM i.e., address range from 0000H to IFFFH.
• The next 48 Kbytes of addresses assigned to RAM i.e.,
address range from 2000H to DFFFH.
• The next highest 2 Kbytes to input/output devices i.e.,
address range from E000H to E7FFH and the remaining 6K
bytes are un used i.e., the address range from E800H to
FFFH.
• The separate decoding circuit decides the address map of
the entire address space.
6. MEMORY MAPPING
• A memory map is a pictorial representation of memory devices are
located in the entire range of address.
• The number of address lines of the microprocessor unit determines its
memory capacity.
• The 8085 Microprocessor unit has 16 address lines and it capable of
indentifying 2^16 = 64 k bytes (65, 536) memory registers.
• The entire memory address can range from 0000 to FFFF in Hex.
• Memory address provide the locations of various devices in the system.
• The 8085 microprocessor can access 64 Kbytes memory but it is not
necessary to use full 64 Kbytes address space, so the total memory size
depends upon the various applications.
8. I/O MAPPING (INPUT/OUTPUT MAPPING)
• I/O Mapping is defined as the assignment of
addresses to various I/O devices in a memory chip.
• The microprocessor unit can identify and
communicate with the I/O devices by using two
methods.
1. Memory Mapped I/O
2. I/O Mapped I/O
9. 1. Memory Mapped I/O
• In memory mapped I/O, the Input / Output devices are
indentified with a 16 bit addresses.
• In 8085 microprocessor the 64 Kbyte memory map is
shared between memory and I/O devices.
• The address assigned for l/o device can not be used for
memory register. The memory related Instructions (such as
STA, LDA, MOV R,M, MOV M,R) are used to data transfer
between microprocessor and I/O devices.
• A 64 Kbyte address space memory mapped I/O is shown in
figure.3.3
12. • In the above figure 3.4, there are four address
spaces in the 64 k byte of memory mapped I/O.
• The Lowest 8 k byte address are allotted to the
EPROM.
• The next highest 48 k byte addresses are allotted to
the RAM.
• The next highest 2 k byte addresses are allotted to
I/O port and remaining 6 k byte addresses are
unused.
13. Advantages of Memory Mapped I/O
1. It provides more input and output ports.
2. High Speed.
3. Powerful instructions are used to access I/O device.
4. No special instructions are needed.
Disadvantages of Memory Mapped I/O
1. Length of program is increased
2. Increased interface hardware
3. Limited number of I/O ports connected
4. Less memory requirement
14. 2. I/O Mapped I/O
• In I/O mapped I/O, the devices have separate addresses.
• The microprocessor unit uses 8-address lines to identify the
microprocessor.
• The microprocessor can identify 256 input devices and 256
output devices (28 = 256)
• The input and output devices are differented by the control
signals I/O Road (IOR) and I/O write (IOW).
• In this method, I/O addresses range from 00H to FFH.
• A 64 k byte address space for I/O mapped I/O is shown in
figure.3.5
17. I/O Mapped I/O
• The 8085 microprocessor has two instructions.
(IN & OUT) It can be used for data transfer
between I/o device and the processor.
• When IN instruction is used to data transfer
from the accumulator to an output device.
18. Advantages of I/O mapped I/O
1. Simple program.
2. Memory address is not affected by I/O.
3. More space for memory.
Disadvantages of I/O mapped 1/0
1. It is less powerful less flexible than Memory mapped I/O
2. Two additional control lines are needed for issuing IOR
and IOW signals.
