Page 45 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 3: Assembly Language Programming of 8085

Notes
IN 8-bit port address Input from Port (Read a data byte from a input
device)
(a) It is a 2-byte instruction.
(b) Reads data from the input port specified in the
second byte and loads into Accumulator.

HLT Halt
(a) It is 1-byte instruction.
(b) Processor stops executing and enters wait state.
(c) The address bus and data bus are placed in
high impedance state. No register contents are
effected.

NOP No Operation
(a) It is a 1-byte instruction.
(b) No operation is performed.
(c) Used to increase processing time or substitute
in place of an instruction.

The symbols R, R and R represent any of the 8085 registers as A, B, D, E, H and L.
d S
The two machine control operations: HLT and NOP are necessary to execute the program. NOP
is generally used when an error occurs in a program and an instruction needs to be eliminated. It
is more convenient to substitute NOP than to reassemble the whole program. When HLT opcode
is used then address bus and data bus are placed in the high-impedance state or tri-state. No
registers contents are affected by HLT.
As Table 3.2 states that the OUT and IN opcodes are 2-byte instructions the second byte is not
defined. The question arises: what is the second byte in the instructions IN and OUT? The answer
is the second byte is the I/O port address. Each I/O port is identified by a unique number. The
second byte: 8-bits, i.e. 256 (2 ) combination. Thus 256 input ports and 256 output ports N addresses
8
from OOH to FFH can be connected to the system.
Key Points about Data Transfer (Copy) Operations:
1. Registers are used to load data directly or to save data bytes.
2. The destination register is modified whereas the source register is not affected.

3. Data copy instructions do not affect any of the status flags. This is because it involves two
different registers or one register and one data byte. As only the Accumulator register can
change or alter the status flag, all other registers B, C, D, E, H and L does not affect the flag
register. Flags are affected by the operations in the ALU; therefore operations that take
place outside the ALU do not affect flags.
4. The 8085 transfer’s data from an input port to the Accumulator (IN) and from the
Accumulator to an output port (OUT).
5. The OUT instruction cannot send data from any other register except the Accumulator.
6. The IN instruction can only load data into Accumulator.

In 8085 processor, data transfer instructions do not affect any flag.

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