In this program we will see how to add three 16-bit numbers stored in register pairs.Problem StatementWrite 8085 Assembly language program to add three 16-bit numbers stored in register pair BC, DE and HL. Store the result at DE register pair.DiscussionIn this program we are storing the 16-bit numbers into BC, DE and HL pair. We have DAD D instruction which helps to add HL and DE register pair, and store the result into HL pair. After that copy BC to DE, then again perform DAD D to add them. Finally using XCHG we are storing them into DE register ... Read More

The Program and data which are stored in the memory, are used externally to the microprocessor for executing the complete instruction cycle. Thus to execute a complete instruction of the program, the following steps should be performed by the 8085 microprocessor.Fetching the opcode from the memory;Decoding the opcode to identify the specific set of instructions;Fetching the remaining Bytes left for the instruction, if the instruction length is of 2 Bytes or 3 Bytes;Executing the complete instruction procedure.The given steps altogether constitute the complete instruction cycle. These above mentioned steps are described in detail later. The above instructions are assumed by ... Read More

The select unit of the register in the 8085 selects any one of the register pairs (BC, DE, HL, SP, PC, or WZ) for sending them to it to the latch unit specified for addressing. For example, the contents of the PC be C200H. If the selection unit be the register which selects the PC, and sends C200H from the PC to the address latch internally thereafter the latch holds the specified value and sends directly out to the pins of the address after buffering. The Most Significant Byte of the address i.e. the C2H, is sent out to the ... Read More

Address and data buffers are used for bidirectional data transfer. They perform the unidirectional data transfer when they send out the Least Significant Byte of the address. These buffers are only used for increasing the driving capacity of the current. Through the internal bus data goes to the buffers. The Least Significant Byte of the address goes to the buffers from the internal address latch to the other.Hence the address or data are sent out on the address ranging from AD7 to AD0 can drive every external chips, like chips of RAM, chips of EPROM, and other peripheral chips meant for ... Read More

Let us consider the instruction to be executed as “MOV A, C”. Here in this case the value of 8 bit in the register C must be moved to the register. The given set of registers namely B, C, D, E, H, and L must be connected to the internal bus by means of a multiplexer (many input but only one output) or demultiplexer the reverse of multiplexer. The register meant to carry the work selects the specific unit and sends the appropriate code to the multiplexer such that the contents of register C are sent out to the multiplexer ... Read More

Temporary register is also an 8-bit register which the programmer can’t access at all. It is temporarily stored inside 8085 microprocessor which is 8 bit operand to the instruction set. For example, when the fetching of instructions “MVI M, 34H” is done the instruction register IR register will receive the opcode for MVI M, and the Temp register receives 34H.The operations of arithmetic and logical sequence carried out involves two operands, among which one is operand is provided by the accumulator, and the other operand is provided by the Temp register. For example, in the addition process the instruction to ... Read More

We use Timing and Controlling unit in 8085 for the generation of timing signals and the signals to control. All the operations and functions both interior and exterior of a microprocessor are controlled by this unit. X2 and CLK output pins: To do or rather perform the operations of timing in the microcomputer system, we have a generator called clock generator in the CU of 8085. Other than the quartz crystal the complete circuit of the oscillator is within the chip. The two pins namely X1 and X2 are taken out from the chip to give the connection to the ... Read More

The main differences between a JMP instruction and a CALL instruction is as follows –If a JMP instruction is executed, we jump to the destination location, and the execution carries on from there, without bothering to come back later to the instruction after the JMP. On the other hand, if a CALL instruction is executed, we jump to the subroutine, and the execution carries on from there till the RET instruction is executed in the subroutine, and then we come back to the instruction after the CALL in the main program.The address of the next instruction after the CALL instruction ... Read More

To define Temporary Register, we can mention that it is an 8-bit non-programmable resister used to hold data during an arithmetic and logic operation (temporary resister is used to hold intermediate result). The result is stored in the accumulator, and the flags (flip-flops) are set or reset according to the result of the operation.W and Z are two 8-bit temporary registers of 8085 microprocessor, which is not accessible to the user. They are exclusively used for the internal operation by the microprocessor. These registers are used either to store 8-bit of information in each W and Z registers or a ... Read More

IR (Instruction Register) is a special purpose register, which is used to receive the 8-bit opcode portion of an instruction. It is not accessible to the programmer. What it means is that there are no instructions by which the programmer can load it with values of his choice. For example, instructions like ‘MOV IR, D’ or ‘MVI IR, 45H’ are not present in the instruction set of 8085. Thus, IR register is not shown in the programmer's view of 8085.Let us consider one example. First of all, PC is loaded with the value 8000H. This is done by typing the ... Read More