A two-byte instruction gives the specific function instruction in two bytes, or two words. The first specifies the opcode, which tells the microprocessor what operation will occur. The second specifies the operand, or the data that the operation is done on.
A 3 byte instruction is an instruction with 3 bytes, usually 1 opcode byte and 2 data bytes. Examples are LHLD 1234H, for the 8085, and MOV AX,1234H, for the 8086/8088.
1byte instruction = MOV A,B 2byte instruction = MVI A,01H 3byte instruction = STA 2030H
A microprocessor know whether the next byte is an instruction or data because the microprocessor knows for what it is looking. The bus, on the other hand, for an 8085 based system, knows an opcode from data by looking at S0 and S1 when IO/M- is low. If both are high, it is an opcode, otherwise it is data.
In the 8085 microprocessor, the MOV instruction copies data between two registers, or between a register and memory. The MVI instruction differs only in that the source data is contained in the byte immediately following the opcode byte.
1) in 8088 we hav 4 byte long queue.So in order to increase the speed of execution manufacturer has increased the length of queue to 6 bytes in 8086. 2) bcoz the longest instruction in instruction set is 6bytes long. hence with a six byte long queue it is possible to prefetch even longest instruction in the instruction set.
RST 2 is one example of a one byte call instruction. It points to 0010H.
fetch SHLD opcode bytefetch direct address least significant bytefetch direct address most significant bytewrite L register contents byte to memorywrite H register contents byte to memory
Because that's how Intel designed it. The size of the instruction prefetch queue is a design tradeoff between complexity and performance, given a certain target instruction mix and desired queue hit/miss ratio.
The 8085 instruction set is classified into three groups according to its Word size. They are 1. One word /1 byte instructions 2. Two word / 2 byte instructions 3. Three word / 3 byte instructions
It fetches the next instruction.
microprocessor is a program controlled device,which fetches, decodes and executes instruction codes.
it goes to queue for next instruction
Suppose we give a 8-bit instruction ADD B to the microprocessor then this instruction is not at all understood by microprocessor as it only accepts binary inputs so first of all it stores the instruction in the INSTRUCTION REGISTOR then it decodes this instruction ADD B to its suitable binary code 80H in the INSTRUCTION DECODER.. after converting to 80H then the microprocessor understands that .. yes i have to add the content of the resistor B with that of A(accumulator) and store the result in the accumulator A this is a small example how microprocessor operates facing the instructions
there are four types of instructions in 8085 microprocessor
Depending on the particular microprocessor, a machine cycle is the fetch or store of one (typically, one byte) native word. In the 8085, this is a byte fetch or store, plus the overhead in decoding and processing the instruction. In this case, the first machine cycle is four clock cycles, or T states, and subsequent machine cycles are three clock cycles, although certain instruction sequences, such as DAD, require two extra clock cycles.
In 8086 the instruction queue is 6 byte long. This is because even the longest 8086 instruction is 6 byte long. Thus it is possible to prefetch even the longest instruction in the instruction set.
A series of data bytes or words available in a memory at consecutive locations, to be referred to collectively or individually, are called as byte strings or word strings.
SJMP is short jump. In this 2-byte instruction the first byte is opcode and second byte is relative address of target location. This can save some byte of memory in many applications where memory space is in short supply where as LJMP (Long Jump) is 3-byte instruction in which 1st byte is opcode and 2nd and 3rd byte represent the 16- bit address of target location.
The 8085 instruction set is classified into the following three groups according to word size: 1. One-word or 1-byte instructions 2. Two-word or 2-byte instructions 3. Three-word or 3-byte instructions
Special restart instruction used with interrupts
We can implement "Divided by '2' " operation by using RRC.
The CALL InstructionOpcode OperandCALL 16-bit memory addressof a subroutineIt is a 3-byte instruction that transfers the program sequence to a subroutineSaves the content of the PC (Program Counter-16-bit register) , the address of the next instruction , on the stackDecrements the stack pointer register by 2Jumps unconditionally to the memory location specified by the 2nd and 3rd bytes.This instruction is accompanied by a RETURN instruction in the subroutine
Because the longest instruction in instruction set is 6 bytes long. hence with a6 byte long queue it is possible to prefetch even longest instruction in the instruction set.
to increase the speed of microprocessor