Instruction cycle is defined, as the time required completing the execution of an instruction.
An Instruction Cycle will have one to six machine cycles.
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.
There are no instructions in the 8085 that execute in only one clock pulse. The minimum number of clock cycles is four; three for instruction fetch and one for instruction decode/execute.
It is not possible to calculate the number of machine cycles from first principles without going into design details of the CPU in question. You will need the reference book or card for the specific microprocessor model you are using. That will give you the actual number of cycles that are required for each instruction family. Note that many instructions take a variable number of cycles based on where they fetch their operands from. In the 8088 and 8086, in many cases the number of machine cycles is given in the form "4+EA". This means that you have to look up the number of cycles for a specific Effective Address, which is part of the op code, and add it to the number of cycles to execute the op code. There will be a table of machine stated for each type of Effective Address determination as well. Older machines like the 8085, 8086, and 8088 will actually have a few instructions, notably rotate and shift instructions, where the time is given as something like "4+2s". In these cases, the value "s" is the number of positions you are shifting the operand; to shift it 7 places takes 14 machine states, over and above the initial 2.
In order to determine the instructions per second in an 8085 microprocessor, you need to know how long each instruction takes to execute. Some are as short as 4 T cycles. Some are as long as 18 T cycles. This is dependent on how the program is written. Add up the T cycles for each instruction. Divide the clock frequency in hertz by the number of T cycles, and you get instructions per second. Note that clock frequency is one half of the crystal frequency. Note also that you must include Twait cycles in your calculation.
9
In the context of the 8085 microprocessor, the ADD instruction takes 1 machine cycle to execute, as it operates directly on the accumulator and the specified register. On the other hand, the LHLD (Load H and L Direct) instruction requires 3 machine cycles, as it involves reading data from a specified memory address into the L and H registers.
The crystal frquency in an 8085 system is twice the desired clock frequency, so a crystal of 2.2 MHz is required to operate at 1.1 MHz.Note: Clock frequency is not the same as instructions per second, because the instructions in an 8085 take a variable number of clock cycles, between 4 and 18, to execute.
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
The 8085 instruction MOV M,A requires two machine cycles and 7 T states. Cycle one is 3 T states for opcode fetch, plus 1 T state for opcode decode. Cycle two is 3 T state for operand store. These numbers do not include WAIT states. WAIT states are interposed between T2 and T3 of any memory access cycle, and the total number of WAIT states depends on the READY line.
There are 74 instructions in the 8085 microprocessor.
7
There is no protected mode in the 8085 microprocessor, therefore, there are no instructions that are considered privileged.