8085 microprocessor architecture pdf

RLC vs. RLA Logical Operations Branch Operations Unconditional Branch Machine Control Operand Types Instruction with Immediate Date Addressing Modes Data Formats Using a Register Pair as a Loop Counter Nested Loops for Delay The Non-Vectored Interrupt Process Mode 0 Simple Input or Output Download process is simple click to the attachment download will start automatically.

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Disclaimer : BooksWarrior does not own this book pdf, neither created nor scanned. Usually un-used in small systems. There are 3 important pins in the frequency control group.

X0 and X1 are the inputs from the crystal or clock generating circuit. The frequency is internally divided by 2. To understand how the microprocessor operates and uses these different signals, we should study the process of communication between the microprocessor and memory during a memory read or write operation. Lets look at timing and the data flow of an instruction fetch operation. Example 3. Lets assume that we are trying to fetch the instruction at memory location That means that the program counter is now set to that value.

The following is the sequence of operations: The program counter places the address value on the address bus and the controller issues a RD signal.

The memorys address decoder gets the value and determines which memory location is being accessed. The value in the memory location is placed on the data bus. The value on the data bus is read into the instruction decoder inside the microprocessor.

After decoding the instruction, the control unit issues the proper control signals to perform the operation. Now, lets look at the exact timing of this sequence of events as that is extremely important. The signal remains low active for two clock periods to allow for slow devices.

During T3 the RD signal is Disabled goes high. This turns off the output Tri-state buffers in the memory. That makes the AD7 AD0 lines go to high impedence mode. Demultiplexing AD7-AD0 From the above description, it becomes obvious that the AD7 AD0 lines are serving a dual purpose and that they need to be demultiplexed to get all the information.

The high order bits of the address remain on the bus for three clock periods. However, the low order bits remain for only one clock period and they would be lost if they are not saved externally. Also, notice that the low order bits of the address disappear when they are needed most. To make sure we have the entire address for the full three clock cycles, we will use an external latch to save the value of AD7 AD0 when it is carrying the address bits.

We use the ALE signal to enable this latch. Given that ALE operates as a pulse during T1, we will be able to latch the address. Sub: MALP. From the above discussion, we can define terms that will become handy later on: T- State: One subdivision of an operation. A T-state lasts for one clock period. An instructions execution length is usually measured in a number of T-states.

This cycle may consist of 3 to 6 T-states. Instruction Cycle: The time required to complete the execution of an instruction. In the , an instruction cycle may consist of 1 to 6 machine cycles. The generates a single RD signal. A closer look at the Architecture Previously we discussed the from a programmers perspective.

Now, lets look at some of its features with more detail. Also, the ALU includes a temporary register used for holding data temporarily during the execution of the operation. This temporary register is not accessible by the programmer.

S-sign flag The sign flag is set if bit D7 of the accumulator is set after an arithmetic or logic operation. Z-zero flag Set if the result of the ALU operation is 0. Otherwise is reset. This flag is affected by operations on the accumulator as well as other registers. DCR B. This flag is used only internally for BCD operations. Section Otherwise it is cleared. So, the flag can be used to indicate even parity. CY-carry flag Discussed earlier.

The executes several types of instructions with each requiring a different number of operations of different types. However, the operations can be grouped into a small set. The three main types are: Memory Read and Write. Request Acknowledge. These can be further divided into various operations machine cycles. The first step of executing any instruction is the Opcode fetch cycle.

In this cycle, the microprocessor brings in the instructions Opcode from memory. This machine cycle has four T-states. The uses the first 3 T-states to fetch the opcode.

T4 is used to decode and execute it. It is also possible for an instruction to have 6 T-states in an opcode fetch machine cycle.