UNIT 1 REFERENCE 1 PREPARED BY S.RAVINDRAKUMAR, LECT/ECE, CHETTINAD COLLEGE OF ENGG AND TECH, KARUR

Size: px

Start display at page:

Download "UNIT 1 REFERENCE 1 PREPARED BY S.RAVINDRAKUMAR, LECT/ECE, CHETTINAD COLLEGE OF ENGG AND TECH, KARUR"

Ellen Poole
5 years ago
Views:

1 UNIT 1 REFERENCE 1

2 PROGRAMMING THE 8085 DEVELOPMENT OF PROGRAM A program is a sequence of instructions written to tell a computer to perform a specific function. The instructions are selected from the instruction set of the microprocessor. The first step in writing a program is to think very carefully about the problem that you want the program to solve. In other words, ask yourself many times, What do I really want this program to do? If you don t do this, you may write a program that works great but does not do what you need it to do. As you think about the problem, it is a good idea to write down exactly what you want the program to do and the order in which you want the program to do it. At this point, you do not write down the instructions, you just divide the given problem in small steps in terms of the operations the 8085 can perform. This sequence of operations used to solve a programming problem is often called the algorithm. For more complex problems, however, we develop a more extensive outline before translating the algorithm into instructions. The most common way of representing the algorithm for a program is flowchart. The steps necessary to write the program can be represented in a pictorial format, called a flowchart. Generally, a flowchart is used for two purposes: to assist and clarify the thinking process and to communicate the programmer s thoughts and logic to others. The below figure shows some of the common flowchart symbols:

3 Writing and Executing a program in a Single Board Microcomputer Writing a simple program of adding two hexadecimal numbers 23H and 84H and saving the result in a register in assembly language is illustrated below: Algorithm 1. Load the 1 st number 23H in one register. 2. Load the 2 nd number 84H in another register. 3. Add the contents of the two registers. 4. Save the result in any register. 5. End the program. Flow Chart Assembly Language The steps described in the flowchart are translated into an Assembly Language as follows: Machine Language Now, the above mnemonics should be converted into machine language. By looking up the machine code for each instruction in the instruction set, we can translate the program into machine language as follows:

4 INSTRUCTION CYCLE: An instruction is a command given to the computer to perform a specified operation on the given data. Sequence of instructions written for a computer to perform a particular task is called program. Program & data are stored in the memory. The microprocessor fetches one instruction from the memory at a time & executes it. It executes all the instructions of the program one by one to produce the final result. The necessary steps that a microprocessor carries out to fetch an instruction & necessary data from the memory & to execute it constitute an instruction cycle. In other words, an instruction cycle is defined as the time required completing the execution of an instruction. An instruction cycle consists of a fetch cycle & an execute cycle. The time required to fetch an opcode (fetch cycle) is a fixed slot of time while the time required to execute an instruction (execute cycle) is variable which depends on the type of instruction to be executed. FETCH OPERATION: The first byte of an instruction is its opcode. An instruction may be more than one byte long. The other bytes are data of operand address. The program counter (PC) keeps the memory address of the next instruction to be executed. In the beginning of a fetch cycle the content of the program counter, which is the address of the memory location where opcode is available, is sent to the memory. The memory places the opcode on the data bus so as to transfer it to the microprocessor. The entire operation of fetching an opcode takes three clock cycles. EXECUTE OPERATION: The opcode fetched from the memory goes to the instruction register (IR). From the instruction register it goes to the decoder circuitry which decodes the instruction. After the instruction is decoded, execution begins. If the operand is in general purpose registers execution is immediately performed. The time taken for decoding and execution is one clock cycle. If an instruction contains data or operand and address which are still in the memory, the microprocessor has to perform some read operations to get the desired data. After receiving the data it performs execute operation. A read cycle is similar to a fetch cycle. In case of a read cycle the quantity received from the memory are data or operand address instead of an opcode. In some instructions write operation is performed. In write cycle data are sent from the microprocessor to the memory or an output device. Thus we see that in some cases an execute cycle may involve one or more read or write cycles or both.

5 MACHINE CYCLE: Machine cycle is defined as the time required completing the operation of accessing either memory or I/O. In the 8085, the machine cycle may consist of three to six T states. T-State: T-State is defined as one sub-division of the operation performed in one clock period. These sub-divisions are internal states synchronized with the system clock. PROGRAMMING TECHNIQUES: Microprocessor is very fast and accurate in processing the data. It is more efficient than human beings when it is required to perform the repeated tasks. To perform repetitions tasks programmer must use different programming techniques such as looping, indexing etc. Looping: Looping is the programming technique which is used to tell the processor to repeat the task. A loop can be constructed by using jump instructions. Continuous loops can be constructed by using unconditional jump instructions. Unless you reset the system continuous loop does not stop repeating the tasks. For example: modulo ten counter which will be discussed later. Conditional loops can be constructed by using conditional jump instructions. The specified tasks will be repeated only when the conditions are met. For example: the delay loop, which will be discussed in the next section. INDEXING: Indexing is the programming technique in which objects will be sorted in a sequential manner. In this data bytes are stores in memory locations sequentially & those data bytes are referred to by their memory address. PROGRAMS 1.Write an Assembly Language Program to add 2-16 bit numbers: LHLD 9501H XCHG LHLD 9503H MVI C, 00 DAD D JNC LOOP1 INR C LOOP1 SHLD 9505H MOV A, C STA 9507H

6 2.Write an ALP to add n- 8 bit numbers: MVI D, 00 MVI C, 05 LXI H, 8030 MOV A, M DCR C AHEAD ADD M JNC LOOP-1 INR D LOOP-1 DCR C JNZ AHEAD STA 8056 MOV A, D STA Write an ALP to perform 32-bit addition: LXI H, 8500 MOV C, M LXI D, 8600 XRA A LOOP-1 LDAX D ADC M MOV M, A INX D DCR C JNZ LOOP-1 MVI A, 00 RAL MOV M, A RST 5 4.Write an ALP to subtract 2-8 bit numbers: LXI H, 8501 MOV A, M SUB M JNC LOOP-1 INR C LOOP-1 MOV M, A

7 MOV A, C MOV M, C RST5 5.Write an ALP to ADD 2-BCD numbers: LXI H, 8A00 MVI D, 00 MOV A, M ADD M DAA JNC LOOP-1 INR D LOOP-1 STA 8A03 MOV A, D STA 8A04 RST5 6.Write an ALP to SUBTRACT 2-16 BIT NUMBERS: LHLD 8100 XCHG LHLD 8102 MOV A, E SUB L STA 8104 JNC LOOP DCR D LOOP MOV A, D SUB H STA 8105 RST5

8 7.Write an ALP to multiply 2-8bit numbers: LXI H, 8A00 MOV B, M XRA A MOV C, A AHEAD ADD M JNC LOOP-1 INR C LOOP-1 DCR C JNZ AHEAD MOV M, A MOV M, C RST5 8.Write an ALP to perform division of 1-8 bit number by another 8-bit number: LXI H, 8900H MOV A, M MOV B, M MVI C, FF LOOP INR C SUB B JNC LOOP ADD B STA 8902H MOV A, C STA 8903H TIME DELAYS: Counters are constructed using software instructions to keep track of the events. Since the counting is performed at such high speed, only the last count can be observed. To observe all the counts, there must be an appropriate time delay between counts. Designing a delay is very simple. A register is loaded with a number, depending on the delay required, and then the register is decremented until it reaches zero by setting up a loop with a conditional JUMP instruction. The loop causes the delay, depending upon the clock period of the system.

9 Single Register Delay: A count is loaded in a register and the loop is executed until the count reaches zero. The set of instructions necessary to set up a delay loop are: LOOP MVI B, FF DCR B JNZ LOOP To calculate the time delay we must consider the T-states required for each instruction, and for the number of times the instruction are executed in the loop. The clock frequency of 8085 is 3MHZ. Clock period T= 1/f = 1/3*106 = 0.33*10-6 Register B is loaded with FFH (25510) therefore the loop is repeated 255 times. The time delay can be calculated as follows: TL=(T*Loop T states*n10) Where: TL=Time Delay in loop T= System Clock Period. N10=Equivalent decimal number of Hexadecimal count loaded in the delay register. DCR & JNZ forms a 0 loop with a total of 14 (4+10) T- states. Therefore: TL= (0.33*10-6*14*255) TL= *10-3 Eg: Write an ALP to display FF and 00 alternatively with a delay: MVI A, FF BACK PUSH PSW STA 8FF1 CALL UPDDT CALL DELAY POP PSW CMA JMP BACK

10 Other Sample Programs Write an assembly program to add two numbers Program MVI D, 8BH MVI C, 6FH MOV A, C ADD D OUT PORT1 Write an assembly program to multiply a number by 8 Program MVI A, 30H RRC RRC RRC OUT PORT1 Write an assembly program to find greatest between two numbers Program MVI B, 30H MVI C, 40H MOV A, B CMP C JZ EQU JC GRT OUT PORT1 EQU: MVI A, 01H OUT PORT1 GRT: MOV A, C OUT PORT1

Assembly Language Programming of 8085

Assembly Language Programming of 8085 Topics 1. Introduction 2. Programming model of 8085 3. Instruction set of 8085 4. Example Programs 5. Addressing modes of 8085 6. Instruction & Data Formats of 8085