Paper: Microprocessor and Computer Programming

Transcription

1 Paper: Microprocessor and Computer Programming Lesson: Microprocessor 8085 architecture Author: Priyamvada Bhardwaj College/Department: Ramjas College / Dept. of Physics, University of Delhi 1

2 Table of Contents Chapter 4: Microprocessor 8085 architecture 3.1 Introduction 3.2 Block diagram of system bus Pin description 3.5 Microprocessor operations Summary Exercise Glossary References 2

3 Chapter 3: Microprocessor 8085 Architecture 3.1 Introduction Intel 8085 is the most popular 8-bit microprocessor. It is a 40-pin IC fabricated on a single VLSI chip. It requires a single +5V power supply. It has in-built clock generation, system control and interrupt circuitry. The 8085 µp is a programmable digital device which has got a set of instructions designed internally to manipulate the data and communicate with peripherals. The internal logic organization or design is called architecture of the µp. It is important to know the architecture because it is the internal architecture of a µp which determines as to what happens to the data inside the µp and how and what type of operations can be performed with the data. 3.2 Block diagram of 8085 To understand the architecture of 8085 µp it is necessary to discuss the block diagram. Fig. 3.1 below shows the block diagram of 8085 µp. It mainly consists of three sections (a) Register unit (b) Arithmetic and logic unit (c) Timing and control unit These important sections will be discussed in detail in the subsequent sections. Register unit The micro processor 8085 consists of different types of registers (a) Accumulator (b) General purpose register (c) Flag register (d) Temporary register (e) Program counter (f) Stack pointer Accumulator: It is an 8-bit register which is used for performing all the arithmetic s and logical operations. Accumulator is also known as register A. During any mathematical or logical operation one of the operands should be present in the accumulator. The final result is also stored in the accumulator. 3

4 INTERRUPT SIGNALS SOD SID INTERRUPT CONTROL SERIAL I/O CONTROL ACCUM ULATOR TEMPOR ARY REGISTER STATUS FLAGS INSTRUC TION REGISTER B D C E H L ARITHMETIC and LOGIC UNIT (ALU) INSTRUCTION DECODER STACK POINTER PROGRAM COUNTER X1 X2 TIMING and CONTROL UNIT CLOCK CONTROL SIGNALS ADDRESS BUFFER DATA/ADDRESS BUFFER A 8 -A 15 ADo-AD 7 Fig. 3.1 Functional Block diagram 4

5 General purpose registers: There are six 8-bit general purpose registers B, C, D, E, H & L. General purpose registers are used for temporary storage of data and intermediate results while the processor is executing the program. Two eight bit registers can be combined for handling 16-bit data. Combination of two 8-bit registers is known as pair. Valid register pairs are B-C, D-E, H-L. The H-L pair is used to address memories. Flag register: It is an 8-bit register in which five flip flops are used for checking condition. These flip flops are called flags. Each of these flags can have the value either one or zero to indicate certain condition after arithmetic and and logical operation. The five flags present in 8085 µp are: (a) Sign flag (b) Zero flag (c) Auxiliary carry flag (d) Parity flag (e) Carry flag Sign flag (S): After the execution of an arithmetic or logical operation, if bit D 7 of the result is 1, the sign flag is set. This flag is used with signed numbers and has no relevance for unsigned numbers. In a given byte, if D 7 is 1, the number is said to be negative and if it is 0, the number is considered as positive. Zero Flag (Z): This flag is set if the result after arithmetic or logical operation turns out to be zero and is reset if the result is non-zero. Auxiliary carry (AC): This flag is set if in an arithmetic operation a carry is generated at bit D 3 and passed onto D 4. This flag is used internally for BCD operations and is not available to the user unlike other flags. Parity (P): After an arithmetic or logical operation, if the result has an even number of 1s, the flag is set and if there is odd number of 1s the flag is reset. Carry (CY): If an arithmetic operation results in a carry, the flag is set otherwise it is reset. It also serves as a borrow flag for subtraction. The bit position reserved for these flags in the flag register are as follows: S Z AC P CY D 0 D 1 D 2 D 3 D 4 D 5 D 6 D 7 Temporary register:- Register W and Z are known as temporary registers. They are used by the µp for storing the data temporarily during execution of a program. They are 8-bit registers and are not accessible to the user. 5

6 Program Counter: It is a 16-bit register which holds the address of the instructions. Initially it indicates towards the starting address of the program but after the first instruction is fetched the program counter automatically gets incremented by one and points towards the next instruction. This process continues till the end of the program. Stack pointer: The stack pointer is a 16-bit register which basically serves two purposes (a) (b) Points towards the stack memory. Initially it indicates the beginning of the stack memory. Whenever something is added to the stack, the stack pointer is decremented and whenever something is removed from the stack the stack pointer is incremented. Hence the stack pointer always points to the top of the stack. Stack pointer also points towards the memory location where the µp has to go after attending an interrupt or a subroutine; therefore it acts as a bookmark. Arithmetic and logic unit: This is the unit where all the arithmetic operations like addition, subtraction, multiplication and division and logical operations like AND, OR, Ex-OR, complement, compare etc. are performed. It includes the accumulator, the temporary register, the arithmetic and logic circuits and five flags. The temporary registers are used to store data temporarily during an arithmetic/logical operation. The result is stored in the accumulator and the flags are set or reset according to the result of the operation. Timing and control unit: The timing and control unit generates timing and control signals which are necessary for the execution of instruction. It controls data flow between CPU and peripherals. It provides status, control and timing signals which are required for the operation of memory and I/O devices System Bus System bus is basically a group of communication lines/wires that are responsible for transferring information between different units of the device or peripherals. A typical microprocessor communicates with memory and other devices using three buses: address bus, data bus and control bus. Address bus: Address bus is a unidirectional group of 16 lines i.e. bits flow in one direction from the µp to the peripheral devices. The 8085 µp with it s 16 address lines is capable of addressing 2 16 =65536 (64K) memory locations. Data bus: Data bus carries data in binary form, between microprocessor and peripheral devices as well as memory. It is a group of 8-bits and is bidirectional. Data bus also carries instructions from memory to the microprocessor. Size of the bus therefore limits the number of possible instructions. The 8085 µp has 246 bit patterns amounting to 74 different instructions. These 74 different instructions are therefore called its instruction set. 6

7 Control bus: The control bus is combination of various single lines that carry control signals. The control lines are not group of lines like address and data bus but are individual lines. Microprocessor generates specific control signals for every operation it performs. The bus system of 8085 is shown in figure 3.2. Figure Bus Structure Value addition: Evolution of microprocessor: With the advances in L.S.I. (Large Scale Integration) and V.L.S.I. (Very Large scale integration) technology it became possible to build the whole CPU of a digital computer on a single IC (Integrated circuit). A CPU built on a single chip is called microprocessor. The first microprocessor INTEL 4004 a 4-bit µp was introduced in 1971 by INTEL corporation. In 1972 INTEL introduced the first 8-bit microprocessor These microprocessors used PMOS technology. A more powerful and faster µp the INTEL 8080 using NMOS technology was introduced in A number of companies like Motorola, Zilog, National Semiconductor etc. INTEL introduced 8-bit µp 8085 in It is very popular and most widely used microprocessor. 8-bit microprocessors were soon followed by 16-bit microprocessors like INTEL 8086, 80186, Microprocessors like INTEL 80386, are 16-bit microprocessors that were introduced in The most powerful µp (PENTIUM) was introduced in It is a 64-bit microprocessor. Since then there has been a continuous development in this field of semiconductor technology Pin Description 7

8 The 8085 µp is an 8-bit microprocessor capable of addressing 64K of memory. It is a 40-pin IC which requires a power supply of +5V and it s operating frequency is 3MHz. Fig. 3.3 shows the pin configuration of 8085 µp.the pin configuration of 8085 µp can be classified into six groups namely: High order Address bus Multiplexed Address/Data bus Control and status signals Power supply and frequency signals Externally initiated signals and Serial I/O ports Figure 3.2 Pin Configuration of 8085 (1) High order address bus: The address bus is a group of 16-lines generally identified as A 0 -A 15. The address bus is split into two segments AD O -AD 7 and A 8 -A 15.The buses from AD O -AD 7 are low order address buses and A 8 -A 15 buses are high order address buses. (2) Multiplexed Address/Data bus: The data bus is a group of 8-lines used for transfer of data. These lines are bidirectional i.e. data flows in both the directions from µp to peripherals and vice-versa. They are represented as AD O -AD 7 because they serve dual purpose. They are used as low-order address bus as well as data bus and therefore they are known as multiplexed address/data bus. (3) Control and status signals: 8

9 This group of signals includes two control signals, three status signals and one special signal. These signals are as follows: Control signal: - and are the two control signals which indicate that the data is to be read from or written into a selected memory or I/O location. Both are active low signals. Status signal: -, S 1 and S O are the three status signals. is used to differentiate between I/O and memory operations. When it is high it indicates I/O operation and when it is low it indicates memory operation. S 1 and S O are also status signals but are rarely used in small systems. Special signal: - ALE (Address Latch Enable) is a special signal used for demultiplexing address and data bus (AD O -AD 7 ). It is a positive going pulse generated every time a machine cycle begins and so long as it remains positive it indicates that the bits on AD O - AD 7 are address bits. (4) Power supply and clock frequency: The power supply required for 8085 µp is +5V. As shown in pinout diagram V CC is connected to +5V and V SS is connected to the ground of the power supply. The µp operates on frequency of 3MHz, therefore an oscillator of frequency 6MHz is connected between pin no. 1 and 2 as the frequency is internally divided by two. CLK (OUT) i.e. Pin no. 37 is used as a system clock for other devices. (5) Externally initiated signals: There are certain operations which can be initiated by external devices (or signals). For these externally initiated operations there are individual pins assigned on the microprocessor chip. Interrupts are also considered as externally initiated signal. Here is a brief explanation of interrupts and other externally initiated signals. Interrupts: - The 8085 has five interrupt signals that can be used to interrupt a program execution. These interrupts are (a) TRAP: The interrupt with highest priority. It is non-maskable and vectored interrupt i.e. the µp has to attend this interrupt immediately. (b) RST 7.5, RST 6.5, RST 5.5: These are known as Restart interrupts and have lower priority than TRAP but have higher priority than the INTR interrupt. They are vectored and maskable interrupts. Among the three the priority order is RST7.5>RST6.5>RST5.5 (c) INTR: It is a general purpose interrupt. It is maskable and non-vectored interrupt and has the least priority. Reset: - When RESET pin is activated, the µp suspends all the internal operations and the program counter is cleared. Now the program execution can again begin at the zero memory address. 9

10 Ready: - The Ready signal is used to synchronize slower peripherals with the microprocessor. If the signal at READY pin is low the microprocessor enters into a wait state. Hold: - This signal is used by the external devices to request the microprocessor for using the buses. When this signal is activated the µp leaves is control over the buses and makes them free for the peripherals to use. : - Interrupt Acknowledge. This signal is used to acknowledge an interrupt. HLDA: - Hold Acknowledge. This signal is used to acknowledge the HOLD request. : - This is an active low signal. When it is activated the buses are tristated, the program counter is cleared and the microprocessor is reset. RESET OUT: This signal indicates that the microprocessor is being reset. It can be used to reset other devices also. (6) Serial I/O Ports: To send and receive data serially microprocessor has two pins SID and SOD by using these pins the µp can communicate with other µp and peripheral devices. In serial transmission, data bits are sent over a single line, one bit at a time. SID is Serial Input Data and SOD stands for Serial Output Data. SID (Input) On execution of the RIM instruction the data on this line is loaded into the seventh bit of the accumulator. SOD(output)- When SIM instruction is executed the 7 th bit of the accumulator is output on SOD line. 3.5 Microprocessor operations The process of data manipulation and communication with peripherals is determined by the logic design of the microprocessor. This logic design is called architecture. All the operations of microprocessor can be classified into following types: (a) Microprocessor initiated operations (b) Internal operations (c) Peripheral or externally initiated operations Microprocessor initiated operations operations : Microprocessor initiated operations are usually one of the following (a) Memory read: Reads data from memory. (b) Memory write: Writes data into the memory. (c) I/O Read: Accepts data from input devices. (d) I/O Write: Sends data to output devices All these operations are initiated by microprocessor and executed by peripheral devices as a part of communication process between microprocessor and memory or 10

11 peripheral devices. To perform these communication operations, the MPU needs to perform following steps (a) Identify the memory location or the peripheral devices. (b) Provide synchronization signal i.e. timing signals. (c) Transfer the binary information (data and instruction). All these functions are performed with the help of communication lines called system bus. The details of the three types of buses i.e. Address bus, data bus and control bus have been discussed in section Value addition: Data flow between memory and MPU: Figure below explains the flow of data between microprocessing unit and memory. First of all the 16-bit address is placed on the address bus from the program counter. In the figure it can be seen that the address is 2005H where the data is placed. The higher order address i.e. 20H is placed on the address bus A 8 -A 15 while the lower order address is placed on the multiplexed address and data bus AD O -AD 7. The lower order address continues to remain on this address bus so long as ALE (Address Latch Enable) remains positive. Once ALE goes low it carries data. The control unit sends the signal to indicate what type of operation is to be performed. Since the data is to be read from the memory therefore it sends to enable the memory chip. The byte from the memory location is then placed on the data bus i.e. 4F saved in location 2005H is placed on the data bus and sent to the instruction decoder. The instruction is decoded and accordingly the task is performed by the ALU i.e. Arithmetic and logic unit. Figure: Data flow between memory and MPU 11

12 Internal data operations These are the operations which are internally performed by the microprocessor. They are classified into five groups: 1) Store 8-bit data 2) Perform arithmetic and logical operation 3) Test for the conditions 4) Sequence the execution of instructions 5) Store data temporarily during execution in the R/W memory locations called the stack. To perform these operations microprocessor requires registers, Arithmetic and logic unit, buses and control unit. To store 8-bit data microprocessor has six general purpose registers. During program execution, these registers are used to store 8- or 16-bit data. The details of these registers i.e. B, C, D, E, H and L have already been discussed in register unit. Arithmetic and logical operations are performed in accumulator and the conditions are tested by the flag register. Program counter is used to sequence the execution of instructions and stack memory is used for temporary storage of data during execution and stack pointer is used to address the stack memory. Peripheral or Externally initiated operations There are four externally initiated operations, for which individual pins on the microprocessor chip are assigned. These operations are RESET, READY, HOLD and INTERRUPT. The details of all these operations have been discussed in section

13 Value addition: 8085 Programming Model: A programming model is a conceptual representation of a real object and can be in the form of a text, a drawing or a built structure. Here is a programming model of 8085 which gives information required to write a program. It consists of some segments of ALU and the registers. This model includes six general purpose registers, accumulator, flag register, program counter and stack pointer. This model is not a reflection of physical structure of the microprocessor but gives all the information that is relevant for writing an assembly language program. Figure below shows the programming model specific to the 8085 microprocessor. Figure: 8085 Programming model 13

14 SUMMARY This chapter describes the architecture of the 8085 microprocessor. The important concepts which have been explained in this chapter are summarized below: Various functional units present in 8085 like register unit, arithmetic and logic unit and timing and control unit have been explained with the help of block diagram. Register unit consists of six general purpose registers namely B, C, D, E, H and L, two temporary registers W and Z, two 16-bit registers SP(stack pointer) and PC(program counter). Accumulator is register A where all the mathematical and logical operations are performed and final result is stored Besides pinout diagram the signals of microprocessor 8085 have been discussed. The signals of 8085 µp can be classified into six groups: High order Address bus, Multiplexed Address/Data bus, Control and status signals, Power supply and frequency signals, Externally initiated signals and Serial I/O ports. The operations performed by microprocessor can be classified into three types (a) Microprocessor initiated operations (b) Internal data operations (c) Externally initiated operations. This chapter also explains the process of data flow between microprocessor and memory. The programming model which is very important to understand programming of microprocessor has also been discussed. 14

15 EXERCISES Question Number Type of question 1 Multiple choice questions (1) What is the clock frequency for 8085 μp? (a). 2 MHz (c) 4 MHz (b) 3 MHz (d) 6 MHz. (2) Signal used for demultiplexing of address and data bus? (a) HOLD (c) READY (b) ALE (d) INTR (3) Which of the following is 16 bit register? (a) PC (c) None of these (b) SP (d). Both (a) and (b). (4) How many flags are available in 8085? (a) 4 (b) 6 (c) 5 (d) 7 (5) Stack pointer register is used for accessing (a) Stack (c) Strings (b) Memory (d) None of above (6) Which of the following interrupt has the highest priority? (a) RST 7.5 (b) RST 6.5 (c) RST 5.5 (d) TRAP (7) Which register is not available for user in microprocessor? (a) A (c) D (b) B (d) W 15

16 (8) Which signal is used for synchronizing the speed of 8085 with slower peripheral devices? (a) RESET (c) HOLD (b) READY (d) INTR (9) The register used for sequencing the execution of instructions is (a) Stack Pointer (c) Program counter (b) Accumulator (d) W and Z (10) Maximum memory which 8085 microprocessor can address (a) 64KB (c) 4KB (b) 32KB (d) 16KB Correct answers (1) b (6) d (2) b (7) d (3) d (8) c (4) c (9) c (5) a (10) a Question Number Type of question 2 Fill in the blanks 1) 8085 μp in an bit microprocessor.. 2) 8085 μp is pin microprocessor. 3) There are address lines in 8085 μp. 4) All the arithmetic and logical operations are performed in. 5) flag is used for BCD operation. 16

17 Correct answers (1) 8-bit (2) 40 (3) 16 (4) Accumulator (5) Auxiliary carry Question Number Type of question 3 Subjective questions 1) Explain the function of ALE signal. 2) Explain the BUS structure in 8085 microprocessor. 3) Why are PC and SP 16-bit registers? 4) Name the various programmable registers used in microprocessor ) Give the labeled pin-out diagram of microprocessor ) Name the various programmable registers used in ) What is stack pointer? State its function. 8) Explain the purpose of the flag register in microprocessor 8085 and show its bit pattern. 9) What is the function of the following in microprocessor (a) RESET (b) READY (c) HOLD (d) HALT 10) Give the programming model of 8085 microprocessor. 17

18 GLOSSARY VLSI: Very Large Scale Integration Technology: In this technology a million transistors are created on a single silicon chip. It was developed in 1980s. Accumulator: It is an eight bit register in which all the arithmetic and logical operations are performed. It is also known as register A. The final result is also stored in this register. Program counter: It is a 16- bit register which stores the address of the next instruction to be executed. It helps in sequencing the execution of instruction. Stack Pointer: It is a 16-bit register which stores the address of the stack memory. It also acts as a bookmark incase of in case of interrupt and call of a subroutine. Flag register: It is an 8- bit register in which only five bits are used for testing the various conditions like sign, carry, parity etc. Interrupt: It is an externally initiated signal which alters the sequence of execution by requesting the microprocessor to perform some task other than the program it is executing. Address bus: It is a group of lines used for sending address from the microprocessor to the memory or the peripherals. There are 16 address lines in 8085 microprocessor. This is a unidirectional bus Data bus: It ia group of lines used for transferring data between MPU and memory or peripherals. It is bidirectional. There are 8 address lines in 8085µP Control bus: The control bus is a combination of various single lines that carry control signals or synchronization signals. These control lines are not group of lines like address or data buses but individual lines that provide a signal to indicate µp operation. Assembly language: It is a language in which programs are written using mnemonics (instructions in simple English language). 18

19 References 1. Suggested readings Microprocessor architecture, Programming and applications with the 8085 Ramesh S. Gaonkar Fundamentals of Microprocessor and Microcomputers B.Ram Microprocessor 8085 and its interfacing Sunil Mathur 8085 microprocessor: Programing and interfacing Srinath N. K. 19