Introduction to Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit (CPU) on a single integrated circuit, (IC) or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output. It is an example of sequential digital logic, as it has internal memory. Microprocessors operate on numbers and symbols represented in the binary numeral system.

The microprocessors can be classified based on the following features.

Instruction Set

It is the set of the instructions that the Microprocessor can execute.

Bandwidth

The number of bits processed by the processor in a single instruction.

Clock Speed

Clock speed is measured in the MHz and it determines that how many instructions a processor can processed.

The speed of the microprocessor is measured in the MHz or GHz. The processor is also known as the CPU (Central Processing Unit). It contains the control unit and the arithmetic unit and both works together to process the commands. CPU is used in every computer whether it is a workstation, server or a laptop. CPU is a complete computational engine that is designed as a chip. It starts the work when you turn on your computer.

CPU is designed to perform the arithmetic and logical operations inside the computer. Common operations inside the computer include adding, subtracting, multiplying, comparing the values and fetching the different numbers to process them. The higher the CPU clocks’ speed the more efficient will be the performance of the computer.

Computer’s performance is also influenced by the system bus architecture, memory used, type of the processor and software program being running. Pentium 4 is the fastest type of the Intel’s processor that contains 125,000,000 transistors and operates at the speed of 3.6 GHz.

4 components of microprocessor

- The physical components of this system are called hardware.

- A set of instructions written for the microprocessor to perform a task is called a program.

Input

Microprocessor

Memory

Output

- A group of programs is called software.

The microprocessors applications are classified primarily in two categories: reprogrammable systems and embedded systems. In reprogrammable systems, such as microcomputers, the microprocessor is used for computing and data processing, a Personal Computer (PC) is a typical illustration. In an embedded system, the microprocessor is a part of a final product and is not available for reprogramming to the end user.

The microprocessor operates in binary digits, 0 and 1, also known as bits. Bit is an abbreviation for the term binary digit. These digits are represented in terms of electrical voltages.

Each microprocessor recognizes and processes a group of bits called the word, and microprocessor are classified according to their word length. The fact that the microprocessor is programmable means it can be instructed to perform given tasks within its capability. The instructions are entered or stored in a storage device called memory, which can be read by the microprocessor.

Memory is like the pages of a notebook with space for a fixed number of binary numbers on each line.

Microprocessor Microcomputer

The microprocessor is the heart of many products, but the microprocessor is never a complete, working by itself. It still needs I/O, memory, data storage or program storage and power.

What is a microcomputer?

The words “Microprocessor” and “Microcomputer” are used to mean the same thing, but in fact these words have different meanings. The microprocessor is an IC (data processing and control). The microcomputer is a complete computing system built around a microprocessor.

What is the power of a microprocessor?

Almost all microprocessors are made on silicon die (IC). These ICs are about ¼ inch (in), or 0.64 centimeters (cm), on a side. The power of a microprocessor is its capacity to process data. These are three main measures of the power of a microprocessor: the length of the microprocessor’s data word; the number of memory words that the microprocessor can address; and the speed with which the microprocessor can execute an instruction.

The lengths of microprocessor’s data words are including 4 bits, 8 bits, 16 bits, 32 bits and 64 bits. The 8 bits data word is so common that it has been given the special name byte. Because the byte is so commonly used, 16 bit microprocessors often have instructions that let them process their 16 bits data word in two 8 bit bytes.

Figure: A 16bit digital word showing the high and low byte breakdown

The 4 bit microprocessor was the first one developed. Microprocessors of this word length are still popular in some types of work. Four bits is the length needed for a binary-coded decimal (BCD) numbers. In some applications, including; calculators, simple consumer products and toys, the microprocessor deals only with BCD numbers. The 8 bits word length was the next developed after the 4 bits.

The advantages of 8 bit microprocessor

1. The 8 bit word length is twice 4 bits.

2. The 8 bit word length allows two BCD numbers for each CPU data word

3. The 8 bit word length can hold all the data needed for one character in

American Standard Code for Information Interchange (ASC II), ASCII characters are used widely in data processing to represent numbers, letters, and many special symbols.

Each time the microprocessor’s word length doubles, the processor becomes more powerful. Greater word lengths have required improved LSI technology. For example, the LSI used to develop some of the new 64 bit microprocessors uses a similar sized chip, but it comes over 23 million transistors.

Another common measure of microprocessor power is the number of memory bytes that the microprocessor can address. For example, a 4 bit microprocessor stores 4 bit word in memory. The length of the data word is the same as the length of the data word used by the microprocessor. Each word in memory is assigned a location number or address.

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Low byteHigh byte

Binary Address Memory contents

(4 bits long)

1111 Data word 15

1110 Data word 14

1101 Data word 13

1100 Data word 12

1011 Data word 11

1010 Data word 10

1001 Data word 9

1000 Data word 8

0111 Data word 7

0110 Data word 6

0101 Data word 5

0100 Data word 4

0011 Data word 3

0010 Data word 2

0001 Data word 1

0000 Data word 0

Figure: A 16bit word memory addressed by a 4bit

Figure shows the memory-addressing power of single 4 bit word. 4 bits can address 16 words in memory. We number these 16 words from 0 to 15. A single 8 bit word has an address range of 256 memory words. A 16 bit word has an address range of 65,536 memory words. Most microprocessors can use more than a single word to address memory. Therefore the memory address range is not limited by the length of the microprocessor’s data word.

A shorthand notation is used in specifying the number of bytes. The symbol “K” is used to say times 1000”. The symbol “M” means “times 1 million”. The symbol “G” means “times 1 billion”.

Data word length

4bit 8bit 16bit 32bit

4096k

Memory address range

8192k

65,536(64k)

32,768(32k)

65,536(64k)

1,048,576(1M)

2,097,152(2M)

4,194,304(4M)

4,294,967,296(4G)

34,359,738,367(32G)

A third common measure of microprocessor power is the speed with which microprocessor executes an instruction. Speed is determined by the time it takes the microprocessor to complete the fetch / execute cycle for one program step. Some microprocessors are 20 to 100 times faster than others. Each one has oscillator circuit is called the microprocessor’s clock. Slow microprocessors may use a clock that at a few hundred kilohertz (KHz). It takes such a microprocessor 10 to 20 microseconds (s) to execute one instruction.

Microprocessor Instruction Set and Computer Languages

The word (or word length), is the number of bits the microprocessor recognizes and processes at a time. The word length ranges from 4 bits for small microprocessor, to 64 bits for high-end microcomputers. The byte is defined as a group of eight bits. The term “nibble”, which stands for a group of four bits, is also found in popular computer magazines and books.The instruction is defined as a complete task that microprocessor can perform. Instructions must be written in binary language, also know as machine language.

Assemble language is the program language that programmer can easily programs like as English words.

High-level languages (HLL) are the general-purpose languages as Visual BASIC, PASCAL, JAVA, C and C++. A program written in these languages can be machine independent.

Microprocessor-Based System with Bus Architecture

ALU (Arithmetic/Logic Unit) – It performs such arithmetic operations as addition and subtraction, and such logic operations as AND, OR, and XOR. Results are stored either in registers or in memory.

Register Array – It consists of various registers identified by letter such as B, C, D, E, H, L, IX, and IY. These registers are used to store data and addresses temporarily during the execution of a program.

Control Unit – The control unit provides the necessary timing and control signals to all the operations in the microcomputer. It controls the flow of data between the microprocessor and memory and peripherals.

Input – The input section transfers data and instructions in binary from the outside world to the microprocessor. It includes such devices as a keyboard, switches, a scanner, and an analog-to-digital converter.

Output – The output section transfers data from the microprocessor to such output devices as LED, CRT, printer, magnetic tape, or another computer.

Memory – It stores such binary information as instructions and data, and provides that information to the microprocessor. To execute programs, the microprocessor reads instructions and data from memory and performs the computing operations in its ALU section. Results are either transferred to the output section for display or stored in memory for later use.

System bus – It is a communication path between the microprocessor and peripherals. The microprocessor communicates with only one peripheral at a time. The timing is provided by the control unit of the microprocessor

Programming With 8085

Internal Architecture of 8085 Microprocessor

Control Unit

Generates signals within uP to carry out the instruction, which has been decoded. In reality causes certain connections between blocks of the uP to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

Arithmetic Logic Unit

The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’, ‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform arithmetic. Always stores result of operation in Accumulator.

Registers

Control

ALURegister

Array

Input Output

ROM R/WM

System Bus

Microprocessor

Memory

The 8085/8080A-programming model includes six registers, one accumulator, and one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer and the program counter. They are described briefly as follows. The 8085/8080A has six general-purpose registers to store 8-bit data; these are identified as B,C,D,E,H, and L as shown in the figure.

They can be combined as register pairs - BC, DE, and HL - to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instructions.

Accumulator

The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. The accumulator is also identified as register A.

Flags

The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are listed in the Table and their bit positions in the flag register are shown in the Figure below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator id larger than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag register, adjacent to the accumulator. However, it is not used as a register; five bit positions out of eight are used to store the outputs of the five flip-flops. The flags are stored in the 8-bit register so that the programmer can examine these flags (data conditions) by accessing the register through an instruction.

These flags have critical importance in the decision-making process of the micro- processor. The conditions (set or reset) of the flags are tested through the software instructions. For example, the instruction JC (Jump on Carry) is implemented to change the sequence of a program when CY flag is set. The thorough understanding of flag is essential in writing assembly language programs.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location.

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading 16-bit address in the stack pointer. The stack concept is explained in the chapter "Stack and Subroutines."

Instruction Register/Decoder

Temporary store for the current instruction of a program. Latest instruction sent here from memory prior to execution. Decoder then takes instruction and ‘decodes’ or interprets the instruction. Decoded instruction then passed to next stage.

Memory Address Register

Holds address, received from PC, of next program instruction. Feeds the address bus with addresses of location of the program under execution.

Control Generator

Generates signals within uP to carry out the instruction which has been decoded. In reality causes certain connections between blocks of the uP to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

Register Selector

This block controls the use of the register stack in the example. Just a logic circuit which switches between different registers in the set will receive instructions from Control Unit.

General Purpose Registers

uP requires extra registers for versatility. Can be used to store additional data during a program. More complex processors may have a variety of differently named registers.

Microprogramming

How does the µP knows what an instruction means, especially when it is only a binary number? The microprogram in a uP/uC is written by the chip designer and tells the uP/uC the meaning of each instruction uP/uC can then carry out operation.

8085 System Bus

Typical system uses a number of busses, collection of wires, which transmit binary numbers, one bit per wire. A typical microprocessor communicates with memory and other devices (input and output) using three busses: Address Bus, Data Bus and Control Bus.

Address Bus

One wire for each bit, therefore 16 bits = 16 wires. Binary number carried alerts memory to ‘open’ the designated box. Data (binary) can then be put in or taken out.The Address Bus consists of 16 wires, therefore 16 bits. Its "width" is 16 bits. A 16 bit binary number allows 216 different numbers, or 32000 different numbers, ie 0000000000000000 up to 1111111111111111. Because memory consists of boxes, each with a unique address, the size of the address bus determines the size of memory, which can be used. To communicate with memory the microprocessor sends an address on the address bus, eg 0000000000000011 (3 in decimal), to the memory. The memory the selects box number 3 for reading or writing data. Address bus is unidirectional, ie numbers only sent from microprocessor to memory, not other way.

Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits), how many wires does the address bus need, in order to be able to specify an address in this memory? Note: the memory is organized in groups of 8 bits per location, therefore, how many locations must you be able to specify?

Data Bus

Data Bus: carries ‘data’, in binary form, between µP and other external units, such as memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and µP size helps determine performance of µP. The Data Bus typically consists of 8 wires. Therefore, 28 combinations of binary digits. Data bus used to transmit "data", ie information, results of arithmetic, etc, between memory and the microprocessor. Bus is bi-directional. Size of the data bus determines what arithmetic can be done. If only 8 bits wide then largest number is 11111111 (255 in decimal). Therefore, larger number have to be broken down into chunks of 255. This slows microprocessor. Data Bus also carries instructions from memory to the microprocessor. Size of the bus therefore limits the number of possible instructions to 256, each specified by a separate number.

Control Bus

Control Bus are various lines which have specific functions for coordinating and controlling uP operations. Eg: Read/NotWrite line, single binary digit. Control whether memory is being ‘written to’ (data stored in mem) or ‘read from’ (data taken out of mem) 1 = Read, 0 = Write. May also include clock line(s) for timing/synchronising, ‘interrupts’, ‘reset’ etc. Typically µP has

10 control lines. Cannot function correctly without these vital control signals. The Control Bus carries control signals partly unidirectional, partly bi-directional. Control signals are things like "read or write". This tells memory that we are either reading from a location, specified on the address bus, or writing to a location specified. Various other signals to control and coordinate the operation of the system. Modern day microprocessors, like 80386, 80486 have much larger busses. Typically 16 or 32 bit busses, which allow larger number of instructions, more memory location, and faster arithmetic. Microcontrollers organized along same lines, except: because microcontrollers have memory etc inside the chip, the busses may all be internal. In the microprocessor the three busses are external to the chip (except for the internal data bus). In case of external busses, the chip connects to the busses via buffers, which are simply an electronic connection between external bus and the internal data bus.

The 8085 Programming Model

we described the 8085 microprocessor registers in reference to the internal data operations. The same information is repeated here briefly to provide the continuity and the context to the instruction set and to enable the readers who prefer to focus initially on the programming aspect of the microprocessor.

The 8085 programming model includes six registers, one accumulator, and one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer and the program counter. They are described briefly as follows.

Registers

The 8085 has six general-purpose registers to store 8-bit data; these are identified as B,C,D,E,H, and L as shown in the figure. They can be combined as register pairs - BC, DE, and HL - to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instructions

Accumulator

The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. The accumulator is also identified as register A.

Flags

The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; their bit positions in the flag register are shown in the Figure below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data

conditions. For example, after an addition of two numbers, if the sum in the accumulator id larger than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag register, adjacent to the accumulator. However, it is not used as a register; five bit positions out of eight are used to store the outputs of the five flip-flops. The flags are stored in the 8-bit register so that the programmer can examine these flags (data conditions) by accessing the register through an instruction.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading 16-bit address in the stack pointer.

The 8085 Addressing Modes

The instructions MOV B, A or MVI A, 82H are to copy data from a source into a destination. In these instructions the source can be a register, an input port, or an 8-bit number (00H to FFH). Similarly, a destination can be a register or an output port. The sources and destination are operands. The various formats for specifying operands are called the ADDRESSING MODES. For 8085, they are:

1. Immediate addressing.

2. Register addressing.

3. Direct addressing.

4. Indirect addressing.

Immediate addressing

Data is present in the instruction. Load the immediate data to the destination provided. Example: MVI R,data

Register addressing

Data is provided through the registers.

Example: MOV Rd, Rs

Direct addressing

Used to accept data from outside devices to store in the accumulator or send the data stored in the accumulator to the outside device. Accept the data from the port 00H and store them into the accumulator or Send the data from the accumulator to the port

01H.

Example: IN 00H or OUT 01H

Indirect Addressing

This means that the Effective Address is calculated by the processor. And the contents of the address (and the one following) is used to form a second address. The second address is where the data is stored. Note that this requires several memory accesses; two accesses to retrieve the 16-bit address and a further access (or accesses) to retrieve the data which is to be loaded into the register.

Instruction Set Classification

An instruction is a binary pattern designed inside a microprocessor to perform a specific function. The entire group of instructions, called the instruction set, determines what functions the microprocessor can perform. These instructions can be classified into the following five functional categories: data transfer (copy) operations, arithmetic operations, logical operations, branching operations, and machine-control operations. Data Transfer (Copy) Operations This group of instructions copy data from a location called a source to another location called a destination, without modifying the contents of the source. In technical manuals, the term data transfer is used for this copying function. However, the term transfer is misleading; it creates the impression that the contents of the source are destroyed when, in fact, the contents are retained without any modification.

Arithmetic Operations

These instructions perform arithmetic operations such as addition, subtraction, increment, and decrement.

Addition - Any 8-bit number, or the contents of a register or the contents of a memory location can be added to the contents of the accumulator and the sum is stored in the accumulator. No two other 8-bit registers can be added directly (e.g., the contents of register B cannot be added directly to the contents of the register C). The instruction DAD is an exception; it adds 16-bit data directly in register pairs.

Subtraction - Any 8-bit number, or the contents of a register, or the contents of a memory location can be subtracted from the contents of the accumulator and the results stored in the accumulator. The subtraction is performed in 2's compliment, and the results if negative, are expressed in 2's complement. No two other registers can be subtracted directly.

Increment/Decrement - The 8-bit contents of a register or a memory location can be incremented or decrement by 1. Similarly, the 16-bit contents of a register pair (such as BC) can be incremented or decrement by 1. These increment and decrement operations differ from addition and subtraction in an important way; i.e., they can be performed in any one of the registers or in a memory location.

Logical Operations

These instructions perform various logical operations with the contents of the accumulator.

AND, OR Exclusive-OR - Any 8-bit number, or the contents of a register, or of a memory location can be logically ANDed, Ored, or Exclusive-ORed with the contents of the accumulator. The results are stored in the accumulator.

Rotate- Each bit in the accumulator can be shifted either left or right to the next position.

Compare- Any 8-bit number, or the contents of a register, or a memory location can be compared for equality, greater than, or less than, with the contents of the accumulator.

Complement - The contents of the accumulator can be complemented. All 0s are replaced by 1s and all 1s are replaced by 0s.

Branching Operations

This group of instructions alters the sequence of program execution either conditionally or unconditionally. Jump - Conditional jumps are an important aspect of the decision-making process in the programming. These instructions test for a certain conditions (e.g., Zero or Carry flag) and alter the program sequence when the condition is met. In addition, the instruction set includes an instruction called unconditional jump.

Call, Return, and Restart - These instructions change the sequence of a program either by calling a subroutine or returning from a subroutine. The conditional Call and

Return instructions also can test condition flags.

Machine Control Operations

These instructions control machine functions such as Halt, Interrupt, or do nothing. The microprocessor operations related to data manipulation can be summarized in four functions:

1. copying data

2. performing arithmetic operations

3. performing logical operations

4. testing for a given condition and alerting the program sequence

Instruction Format

An instruction is a command to the microprocessor to perform a given task on a specified data. Each instruction has two parts: one is task to be performed, called the operation code (opcode), and the second is the data to be operated on, called the operand. The operand (or data) can be specified in various ways. It may include 8-bit (or 16-bit ) data, an internal register, a memory location, or 8-bit (or 16-bit) address.

In some instructions, the operand is implicit. Instruction word size 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

In the 8085, "byte" and "word" are synonymous because it is an 8-bit microprocessor. However, instructions are commonly referred to in terms of bytes rather than words.

One-Byte Instructions

A 1-byte instruction includes the opcode and operand in the same byte. Operand(s) are internal register and are coded into the instruction.

MOV rd, rs

rd <-- rs copies contents of rs into rd.

Coded as 01 ddd sss where ddd is a code for one of the 7 general registers which is the destination of the data, sss is the code of the source register.

Example: MOV A,B

Coded as 01111000 = 78H = 170 octal (octal was used extensively in instruction design of such processors).

ADD r

A <-- A + r

Two-Byte Instructions

In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. Source operand is a data byte immediately following the opcode

MVI r,data

r <-- data

Example: MVI A,30H coded as 3EH 30H as two contiguous bytes. This is an example of immediate addressing.

ADI data

A <-- A + data

OUT port

Three-Byte Instructions

In a three-byte instruction, the first byte specifies the opcode, and the following two bytes specify the 16-bit address. Note that the second byte is the low-order address and the third byte is the high-order address. opcode + data byte + data byte

Three byte instructions - opcode + data byte + data byte

LXI rp, data16

rp is one of the pairs of registers BC, DE, HL used as 16-bit registers. The two data bytes are 16-bit data in L H order of significance.

rp <-- data16

Example:

LXI H,0520H coded as 21H 20H 50H in three bytes. This is also immediate addressing.

LDA addr

A <-- (addr) Addr is a 16-bit address in L H order. Example: LDA 2134H coded as 3AH 34H 21H. This is also an example of direct addressing

Sample Programs

Write an assembly program to add two numbers

Program

MVI D, 8BH

MVI C, 6FH

MOV A, C

1100 0011

1000 0101

0010 0000ADD D

OUT PORT1

HLT

Write an assembly program to multiply a number by 8

Program

MVI A, 30H

RRC

RRC

RRC

OUT PORT1

HLT

Classification of Interrupts

Interrupt is a process where an external device can get the attention of the microprocessor. The process starts from the I/O device and is asynchronous.

Interrupts can be classified into two types:

-Maskable Interrupts (Can be delayed or Rejected)

-Non-Maskable Interrupts (Can not be delayed or Rejected)

Interrupts can also be classified into:

-Vectored (the address of the service routine is hard-wired)

-Non-vectored (the address of the service routine needs to be supplied externally by the device)

The 8085 has 5 interrupt inputs:

1- The INTR inputThe INTR input is the only non-vectored interrupt.INTR is maskable using the EI/DI instruction pair.2,3,4- RST 5.5, RST 6.5, RST 7.5 are all automatically vectored.RST 5.5, RST 6.5, and RST 7.5 are all maskable.5- TRAP is the only non-maskable interrupt in the 8085 TRAP is also automatically vectored.

System Progamming

System programming (or systems programming) is the activity of programming system software. The primary distinguishing characteristic of systems programming when compared to application programming is that application programming aims to produce software which provides services to the user (e.g. word processor), whereas systems programming aims to produce software which provides services to the computer hardware (e.g. disk defragmenter). It requires a greater degree of hardware awareness.

In system programming more specifically the programmer will make assumptions about the hardware and other properties of the system that the program runs on, and will often exploit those properties (for example by using an algorithm that is known to be efficient when used with specific hardware).

Features:

usually a low-level programming language or programming language dialect is used that:

an operate in resource-constrained environments

is very efficient and has little runtime overhead

has a small runtime library, or none at all

allows for direct and "raw" control over memory access and control flow

lets the programmer write parts of the program directly in assembly language

debugging can be difficult if it is not possible to run the program in a debugger due to resource constraints. Running the program in a simulated environment can be used to reduce this problem.

Systems programming is sufficiently different from application programming that programmers tend to specialize in one or the other. In system programming, often limited programming facilities are available. The use of automatic garbage collection is not common and debugging is sometimes hard to do. The runtime library, if available at all, is usually far less powerful, and does less error checking. Because of those limitations, monitoring and logging are often used; operating systems may have extremely elaborate logging subsystems. Implementing certain parts in operating system and networking requires systems programming (for example implementing Paging (Virtual Memory) or a device driver for an operating system).

Macro Processor

A macro in computer science is a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to an output sequence (also often a sequence of characters) according to a defined procedure. The mapping process that instantiates (transforms) a macro into a specific output sequence is known as macro expansion.

The term is used to make available to the programmer, a sequence of computing instructions as a single program statement, making the programming task less tedious and less error-prone. Thus, they are called "macros" because a big block of code can be expanded from a small sequence of characters. Macros often allow positional or keyword parameters that dictate what the conditional assembler program generates and have been used to create entire programs or program suites according to such variables as operating system, platform or other factors.

Macro Instruction Set

A macro instruction is a line of computer program coding that results in one or more lines of program coding in the target programming language, sets variables for use by other statements, etc.. In the mid 1950s, when assembly language programming was commonly used to write programs for digital computers, the use of macro instructions was initiated for two main purposes: to reduce the amount of program coding that had to be written by generating several assembly language statements from one macro instruction and to enforce program writing standards, e.g. specifying input/output commands in standard ways. Macro instructions were effectively a middle step between assembly language programming and the high-level programming languages that followed, such as FORTRAN and COBOL.

A macro instruction written in the format of the target assembly language would be processed by a macro compiler, which was a pre-processor to the assembler, to generate one or more assembly language instructions to be processed next by the assembler program that would translate the assembly language instructions into machine language instructions.

While macro instructions can be defined by a programmer as a shorthand for any set of native assembler program instructions, typically macros are associated with macro libraries delivered with the operating system allowing access to operating system functions such as peripheral access by access methods (including macros such as OPEN, CLOSE, READ and WRITE) and other operating system functions such as ATTACH, WAIT and POST for subtask creation and synchronization. Typically such macros expand into a list of define constant instructions representing the parameters of the macro instruction (such as a reference to a file and a data area for a READ instruction) followed by either a branch and link instruction to call a routine, or a supervisor call instruction to call an operating system function directly.

In older operating systems such as those used on IBM mainframes, full operating system functionality was only available to assembler language programs, not to high level language programs (unless assembly language subroutines were used, of course), as the standard macro instructions did not always have counterparts in routines available to high-level languages.

In modern operating systems such as Unix and its derivatives, operating system access is provided through subroutines, usually invoking DLL routines. High-level languages such as C offer comprehensive access to operating system functions, obviating the need for assembler language programs for such functionality.

Assembler and Its Passes

Typically a modern assembler creates object code by translating assembly instruction mnemonics into opcodes, and by resolving symbolic names for memory locations and other entities. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include macro facilities for performing textual substitution—e.g., to generate common short sequences of instructions as inline, instead of called subroutines.

Assemblers are generally simpler to write than compilers for high-level languages,and have been available since the 1950s. Modern assemblers, especially for RISC architectures, such as SPARC or POWER, as well as x86 and x86-64, optimize Instruction scheduling to exploit the CPU pipeline efficiently.

There are two types of assemblers based on passes

One-pass assemblers go through the source code once. Any symbol used before it is defined will require "errata" at the end of the object code (or, at least, no earlier than the point where the symbol is defined) telling the linker or the loader to "go back" and overwrite a placeholder which had been left where the as yet undefined symbol was used.

Multi-pass assemblers create a table with all symbols and their values in the first passes, then use the table in later passes to generate code.

In both cases, the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of symbols. This means that if the size of an operation referring to an operand defined later depends on the type or distance of the operand, the assembler will make a pessimistic estimate when first encountering the operation, and if necessary pad it with one or more "no-operation" instructions in a later pass or the errata. In an assembler with peephole optimization addresses

may be recalculated between passes to allow replacing pessimistic code with code tailored to the exact distance from the target.

The original reason for the use of one-pass assemblers was speed of assembly; however, modern computers perform multi-pass assembly without unacceptable delay. The advantage of the multi-pass assembler is that the absence of a need for errata makes the linker (or the loader if the assembler directly produces executable code) simpler and faster.

Loaders and Linkers

In computing, a loader is the part of an operating system that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution. Loading a program involves reading the contents of executable file, the file containing the program text, into memory, and then carrying out other required preparatory tasks to prepare the executable for running. Once loading is complete, the operating system starts the program by passing control to the loaded program code.

All operating systems that support program loading have loaders, apart from systems where code executes directly from ROM or in the case of highly specialized computer systems that only have a fixed set of specialised programs.

In many operating systems the loader is permanently resident in memory, although some operating systems that support virtual memory may allow the loader to be located in a region of memory that is pageable.

In the case of operating systems that support virtual memory, the loader may not actually copy the contents of executable files into memory, but rather may simply declare to the virtual memory subsystem that there is a mapping between a region of memory allocated to contain the running program's code and the contents of the associated executable file. (See memory-mapped file.) The virtual memory subsystem is then made aware that pages with that region of memory need to be filled on demand if and when program execution actually hits those areas of unfilled memory. This may mean parts of a program's code are not actually copied into memory until they are actually used, and unused code may never be loaded into memory at all.

Linkers

Link editors are commonly known as linkers. The compiler automatically invokes the linker as the last step in compiling a program. The linker inserts code (or maps in shared libraries) to resolve program library references, and/or combines object modules into an executable image suitable for loading into memory. On Unix-like systems, the linker is typically invoked with the ld command.

Static linking is the result of the linker copying all library routines used in the program into the executable image. This may require more disk space and memory than dynamic linking, but is both faster and more portable, since it does not require the presence of the library on the system where it is run.

Dynamic linking is accomplished by placing the name of a sharable library in the executable image. Actual linking with the library routines does not occur until the image is run, when both the executable and the library are placed in memory. An advantage of dynamic linking is that multiple programs can share a single copy of the library.

Linkers vs. Loaders

Linkers and loaders perform various related but conceptually different tasks:

Program Loading:

This refers to copying a program image from hard disk to the main memory in order to put the program in a ready-to-run state. In some cases, program loading also might involve allocating storage space or mapping virtual addresses to disk pages.

Relocation:

Compilers and assemblers generate the object code for each input module with a starting address of zero. Relocation is the process of assigning load addresses to different parts of the program by merging all sections of the same type into one section. The code and data section also are adjusted so they point to the correct runtime addresses.

Symbol Resolution:

A program is made up of multiple subprograms; reference of one subprogram to another is made through symbols. A linker's job is to resolve the reference by noting the symbol's location and patching the caller's object code.

Compilers

A compiler is a computer program (or set of programs) that transforms source code written in a programming language (the source language) into another computer language (the target language, often having a binary form known as object code). The most common reason for wanting to transform source code is to create an executable program.

The name "compiler" is primarily used for programs that translate source code from a high-level programming language to a lower level language (e.g., assembly language or machine code). If the compiled program can run on a computer whose CPU or operating system is different from the one on which the compiler runs, the compiler is known as a cross-compiler. A program that translates from a low level language to a higher level one is a decompiler. A program that translates between high-level languages is usually called a language translator, source to source translator, or language converter. A language rewriter is usually a program that translates the form of expressions without a change of language.

A compiler is likely to perform many or all of the following operations: lexical analysis, preprocessing, parsing, semantic analysis (Syntax-directed translation), code generation, and code optimization.

Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementors invest a lot of time ensuring the correctness of their software.

The term compiler-compiler is sometimes used to refer to a parser generator, a tool often used to help create the lexer and parser.

Structure of a compiler

Compilers bridge source programs in high-level languages with the underlying hardware. A compiler requires 1) determining the correctness of the syntax of programs, 2) generating correct and efficient object code, 3) run-time organization, and 4) formatting

output according to assembler and/or linker conventions. A compiler consists of three main parts: the frontend, the middle-end, and the backend.

The front end checks whether the program is correctly written in terms of the programming language syntax and semantics. Here legal and illegal programs are recognized. Errors are reported, if any, in a useful way. Type checking is also performed by collecting type information. The frontend then generates an intermediate representation or IR of the source code for processing by the middle-end.

The middle end is where optimization takes place. Typical transformations for optimization are removal of useless or unreachable code, discovery and propagation of constant values, relocation of computation to a less frequently executed place (e.g., out of a loop), or specialization of computation based on the context. The middle-end generates another IR for the following backend. Most optimization efforts are focused on this part.

The back end is responsible for translating the IR from the middle-end into assembly code. The target instruction(s) are chosen for each IR instruction. Register allocation assigns processor registers for the program variables where possible. The backend utilizes the hardware by figuring out how to keep parallel execution units busy, filling delay slots, and so on. Although most algorithms for optimization are in NP, heuristic techniques are well-developed.

One-pass versus multi-pass compilers

Classifying compilers by number of passes has its background in the hardware resource limitations of computers. Compiling involves performing lots of work and early computers did not have enough memory to contain one program that did all of this work. So compilers were split up into smaller programs which each made a pass over the source (or some representation of it) performing some of the required analysis and translations.

The ability to compile in a single pass has classically been seen as a benefit because it simplifies the job of writing a compiler and one-pass compilers generally perform compilations faster than multi-pass compilers. Thus, partly driven by the resource limitations of early systems, many early languages were specifically designed so that they could be compiled in a single pass (e.g., Pascal).

In some cases the design of a language feature may require a compiler to perform more than one pass over the source. For instance, consider a declaration appearing on line 20 of the source which affects the translation of a statement appearing on line 10. In this case, the first pass needs to gather information about declarations appearing after statements that they affect, with the actual translation happening during a subsequent pass.

The disadvantage of compiling in a single pass is that it is not possible to perform many of the sophisticated optimizations needed to generate high quality code. It can be difficult to count exactly how many passes an optimizing compiler makes. For instance, different phases of optimization may analyse one expression many times but only analyse another expression once.

Splitting a compiler up into small programs is a technique used by researchers interested in producing provably correct compilers. Proving the correctness of a set of small programs often requires less effort than proving the correctness of a larger, single, equivalent program.

Lexical analysis

In computer science, lexical analysis is the process of converting a sequence of characters into a sequence of tokens. A program or function which performs lexical analysis is called a lexical analyzer, lexer, or scanner. A lexer often exists as a single function which is called by a parser or another function.

Parsing

In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens (for example, words), to determine its grammatical structure with respect to a given (more or less) formal grammar. Parsing can also be used as a linguistic term, for instance when discussing how phrases are divided up in garden path sentences.

Parsing is also an earlier term for the diagramming of sentences of natural languages, and is still used for the diagramming of inflected languages, such as the Romance languages or Latin. The term parsing comes from Latin pars, meaning part (of speech).

Parsing is a common term used in psycholinguistics when describing language comprehension. In this context, parsing refers to the way that human beings, rather than computers, analyze a sentence or phrase (in spoken language or text) "in terms of grammatical constituents, identifying the parts of speech, syntactic relations, etc." This term is especially common when discussing what linguistic cues help speakers to parse garden-path sentences.

Parser

In computing, a parser is one of the components in an interpreter or compiler that checks for correct syntax and builds a data structure (often some kind of parse tree, abstract syntax tree or other hierarchical structure) implicit in the input tokens. The parser often uses a separate lexical analyser to create tokens from the sequence of input characters. Parsers may be programmed by hand or may be (semi-)automatically generated (in some programming languages) by a tool.

Types of parser

The task of the parser is essentially to determine if and how the input can be derived from the start symbol of the grammar. This can be done in essentially two ways:

Top-down parsing- Top-down parsing can be viewed as an attempt to find left-most derivations of an input-stream by searching for parse trees using a top-down expansion of the given formal grammar rules. Tokens are consumed from left to right. Inclusive choice is used to accommodate ambiguity by expanding all alternative right-hand-sides of grammar rules.[4]

Bottom-up parsing - A parser can start with the input and attempt to rewrite it to the start symbol. Intuitively, the parser attempts to locate the most basic elements, then the elements containing these, and so on. LR parsers are examples of bottom-up parsers. Another term used for this type of parser is Shift-Reduce parsing.

Intermediate Code Generation

Code generation is the process by which a compiler's code generator converts some intermediate representation of source code into a form (e.g., machine code) that can be readily executed by a machine (often a computer).

Sophisticated compilers typically perform multiple passes over various intermediate forms. This multi-stage process is used because many algorithms for code optimization are easier to apply one at a time, or because the input to one optimization relies on the processing performed by another optimization. This organization also facilitates the creation of a single compiler that can target multiple architectures, as only the last of the code generation stages (the backend) needs to change from target to target. (For more information on compiler design, see Compiler.)

The input to the code generator typically consists of a parse tree or an abstract syntax tree. The tree is converted into a linear sequence of instructions, usually in an intermediate language such as three address code. Further stages of compilation may or may not be referred to as "code generation", depending on whether they involve a significant change in the representation of the program. (For example, a peephole optimization pass would not likely be called "code generation", although a code generator might incorporate a peephole optimization pass.)

Major tasks in code generation

In addition to the basic conversion from an intermediate representation into a linear sequence of machine instructions, a typical code generator tries to optimize the generated code in some way. The generator may try to use faster instructions, use fewer instructions, exploit available registers, and avoid redundant computations.

Tasks which are typically part of a sophisticated compiler's "code generation" phase include:

Instruction selection: which instructions to use.

Instruction scheduling: in which order to put those instructions. Scheduling is a speed optimization that can have a critical effect on pipelined machines.

Register allocation: the allocation of variables to processor registers.

Debug data generation if required so the code can be debugged.

Instruction selection is typically carried out by doing a recursive postorder traversal on the abstract syntax tree, matching particular tree configurations against templates; for example, the tree W := ADD(X,MUL(Y,Z)) might be transformed into a linear sequence of instructions by recursively generating the sequences for t1 := X and t2 := MUL(Y,Z), and then emitting the instruction ADD W, t1, t2.

In a compiler that uses an intermediate language, there may be two instruction selection stages — one to convert the parse tree into intermediate code, and a second phase much later to convert the intermediate code into instructions from the instruction set of the target machine. This second phase does not require a tree traversal; it can be done linearly, and typically involves a simple replacement of intermediate-language operations with their corresponding opcodes. However, if the compiler is actually a language translator (for example, one that converts Eiffel to C), then the second code-generation phase may involve building a tree from the linear intermediate code.

Code Optimization

Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied. The growth of portable computers has created a market for minimizing the power consumed by a program. Compiler optimization is generally implemented using a sequence of optimizing transformations, algorithms which take a program and transform it to produce a semantically equivalent output program that uses less resources.

It has been shown that some code optimization problems are NP-complete, or even undecidable. In practice, factors such as the programmer's willingness to wait for the compiler to complete its task place upper limits on the optimizations that a compiler implementor might provide. (Optimization is generally a very CPU- and memory-intensive process.) In the past, computer memory limitations were also a major factor in limiting which optimizations could be performed. Because of all these factors, optimization rarely produces "optimal" output in any sense, and in fact an "optimization" may impede performance in some cases; rather, they are heuristic methods for improving resource usage in typical programs.

Compiler Vs Interpreter

An interpreter translates some form of source code into a target representation that it can immediately execute and evaluate. The structure of the interpreter is similar to that of a compiler, but the amount of time it takes to produce the executable representation will vary as will the amount of optimization. The following diagram shows one representation of the differences.

Compiler characteristics:

spends a lot of time analyzing and processing the program the resulting executable is some form of machine- specific binary code

the computer hardware interprets (executes) the resulting code

program execution is fast

Interpreter characteristics:

relatively little time is spent analyzing and processing the program the resulting code is some sort of intermediate code

the resulting code is interpreted by another program

program execution is relatively slow