Termbank
  1. A
    1. Abstraction
    2. Alias
    3. Argument
    4. Array
  2. B
    1. Binary code file
    2. Binary number
    3. Bit
    4. Bitwise negation
    5. Bitwise operation
    6. Byte
  3. C
    1. C library
    2. C-function
    3. C-variable
    4. Character
    5. Code block
    6. Comment
    7. Compiler
    8. Complement
    9. Conditional statement
    10. Conditional structure
    11. Control structure
  4. D
    1. Data structure
    2. Duck typing
  5. E
    1. Error message
    2. Exception
  6. F
    1. Flag
    2. Float
  7. H
    1. Header file
    2. Headers
    3. Hexadecimal
  8. I
    1. Immutable
    2. Initialization
    3. Instruction
    4. Integer
    5. Interpreter
    6. Introduction
    7. Iteroitava
  9. K
    1. Keyword
  10. L
    1. Library
    2. Logical operation
  11. M
    1. Machine language
    2. Macro
    3. Main function
    4. Memory
    5. Method
  12. O
    1. Object
    2. Optimization
  13. P
    1. Parameter
    2. Placeholder
    3. Pointer
    4. Precompiler
    5. Precompiler directive
    6. Prototype
    7. Python console
    8. Python format
    9. Python function
    10. Python import
    11. Python list
    12. Python main program
    13. Python variable
    14. Python-for
    15. Pääfunktio
    16. printf
  14. R
    1. Resource
    2. Return value
  15. S
    1. Statement
    2. Static typing
    3. String
    4. Syntax
  16. T
    1. Terminal
    2. Type
    3. Typecast
  17. U
    1. Unsigned
  18. V
    1. Value
  19. W
    1. Warning
    2. while
Completed: / exercises

Pipeline processor

Learning Objectives: Understanding the principles of processor pipelining and resolving problems caused by instruction and data dependencies. Pipeline performance.
In previous material, we introduced the sequential processor and its weaknesses. To enhance processor performance, the concept of pipeline was introduced. In pipelining, all processor subsystems are always active, performing different phases of successive instructions. As the execution of one instruction progresses from one phase to another, the next instruction moves into the preceding phase.
Below is an example of a pipeline in the y86 processor. The letters refer to the different phases of instruction execution: Fetch, Decode, Execute, Memory, and Write Back. Note that the PC update phase is no longer present, an explanation will follow shortly.
Pipeline
With pipelined processors, the efficiency/performance of execution increases because the clock cycle length is determined by the longest subsystem phase, rather than the longest instruction execution time. In the figure, three instructions (instruction_1 + instruction_2 + instruction_3) would take 15 (3 x 5) time units in a sequential processor. With a pipeline like the one shown, they can be executed in just 7 time units! This leads to a significant reduction in program execution time.
Example: Comparing the load word instruction (loading a value from memory into a register) in a sequential and pipelined MIPS processor.
MIPS
In pipelining, the clock cycle time is reduced to a quarter of its original duration: 800ps -> 200ps. The benefit is evident as the processor delivers instruction results four times faster at intervals of approximately 200ps, compared to the sequential processor’s 800ps per instruction.

But Wait...

However, pipelined processors have two fundamental challenges. First, in the microarchitecture of a sequential processor, different subsystems use internal registers and signals during various execution phases! This creates a problem in pipelined implementations when different instructions require the same internal registers simultaneously for their execution.
In pipelined implementations, this issue is solved by introducing additional, separate pipeline registers between the phases. These registers store the input and output of each phase, effectively carrying intermediate results forward with the instruction. This ensures that intermediate results from one instruction do not interfere with those of others, allowing multiple instructions to execute synchronously in different subsystems. However, this slightly increases the duration of each phase, but instructions are still completed at a faster rate.
The second issue is that computer programs are typically written under the assumption that execution proceeds sequentially from one instruction to the next. That is, the output of one instruction often serves as the input to the next. This introduces dependencies between instructions.
Looking at the diagrams above, we can identify cases where the results of a preceding instruction are not available in the Write Back phase before they are needed in the Decode phase of the next instruction! This issue is addressed by creating feedback paths between phases so that intermediate results are available for subsequent instructions before being written to registers/memory.

Dependency Problems

Let’s analyze dependency issues between instructions with examples and their solutions:
  1. Dependency between instruction operands, also known as a data hazard. For example, this occurs when the output of one instruction is the input of another.
  2. Dependency between instructions themselves, also known as a control hazard. For example, conditional branch instructions rely on status flags set by a preceding instruction.

Data Hazard

Consider the following example code, which poses no issues in a sequential processor:
irmovq $10,%rdx  # rdx=10
irmovq $3,%rax   # rax=3
addq %rdx,%rax   # rax=rax+rdx
halt
When executed on a pipelined processor, a problem arises. The first two instructions do not reach the Write Back phase (where their results are written to the destination registers) before the third instruction requires their values in its Decode phase.
The diagram below shows that the register values are needed at clock cycle 4, but they only become available at cycles 5 and 6.
To resolve data hazards, several techniques are available.

Stalling

Execution can be delayed (stalling) by inserting nop instructions until the required inputs are available. The nop instruction is useful because it does not alter the contents of the processor's registers.
The diagram shows that by adding 3 nop instructions, the Write Back phases of the first two instructions are completed before their values are needed in the Decode phase of the third instruction.
Another method is to freeze the execution of instructions in their current phase until it is safe to proceed. This can be done by freezing the PC register and inserting bubbles to preserve the values in pipeline registers. Unlike nop, a bubble is not necessarily an instruction. (Although, bubbles are often implemented as nop instructions.)
Here, the inputs were required at cycle 4, so the PC is frozen, and bubbles are inserted starting from cycle 5. As a result, all subsequent instructions are delayed until the required inputs are available at cycle 7.

Forwarding

A drawback of the above methods is that inserting idle instructions or "idling" the processor reduces its efficiency, wasting clock cycles.
Forwarding (or bypassing) eliminates this problem by allowing the control logic to route intermediate results from internal registers directly to the inputs of the current instruction. In other words, if an input is not yet available, the system checks if the result is already computed/stored somewhere in the pipeline.
This technique can only be applied to signals available during the same clock cycle, which slightly increases the clock cycle duration.
In the diagram, the intermediate results of the first and second instructions (from the pipeline’s valM and valE at cycle 4) are routed to the Decode phase input signals of the addq instruction. Since the input values are needed only in the addq instruction’s Execute phase, they can be accessed in time.

Memory Address Hazard

When an instruction performs memory address operations to fetch an input instead of register calls, a load/use hazard can occur. This happens because the Memory phase is yet to come, but the value fetched from memory is already needed in the next instruction.
In this case, the addq instruction cannot have both operands set at clock cycle 7. The output of the fourth instruction (irmovq) is already available after the Execute phase, but the fifth instruction's output will only be available after the Memory phase. Here, Forwarding cannot be used because reading from memory to the register requires the Memory phase, so both inputs will only be available at clock cycle 8.
The solution is to combine Stalling and Forwarding. A bubble is added, and the addq instruction continues its Decode phase until both inputs are available for Forwarding.

Control Hazards

A control hazard refers to situations where there are dependencies between instructions such that the result of one instruction affects where the program execution continues(control dependency). In other words, what is the next instruction's memory address?

Subroutine Hazard

Let's consider a potential hazard caused by the ret instruction using the following code example:
main:
    call funktio
    irmovq $10,%rdx
    halt
funktio:
    irmovq $3, %rcx
    ret
The program's execution on a pipeline is shown below. Here, the subroutine is called and executed, but the return address is only determined during the ret instruction's Write Back phase. That is, when it has been fetched from the stack (Memory phase) and stored in the PC register (Write Back phase).
The solution here too is to insert bubbles until Forwarding can be used to feed the return address into the Fetch phase.

Conditional Branch

Conditional branching in pipelined processors can be implemented speculatively in two ways: the conditional branch always occurs / the conditional branch never occurs.
The problem with speculation is that depending on the condition's result, it might fetch wrong instructions if the result differs from the prediction.
Here’s an example of a conditional branch in y86:
0000: xorq %rax,%rax
0002: jne target         # Default: assume the branch always occurs!
000b: irmovq $1,%rax
0015: halt
0016: target:
0016:    irmovq $2,%rdx
0020:    irmovq $3,%rbx
002a:    ret
In the example below, the instructions following the branch (shown in red) are fetched speculatively based on the assumption that the branch always occurs. However, the correct branch address is only determined during the instruction's Execute phase. This is because the status flags are checked during the Execute phase.
Example: In the diagram, the jne 0x0016 branch instruction fetches two irmovq instructions from the branch target address to the pipeline in advance. However, this situation is not possible in this processor, as the branch address is resolved in the Execute phase when condition flags are updated.
The solution is to add bubbles to the pipeline until the address is determined.
As we will see later, modern processors prefetch instructions into the pipeline. If the prediction is incorrect, the solution is to remove incorrect instructions from the pipeline and insert bubbles.

Speculative Instruction Reordering

Sometimes, it is possible for the processor (or compiler or programmer...) to modify or change the program execution dynamically so that, instead of a bubble, upcoming instructions can be executed. These are instructions that have been checked and found to have no dependencies with stalled instructions.
As you might expect, this requires quite advanced control logic.

y86 Pipeline Implementation

Let’s now examine the y86-64 processor’s pipeline implementation and how it addresses the aforementioned challenges.
y86 pipeline
It is evident that the implementation (=microarchitecture) has become significantly more complex. First, pipeline registers (blue color) have been added between each phase. Additionally, various new registers and control signals have been incorporated into different phases to enable feedback loops between phases. Now, in the Decode phase, there is a new control logic block (circled area in the diagram) that selects inputs for the current instruction, either from its arguments or from intermediate results of other instructions.
But how do we know which intermediate result to select at any given time? In the y86 pipeline processor, a priority is set for various intermediate results, and inputs are chosen based on the instruction closest to the current phase. For example, when intermediate results are available from both the Execute and Memory phases of previous instructions, the intermediate results from the Execute phase are chosen because they are closer to the current phase (Decode).
Additionally, notice the new control logic attached to the Fetch phase (Select PC) and that the PC Update phase has disappeared. In the pipeline implementation, it is moved to the Fetch phase so that the address of the next instruction is fetched as late as possible. This block implements the prediction of the memory address of the next instruction (branch prediction) to improve performance. More on this later...
Now the pipeline register of the Fetch phase (Pred_PC) contains the predicted memory address of the next instruction based on the following rules:
1. If the instruction is not conditional or a jump, the next instruction’s address is in the current instruction’s valP.
2. If the instruction is conditional, it is assumed that the condition is always true, and the register contains the address of the condition’s success.
3. If the condition is false, the next address is fetched from the intermediate results of the previous instruction valA or valM, either from the Memory or Write Back phases.

Pipeline Performance

The pipeline processor’s performance can be expressed using two computational parameters:
These metrics enable us to design various pipeline solutions that maximize throughput. In microarchitecture, the designer can divide instruction execution into as many phases (theoretically) as needed and insert as many pipeline registers as required. In fact, modern processor implementations now feature up to 18 phases! However, with this increased complexity, the write delay for output or pipeline registers must also be factored into the instruction execution time.
Below is an example comparison of the performance difference between a sequential and a pipelined processor.
1. Sequential Processor
2. Pipelined Processor, where instruction execution is divided into three phases.
Now, 8.333 GIPS / 3.125 GIPS = 2.67 , which shows that the pipelined processor is significantly more efficient for program execution, as instructions are produced at a faster rate, even though the total execution time of an individual instruction is slightly longer: 360 / 320 = 1.125 .

Processor Implementations

Let’s look at real-world examples of sequential and pipelined implementations in different generations of Intel x86 processor architectures.
8088 processor (8-bit architecture): It features an internal instruction queue holding up to four instructions, which are executed sequentially. This technique reduces slow memory accesses.
80286 processor (8/16-bit architecture): Its internal instruction queue automatically fetches 2–3 instructions at a time. Instructions are executed sequentially. If a fetched instruction is a jump, the queue is flushed and refilled from the jump instruction’s address.
80386 processor (32-bit architecture): This implementation is based on a two-phase pipeline, meaning the next instruction is fetched while the current one is being executed. Jump instructions can flush the fetched instruction.
80486 processor (32-bit architecture): An instruction execution cycle takes four clock cycles: fetch from memory, decode, fetch operand from memory, and execute. The 486 pipeline, however, consists of five stages: fetch, decode, address generation, execute, and write-back. The 486 also introduced an integrated math processor for floating-point calculations, whereas earlier models used a separate chip on the same board.
Pentium processors (initially 32-bit architecture): The pipeline has five stages, like the 486. However, Pentium processors use superscalar techniques (more on this later...), allowing multiple independent execution queues and ALUs. Pentium processors have two ALUs for integer calculations: one (V-line) for simple instructions and another (U-line) for all instructions. Additionally, Pentium processors have a separate math coprocessor for floating-point calculations, which has its own pipeline. Thus, Pentium effectively has three distinct ALU pipelines!

Additional bibliography

Please refer to the course book Bryant & O'Hallaron, Computer Systems: A Programmer's Perspective, 3rd edition. Chapter 4.

Conclusion

Pipeline implementations significantly improve processor performance, but the trade-off is longer execution times for individual instructions and a more demanding microarchitecture design.
In real processors, (M)IPS is not a reliable performance metric because it reflects the best-case scenario, and in actual program execution, IPS varies. We will explore processor performance further in future material.
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Abstraction is a process through which raw machine language instructions are "hidden" underneath the statements of a higher level programming language. Abstraction level determines how extensive the hiding is - the higher the abstraction level, the more difficult it is to exactly say how a complex statement will be turned into machine language instructions. For instance, the abstraction level of Python is much higher than that of C (in fact, Python has been made with C).
Alias is a directive for the precompiler that substitus a string with another string whenever encountered. In it's basic form it's comparable to the replace operation in a text editor. Aliases are define with the #define directeve, e.g. #define PI 3.1416
Argument is the name for values that are given to functions when they are called. Arguments are stored into parameters when inside the function, although in C both sides are often called just arguments. For example in printf("%c", character); there are two arguments: "%c" format template and the contents of the character variable.
Array is a common structure in programming languages that contains multiple values of (usually) the same type. Arrays in C are static - their size must be defined when they are introduced and it cannot change. C arrays can only contain values of one type (also defined when introduced).
Binary code file is a file that contains machine language instructions in binary format. They are meant to be read only by machines. Typically if you attempt to open a binary file in a text editor, you'll see just a mess of random characters as the editor is attempting to decode the bits into characters. Most editors will also warn that the file is binary.
Binary number is a number made of bits, i.e. digits 0 and 1. This makes it a base 2 number system.
A bit is the smallest unit of information. It can have exactly two values: 0 and 1. Inside the computer everything happens with bits. Typically the memory contains bitstrings that are made of multiple bits.
Bitwise negation is an operation where each bit of a binary number is negated so that zeros become ones and vice versa. The operator is ~.
Bitwise operations are a class of operations with the common feature that they manipulate individual bits. For example bitwise negation reverses each bit. Some operations take place between two binary values so that bits in the same position affect each other. These operations include and (&), or (|) and xor (^). There's also shift operations (<< and >>) where the bits of one binary number are shifted to the left or right N steps.
Byte is the size of one memory slot - typically 8 bits. It is the smallest unit of information that can be addressed from the computer's memory. The sizes of variable types are defined as bytes.
External code in C is placed in libraries from which they can be taken to use with the #include directive. C has its own standard libraries, and other libraries can also be included. However any non-standard libraries must be declared to the compiler. Typically a library is made of its source code file (.c) and header file (.h) which includes function prototypes etc.
Functions in C are more static than their Python counterparts. A function in C can only have ne return value and its type must be predefined. Likewise the types of all parameers must be defined. When a function is called, the values of arguments are copied into memory reserved for the function parameters. Therefore functions always handle values that are separate from the values handled by the coe that called them.
C variables are statically typed, which means their type is defined as the variable is introduced. In addition, C variables are tied to their memory area. The type of a variable cannot be changed.
Character is a single character, referred in C as char. It can be interpreted as an ASCII character but can also be used as an integer as it is the smallest integer that can be stored in memory. It's exactly 1 byte. A character is marked with single quotes, e.g. 'c'.
Code block is a group of code lines that are in the same context. For instance, in a conditional structure each condtion contains its own code block. Likewise the contents of a function are in their own code block. Code blocks can contain other code blocks. Python uses indentation to separate code blocks from each other. C uses curly braces to mark the beginning and end of a code block.
Comments are text in code files that are not part of the program. Each language has its own way of marking comments. Python uses the # character, C the more standard //. In C it's also possible to mark multiple lines as comments by placing them between /* and */.
A compiler is a program that transforms C source code into a binary file containing machine language instructions that can be executed by the computer's processor. The compiler also examines the source code and informs the user about any errors or potential issues in the code (warnings). The compiler's behavior can be altered with numerous flags.
Complement is a way to represent negative numbers, used typically in computers. The sign of a number is changed by flipping all its bits. In two's complement which is used in this course, 1 is added to the result after flipping.
Conditional statement is (usually) a line of code that defined a single condition, followed by a code block delimited by curly braces that is entered if the condition evaluates as true. Conditional statements are if statements that can also be present with the else keyword as else if. A set of conditional statements linked together by else keywords are called conditional structures.
Conditional structure is a control structure consisting of one or more conditional statements. Most contrl structures contain at least two branches: if and else. Between these two there can also be any number of else if statements. It is however also possible to have just a single if statement. Each branch in a conditional structure cotains executable code enclosed within a block. Only one branch of the structure is ever entered - with overlapping conditions the first one that matches is selected.
Control structures are code structures that somehow alter the program's control flow. Conditional structures and loops belong to this category. Exception handling can also be considered as a form of control structure.
Data structure is a comman name for collection that contain multiple values. In Python these include lists, tuples and dictionaries. In C the most common data structures are arrays and structs.
Python's way of treating variable values is called dynamic typing aka duck typing. The latter comes from the saying "if it swims like a duck, walks like a duck and quacks like a duck, it is a duck". In other words, the validity of a value is determined by its properties in a case-by-case fashion rather than its type.
An error message is given by the computer when something goes wrong while running or compiling a program. Typically it contains information about the problem that was encountered and its location in the source code.
An exception is what happens when a program encounters an error. Exceptions have type (e.g. TypeError) that can be used in exception handling within the program, and also as information when debugging. Typically exceptions also include textual description of the problem.
Flags are used when executing programs from the command line interface. Flags are options that define how the program behaves. Usually a flag is a single character prefixed with a single dash (e.g. -o) or a word (or multiple words connected with dashes) prefixed with two dashes (e.g. --system. Some flags are Boolean flags which means they are either on (if present) or off (if not present). Other flags take a parameter which is typically put after the flag separated either by a space or = character (e.g. -o hemulen.exe.
Floating point numbers are an approximation of decimal numbers that are used by computers. Due to their archicture computers aren't able to process real decimal numbers, so they use floats instead. Sometimes the imprecision of floats can cause rounding errors - this is good to keep in mind. In C there are two kinds of floating point numbers: float and double, where the latter has twice the number of bits.
Header files use the .h extension, and they contain the headers (function prototypes, type definitions etc.) for a .c file with the same name.
Headers in C are used to indicate what is in the code file. This includes things like function prototypes. Other typical content for headers are definition of types (structs etc.) and constants. Headers can be at the beginning of the code file, but more often - especially for libraries - they are in placed in a separate header (.h) file.
Hexadecimal numbers are base 16 numbers that are used particularly to represent memory addresses and the binary contents of memory. A hexadecimal number is typically prefixed with 0x. They use the letters A-F to represent digits 10 to 15. Hexadecimals are used because each digit represents exactly 4 bits which makes transformation to binary and back easy.
In Python objects were categorized into mutable and immutable values. An immutable value cannot have its contents changed - any operations that seemingly alter the object actually create an altered copy in a new memory location. For instance strings are immutable in Python. In C this categorization is not needed because the relationship of variables and memory is tighter - the same variable addresses the same area of memory for the duration of its existence.
When a variable is given its initial value in code, the process is called initialization. A typical example is the initialization of a number to zero. Initialization can be done alongside with introduction: int counter = 0; or separately. If a variable has not been initialized, its content is whatever was left there by the previous owner of the memory area.
Instruction set defines what instructions the processor is capable of. These instructions form the machine language of the processor architecture.
Integers themselves are probably familiar at this point. However in C there's many kinds of integers. Integer types are distinguished by their size in bits and whether they are signed or not. As a given number of bits can represent up to (2 ^ n) different integers, the maximum value for a signed integer is (2 * (n - 1))
Python interpreter is a program that transforms Python code into machine language instructions at runtime.
The moment a variable's existence is announed for the first is called introduction. When introduced, a variable's type and name must be defined, e.g. int number;. When a variable is introduced, memory is reserved for it even though nothing is written there yet - whatever was in the memory previously is still there. For this reason it's often a good idea to initialize variables when introducing them.
Iteroitava objekti on sellainen, jonka voi antaa silmukalle läpikäytäväksi (Pythonissa for-silmukalle). Tähän joukkoon kuuluvat yleisimpinä listat, merkkijonot ja generaattorit. C:ssä ei ole silmukkaa, joka vastaisi Pythonin for-silmukan toimintaa, joten taulukoiden yms. läpikäynti tehdään indeksiä kasvattavilla silmukoilla.
Keywords are words in programming languages that have been reserved. Good text editors generally use a different formatting for keywords (e.g. bold). Usually keywords are protected and their names cannot be used for variables. Typical keywords include if and else that are used in control structures. In a way keywords are part of the programming language's grammar.
A library is typically a toolbox of functions around a single purpose. Libraries are taken to use with the include directive. If a library is not part of the C standard library, its use must also be told to the compiler.
Logical operation refers to Boole's algebra, dealing with truth values. Typical logical operations are not, and, or which are often used in conditional statements. C also uses bitwise logical operations that work in the same way but affect each bit separately.
Machine language is made of instructions understood by the processor. Machine language is often called Assembly and it is the lowest level where it's reasonable for humans to give instructions to computers. Machine language is used at the latter part of this course - students taking the introduction part do not need to learn it.
Macro is an alias that defines a certain keyword to be replaced by a piece of code. When used well, macros can create more readable code. However, often the opposite is true. Using macros is not recommended in this course, you should just be able to recognize one when you see it.
In C the main function is the starting point when the program is run. The command line arguments of the program are passed on to the main function (although they do not have to be received), and its return value type is int. At its shortest a main function can defined as int main().
When programs are run, all their data is stored in the computer's memory. The memory consists of memory slots with an address and contents. All slots are of equal size - if an instance of data is larger, a continuous area of multiple memory slots is reserved.
Method is a function that belongs to an object, often used by the object to manipulate itself. When calling a method, the object is put before the method: values.sort().
Object is common terminology in Python. Everything in Python is treated as objects - this means that everything can be referenced by a variable (e.g. you can use a variable to refer to a function). Objects are typically used in object-oriented languages. C is not one.
Optimization means improving the performance of code, typically by reducing the time it takes to run the code or its memory usage. The most important thing to understand about opimization is that it should not be done unless it's needed. Optimization should only be considered once the code is running too slowly or doesn't fit into memory. Optimization should also not be done blindly. It's important to profile the code and only optimize the parts that are most wasteful.
A parameter is a variable defined alongside with a function. Parameters receive the values of the function's arguments when it's called. This differentation between parameters and arguments is not always used, sometimes both ends of the value transfer are called arguments.
Placeholders are used in string formatting to mark a place where a value from e.g. a variable will be placed. In Python we used curly braces to mark formatting placeholders. In C the % character is used which is followed by definitions, where the type of the value is mandatory. For instance "%c" can only receive a char type variable.
Pointers in C are special variables. A pointer contains a memory address of the memory location where the actual data value is located. In a sense they work like Python variables. A variable can be defined as a pointer by postfixing its type with * when it's being introduced, e.g. int* value_ptr; creates a pointer to an integer. The contents of the memory address can be fetched by prefixing the variable name with * (e.g. *value_ptr. On the other hand, the address of a memory adress can be fetched by prefixing a variable name with &, (e.g. &value.
The C precompiler is an apparatus that goes through all the precompiler directives in the code before the program is actually compiled. These directives include statements which add the source code of the included libraries into the program, and define directives that can define constant values (aliases) and macros.
Directives are instructions that are addressed at the precompiler. They are executed and removed from the code before the actual compilation. Directives start with the # character. The most common one is include which takes a library into use. Another common one is define, which is used e.g. to create constant values.
Prototype defines a function's signature - the type of its return value, its name and all the arguments. A prototype is separate from the actual function definition. It's just a promise that the function that matches the prototype will be found in the code file. Prototypes are introduced at the beginning of the file or in a separate header file. In common cases the prototype definition is the same as the line that actually starts the function introduction.
Interactive interpreter or Python console is a program where users can write Python code lines. It's called interactive because each code line is executed after its been fully written, and the interpreter shows the return value (if any).
The format method of string in Python is a powerful way to include variable values into printable text. The string can use placeholders to indicate where the format method's arguments are placed.
Python functions can have optional parameters that have a given default value. In Python the values of arguments in a function call are transferred to function parameters through reference, which means that the values are the same even though they may have different names. Python functions can have multiple return values.
In Python the import statement is used for bringing in modules/libraries - either built-in ones, thrid party modules or other parts of the same application. In Python the names from the imported module's namespace are accessible through the module name (e.g. math.sin). In C libraries are taken to use with include, and unlike Python import it brings the library's namespace into the program's global namespace.
Python lists were discovered to be extremely effective tools in Elementary Programming. A Python list is an ordered collection of values. Its size is dynamic (i.e. can be changed during execution) and it can include any values - even mixed types. Lists can also include other lists etc.
In Python main program is the part of code that is executed when the program is started. Usually the main program is at the end of the code file and most of the time under if __name__ == "__main__": if statement. In C there is no main program as such, code execution starts with the main function instead.
In Python a variable is a reference to a value, a connection between the variable's name in code and the actual data in memory. In Python variables have no type but their values do. The validity of a value is tested case by case when code is executed. In these ways they are different from C variables, and in truth Python variables are closer to C pointers.
Pythonin for-silmukka vastaa toiminnaltaan useimmissa kielissä olevaa foreach-silmukkaa. Se käy läpi sekvenssin -esim. listan - jäsen kerrallaan, ottaen kulloinkin käsittelyssä olevan jäsenen talteen silmukkamuuttujaan. Silmukka loppuu, kun iteroitava sekvenssi päättyy.
Pääfunktio on C:ssä ohjelman aloituspiste ja se korvaa Pythonista tutun pääohjelman. Oletuksena pääfunktion nimi on main ja se määritellään yksinkertaisimmillaan int main().
Resource referes to the processing power, memory, peripheral devices etc. that are availlable in the device. It includes all the limitations within which programs can be executed and therefore defines what is possible with program code. On a desktop PC resources are - for a programmer student - almost limitless, but on embedded devices resources are much more scarce.
Return value is what a function returns when its execution ends. In C functions can only have one return value, while in Python there can be multiple. When reading code, return value can be understood as something that replaces the function call after the function has been executed.
A statement is a generic name for a single executable set of instructions - usually one line of code.
C uses static typing This means that the type of variables is defined as they are created, and values of different types cannot be assigned to them. The validity of a value is determined by its type (usually done by the compiler). Python on the other hand uses dynamic typing aka.duck typing.
In Python all text is handled as strings and it has no type for single characters. However in C there are no strings at all - there's only character arrays. A character array can be defined like a string however, e.g. char animal[7] = "donkey"; where the number is the size of the array + 1. The +1 is neede because the string must have space for the null terminator '\0' which is automatically added to the end of the "string".
Syntax is the grammar of a programming language. If a text file does not follow the syntax of code, it cannot be executed as code, or in the case of C, it cannot be compiled.
Terminal, command line interface, command line prompt etc. are different names to the text-based interface of the operating system. In Windows you can start the command line prompt by typing md to the Run... window (Win+R). Command line is used to give text-based commands to the operating system.
The data in a computer's memory is just bits, but variables have type. Type defines how the bits in memory should be interpreted. It also defines how many bits are required to store a value of the type. Types are for instance int, float and char.
Typecast is an operation where a variable is transformed to another type. In the elementary course this was primarily done with int and float functions. In C typecast is marked a bit differently: floating = (float) integer}. It's also noteworthy that the result must be stored in a variable that is the proper type. it is not possible to change the type of an existing variable.
Unsigned integer is a an integer type where all values are interpreted as positive. Since sign bit is not needed, unsigned integers can represent twice as large numbers as signed integers of the same size. An integer can be introduced as unsigned by using the unsigend keyword, e.g. unsigned int counter;.
In the elementary programming course we used the term value to refer to all kinds of values handled by programs be it variables, statement results or anything. In short, a value is data in the computer's memory that can be referenced by variables. In C the relationship between a variable and its value is tighter as variables are strictly tied to the memory area where its value is stored.
A warning is a notification that while executing or - in this course particularly - compiling it, something suspicious was encountered. The program may still work, but parts of it may exhibit incorrect behavior. In general all warnings should be fixed to make the program stable.
One way to print stuff in C is the printf function, which closely resembles Python's print function. It is given a printable string along with values that will be formatted into the string if placeholders are used. Unlike Python, C's printf doesn't automatically add a newline at the end. Therefore adding \n at the end is usually needed.
Out of loops, while is based on repetition through checking a condition - the code block inside the loop is repeated until the loop's condition is false. The condition is defined similarly to conditional statements, e.g. while (sum < 21).