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Standard:
Compare levels of abstraction and interactions between application software, system software, and hardware.

Descriptive Statement:
At its most basic level, a computer is composed of physical hardware on which software runs. Multiple layers of software are built upon various layers of hardware. Layers manage interactions and complexity in the computing system. System software manages a computing device's resources so that software can interact with hardware. Application software communicates with the user and the system software to accomplish its purpose. Students compare and describe how application software, system software, and hardware interact. For example, students could compare how various levels of hardware and software interact when a picture is to be taken on a smartphone. Systems software provides low-level commands to operate the camera hardware, but the application software interacts with system software at a higher level by requesting a common image file format (e.g., .png) that the system software provides.

Standard:
Develop guidelines that convey systematic troubleshooting strategies that others can use to identify and fix errors.

Descriptive Statement:
Troubleshooting complex problems involves the use of multiple sources when researching, evaluating, and implementing potential solutions. Troubleshooting also relies on experience, such as when people recognize that a problem is similar to one they have seen before and adapt solutions that have worked in the past. For example, students could create a list of troubleshooting strategies to debug network connectivity problems such as checking hardware and software status and settings, rebooting devices, and checking security settings. Alternatively, students could create troubleshooting guidelines for help desk employees based on commonly observed problems (e.g., problems connecting a new device to the computer, problems printing from a computer to a network printer).

Standard:
Identify and fix security issues that might compromise computer programs.

Descriptive Statement:
Some common forms of security issues arise from specific programming languages, platforms, or program implementation choices. Students read a given a piece of code that contains a common security vulnerability, explain the code's intended function or purpose, provide and explain examples of how a specific input could exploit that vulnerability (e.g., the program accessing data or performing in unintended ways), and implement a change in the code to mitigate this vulnerability. For example, students could review code that takes a date as input, recognize that the code doesn't check for appropriate last days of the month, and modify the code to do that. Alternatively, students could review code that supports entry of patient data (e.g., height and weight) and doesn't prompt users to double check unreasonable values (e.g., height at 6 feet and weight at 20 pounds).

Standard:
Develop and use a series of test cases to verify that a program performs according to its design specifications.

Descriptive Statement:
Testing software is a critically important process. The ability of students to identify a set of important test cases communicates their understanding of the design specifications and potential issues due to implementation choices. Students select and apply their own test cases to cover both general behavior and the edge cases which show behavior at boundary conditions. For example, for a program that is supposed to accept test scores in the range of [0,100], students could develop appropriate tests (e.g, a negative value, 0, 100, and a value above 100). Alternatively, students developing an app to allow users to create and store calendar appointments could develop and use a series of test cases for various scenarios including checking for correct dates, flagging for user confirmation when a calendar event is very long, checking for correct email address format for invitees, and checking for appropriate screen display as users go through the process of adding, editing, and deleting events.

Standard:
Modify an existing program to add additional functionality and discuss intended and unintended implications.

Descriptive Statement:
Modularity and code reuse is key in modern software. However, when code is modified, the programmer should consider relevant situations in which this code might be used in other places. Students create and document modifications to existing programs that enhance functionality, and then identify, document, and correct unintended consequences. For example, students could take an existing a procedure that calculates the average of a set of numbers and returns an integer (which lacks precision) and modify it to return a floating-point number instead. The student would explain how the change might impact multiple scenarios.

Standard:
Evaluate key qualities of a program through a process such as a code review.

Descriptive Statement:
Code reviews are a common software industry practice and valuable for developing technical communication skills. Key qualities of code include correctness, usability, readability, efficiency, and scalability. Students walk through code they created and explain how it works. Additionally, they follow along when someone else is explaining their code and ask appropriate questions. For example, students could present their code to a group or visually inspect code in pairs. Alternatively, in response to another student's presentation, students could provide feedback including comments on correctness of the code, comments on how code interacts with code that calls it, and design and documentation features.

Standard:
Illustrate ways computing systems implement logic through hardware components.

Descriptive Statement:
Computing systems use processors (e.g., a central processing unit or CPU) to execute program instructions. Processors are composed of components that implement the logical or computational operations required by the instructions. AND, OR, and NOT are examples of logic gates. Adders are examples of higher-leveled circuits built using low-level logic gates. Students illustrate how modern computing devices are made up of smaller and simpler components which implement the logic underlying the functionality of a computer processor. At this level, knowledge of how logic gates are constructed is not expected. For example, students could construct truth tables, draw logic circuit diagrams, or use an online logic circuit simulator. Students could explore the interaction of the CPU, RAM, and I/O by labeling a diagram of the von Neumann architecture. Alternatively, students could design higher-level circuits using low-level logic gates (e.g., adders).