Hiring guide for LC-3 Engineers

LC-3 Developer Hiring Guide

**LC-3** is a stack-based computer architecture and assembly language designed by Bell Labs in the 1970s for use in introductory computer science courses. It is a simplified version of the PDP-11 architecture, and was used in the early versions of the MIPS architecture. The LC-3 was first described in a 1975 paper by Bell Labs researchers John Cocke and James Hennessey. **Sources:** * [LC-3 Architecture and Assembly Language](https://en.wikipedia.org/wiki/LC-3) * [LC-3: A Simplified RISC Computer Architecture for Computer Science Education](https://dl.acm.org/doi/10.1145/359054.359070)

Ask the right questions secure the right LC-3 talent among an increasingly shrinking pool of talent.

First 20 minutes

General LC-3 app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in LC-3 application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you define the LC-3 architecture?
LC-3 architecture is a simplified version of computer architecture used primarily for educational purposes. It is a load/store architecture with 16-bit instructions and a 16-bit addressable memory.
What are the main components of LC-3?
The main components of LC-3 are the CPU, memory, registers, and the control unit. The CPU performs the operations, memory stores the data, registers hold the data temporarily during execution, and the control unit controls the flow of data.
Describe the difference between LC-3 and x86 architecture.
LC-3 is a simplified, educational architecture with a limited set of instructions and 16-bit addressable memory. On the other hand, x86 is a complex, commercial architecture with a vast set of instructions and larger memory.
How would you implement a subroutine in LC-3?
A subroutine in LC-3 can be implemented using the JSR or JSRR instructions. These instructions store the return address in the R7 register and then jump to the subroutine.
What are the different addressing modes in LC-3?
LC-3 supports three addressing modes: immediate, direct, and indirect. Immediate mode uses a constant value, direct mode uses the contents of a register, and indirect mode uses the contents of a memory location pointed to by a register.
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What you’re looking for early on

Does the candidate demonstrate a deep understanding of LC-3 architecture?
Can the candidate effectively explain complex LC-3 concepts?
Has the candidate shown the ability to solve problems using LC-3?
Does the candidate have experience with LC-3 Assembly Language?

Next 20 minutes

Specific LC-3 development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

How would you implement a loop in LC-3?
A loop in LC-3 can be implemented using conditional branch instructions. The loop condition is checked at the start or end of the loop, and if the condition is true, the program branches back to the start of the loop.
Describe the difference between TRAP and JSR instructions in LC-3.
TRAP is a system call that transfers control to a predefined routine in the operating system, while JSR is a jump to a subroutine in the user program. TRAP does not return control to the user program, while JSR does.
What are the different types of instructions in LC-3?
LC-3 supports three types of instructions: operational, data movement, and control. Operational instructions perform arithmetic and logical operations, data movement instructions move data between registers and memory, and control instructions alter the flow of execution.
How would you implement an array in LC-3?
An array in LC-3 can be implemented as a contiguous block of memory. The base address of the array is stored in a register, and individual elements are accessed by adding an offset to the base address.
Describe the difference between LDR and STR instructions in LC-3.
LDR is a load instruction that copies data from memory to a register, while STR is a store instruction that copies data from a register to memory. Both instructions use base+offset addressing mode.
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The ideal back-end app developer

What you’re looking to see on the LC-3 engineer at this point.

At this point, a skilled LC-3 engineer should demonstrate strong problem-solving abilities, proficiency in LC-3 programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with LC-3.

What does this simple LC-3 code do?
AND R0, R0, #0
ADD R0, R0, #1
This code initializes the register R0 to zero with the AND instruction, then increments R0 by 1 using the ADD instruction. So, the final value in R0 will be 1.
What does this LC-3 code do?
This code loads the value at memory location LABEL into R1. If R1 is zero, it branches to the label END. LABEL is a memory location that contains the value 10. If R1 is not zero, the program continues to execute until it reaches the HALT instruction.
What does this LC-3 code do for array manipulation?
LDR R1, R0, #0
ADD R1, R1, #1
STR R1, R0, #0
This code loads the address of the array into R0. It then loads the value at the first index of the array into R1. It increments the value in R1 by 1 and then stores the incremented value back at the first index of the array.
What does this LC-3 code do in terms of threading or concurrency?
This code jumps to a subroutine that increments the value in R0 by 1. After the subroutine is executed, it returns to the instruction following the JSR instruction. This demonstrates a simple form of concurrency in LC-3.

Wrap-up questions

Final candidate for LC-3 Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle LC-3 application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What are the different types of registers in LC-3?
LC-3 has eight general-purpose registers (R0 to R7), a program counter (PC), a condition code register (CC), and a memory address register (MAR) and memory data register (MDR) for memory access.
How would you implement a stack in LC-3?
A stack in LC-3 can be implemented using the PUSH and POP instructions. PUSH decrements the stack pointer and stores a value at the top of the stack, while POP retrieves a value from the top of the stack and increments the stack pointer.
Describe the difference between signed and unsigned numbers in LC-3.
Signed numbers in LC-3 can represent both positive and negative values, while unsigned numbers can only represent positive values. Signed numbers use two's complement representation, while unsigned numbers use binary representation.

LC-3 application related

Product Perfect's LC-3 development capabilities

Beyond hiring for your LC-3 engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in LC-3 projects, and can engage in multiple capacities.