Find the contents of register R20 after each of the following instructions: \(\begin{array}{llllll}\text { (a) LDI } & R 20,0 \times 65 & \text { (b) } & \text { LDI } & R 20, \\ \text { LDI } & R 21,0 \times 76 & & \text { LDI } & R 21,0 \times 6 \mathrm{~B} \\ & \text { AND } & R 20, R 21 & & \text { OR } & R 20, & R 21 \\ \text { (c) LDI } & R 20,0 \times 95 & \text { (d) } & \text { LDI } & R 20, & 0 \times 5 \mathrm{D} \\ & \text { LDI } & R 21,0 \times A A & & \text { LDI } & R 21, & 0 \times 75 \\ & \text { EOR } & R 20, R 21 & & \text { AND } & R 20, & R 21 \\ \text { (e) LDI } & R 20,0 \times 0 \mathrm{C} 5 & \text { (f) } & \text { LDI } & R 20, & 0 \times 6 \mathrm{~A} \\\ & \text { LDI } & R 21,0 \times 12 & & \text { LDI } & R 21, & 0 \times 6 \mathrm{E} \\ & \text { OR } & R 20, R 21 & & \text { EOR } & R 20, & R 21\end{array}\) \(\begin{array}{cl}\text { (g) LDI } & \text { R20, 0x37 } \\ \text { LDI } & \text { R21, 0x26 } \\ \text { OR } & \text { R20, R21 }\end{array}\)

The LDI instruction stands for 'Load Immediate' and is a fundamental command used in AVR microcontroller assembly programming. This instruction allows you to load a constant value directly into one of the registers without the need for this value to be located in the microcontroller’s memory. For example, 'LDI R20, 0x65' loads the hexadecimal value 65 into register R20, effectively initializing it with a specific value. It's important to note that while LDI is used for initialization purposes, it can only target the high registers (R16-R31).
In our specific exercise, as seen in step 1: 'LDI R20,0x65', the immediate value 0x65 is loaded into R20, setting R20 to that value. This instruction is essential in setting up the initial state of the registers before performing additional operations.

In AVR programming, register operations are tasks performed using the registers of the microcontroller. Registers are small storage areas inside the CPU that hold data to be processed or manipulated. Because they are located within the CPU, access to registers is much faster than accessing memory, making register operations a swift means of handling data.
For instance, following the LDI instruction, which initializes registers, further register operations like 'AND', 'OR', and 'EOR' can be performed, as shown in the problem. Each of these operations is done between two registers, and the result is stored in one of the registers involved in the operation. These operations are binary operations, meaning they operate bit by bit on the binary representations of the contents in the registers.
Take 'AND R20, R21' as an example. This logical AND operation is performed between corresponding bits of R20 and R21. The result, which will also be a binary value, is stored back into R20. Understanding these operations is crucial as they are building blocks for creating more complex programs.

Logical operations in AVR microcontrollers are the backbone of decision-making processes within a program. These operations make use of boolean algebra principles. There are three primary logical instructions used in AVR assembly language: AND, OR, and EOR (exclusive OR), each having a specific function.

• AND: This operation performs a bitwise AND between two registers. If both bits in a pair are '1', the result is '1'; otherwise, it is '0'. For example, the result of 0x65 AND 0x76 is 0x64, because '01100101' AND '01110110' equals '01100100'.
• OR: On the other hand, the bitwise OR sets the result bit to '1' if at least one of the bits in the pair is '1'. Following this logic, 0x37 OR 0x26 yields 0x37.
• EOR: Exclusive OR sets the result bit to '1' only if the bits in the pair are different; if they're the same, the result is '0'. For example, 0x6A EOR 0x6E results in 0x04.

It's crucial for students to appreciate these logical operations because they determine the flow and outcome of AVR programs. They can be used for tasks such as bit masking, setting, clearing, and toggling of bits, which are common when dealing with microcontroller peripherals.