How do I make an else statement in LC2K?

In LC2K, a simplified assembly language, there isn't a direct equivalent of an else statement like you find in higher-level languages. Instead, you achieve the functionality of an else by using conditional branches and jump instructions. Here's how it works:

The Core Idea

The else functionality is implemented using a combination of:

1. A conditional branch instruction, which checks a condition.
2. A jump instruction to skip the code block for the 'if' part if the condition is not met, and instead execute the code for 'else' part.
3. A jump instruction to move to end of the conditional block after the 'if' part is finished

Example: Implementing an "If-Else" Structure

Let's say you want to implement a logic like this in pseudo code:

if (R1 == 0) {
 R2 = 1;
} else {
  R2 = 2;
}

Here's the corresponding LC2K code, with explanation:

  ; Assume R1 holds the value to be checked.
  BRZ R1, ELSE_PART ; If R1 is zero, jump to the ELSE_PART
  ADD R2, R0, R0 ; Initialize R2 to 0
  ADDI R2, R2, 1 ; R2 = 1 (If block)
  JMP END_IF ; Jump to the end of the if/else block
  ELSE_PART:
  ADD R2, R0, R0 ; Initialize R2 to 0
  ADDI R2, R2, 2 ; R2 = 2 (Else block)
  END_IF:
  ;Rest of program continues here

Explanation:

BRZ R1, ELSE_PART: This instruction checks if the value in register R1 is zero. If it is, the program jumps to the ELSE_PART label. If it's not zero, the program continues to the next instruction.
ADD R2, R0, R0: It zeros the R2 register. ADDI R2, R2, 1: If the condition of the branch is not met, then it adds 1 to R2, so it becomes 1, and then jumps to END_IF.
JMP END_IF: This instruction takes execution to the label END_IF.
ELSE_PART:: This is the label marking the beginning of the code block that executes if R1 was zero.
ADD R2, R0, R0: This zeros the R2 register. ADDI R2, R2, 2: This adds 2 to the R2, so it becomes 2.
END_IF:: This is the end of the if else block.

Important Considerations:

Labels: ELSE_PART and END_IF are labels, marking positions in the code for branch and jump instructions.
Branch Instructions: LC2K has other branch instructions (e.g., BGT, BLT) that can be used based on different condition.
Clarity: When writing your assembly code, it's helpful to comment your logic to make it clear what each part is doing.

By using these conditional branching techniques and jump instructions, you effectively implement the if-else conditional logic in LC2K.

Learn more about this