Selecting the Correct Statements About the LC-3 Trap Mechanism

What are the correct statements about the LC-3 Trap mechanism based on the provided information?

The LC-3 Trap Mechanism

The LC-3 Trap mechanism plays a crucial role in handling system calls and service routines within programs. Let's analyze the statements provided and determine which ones are correct. 1. The return address is placed in R7 when the TRAP instruction executes. This statement is correct. When the TRAP instruction is executed, the return address is stored in register R7. 2. The TRAP mechanism is used to provide system calls or service routines to programs. This statement is also correct. The TRAP mechanism is utilized for providing system calls and service routines to programs. 3. The actual address of a trap routine is found by zero-extending bits 0 to 7 of the TRAP instruction and using this address to access the address of the trap routine. This statement is true. The actual address of a trap routine is obtained by zero-extending bits 0 to 7 of the TRAP instruction.

Incorrect Statements

Now, let's explore the incorrect statements mentioned in the data: 1. There are 128 different possible TRAP addresses. This statement is inaccurate. In reality, there are 256 possible TRAP addresses because the TRAP instruction is 8 bits long, allowing for 256 different values. 2. The LC-3 operating system places the TRAP routines below address ×3000. This statement is incorrect. The TRAP routines are typically stored from address ×0100 to ×01FF, as stated in the data.

Conclusion

To summarize, the correct options regarding the LC-3 Trap mechanism are: 1. The return address is placed in R7 when the TRAP instruction executes. 2. The TRAP mechanism is used to provide system calls or service routines to programs. 3. The actual address of a trap routine is found by zero-extending bits 0 to 7 of the TRAP instruction and using this address to access the address of the trap routine. 4. The table of trap addresses is stored from address ×0100 to ×01FF.

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