UNIT 5 [cmsc 311]

How many registers does the LC-3 have and what is their size?

8 general-purpose (R0-R7), 16 bit size each(1 word)

How many instructions does the LC-3 have and what is their size?

15 -> each 16 bits

Define address space and word size in the LC-3.

total # of memory locations available = 2^16. word size = 16 bits which each memory location stores

Explain how opcodes interpret operands.

Each opcode interprets the bit patterns of its operands based on the type of instruction.

Define the five addressing modes in the LC-3.

• Immediate: operand is part of instruction.

• Register: operand is stored in a specified general-purpose register

• PC-relative: address is found by adding an offset to program counter

• Indirect: instruction points to a memory location that contains another address

• Base + Offset: The address is calculated by adding an offset to a base register.

Explain the difference between LD, LDI, and LDR.

LD: Uses PC-relative addressing & accesses memory once to load data

LDI: Uses indirect addressing & accesses memory twice, 1. get address→ 2.get data

LDR: uses base + offset addressing & calculates address using register + offset

What are the NZP flags and how are they used?

The NZP flags represent negative, zero, and positive values. They are set based on the result of an operation and are used by branch instructions to determine whether a branch should be taken.

What makes a branch conditional vs unconditional?

A branch is conditional when it depends on specific NZP flags (such as BRz or BRp). A branch is unconditional when all flags are included (BRnzp), meaning it always executes.

Describe the differences between load operations in the LC-3.

Load operations differ based on how they calculate memory addresses.

Describe store operations.

Store operations work similarly to load operations but write data to memory instead of reading from it. ST uses PC-relative addressing, STI uses indirect addressing, and STR uses base + offset addressing.

What are the four types of instructions in the LC-3?

• OPERATE

• DATA MOVEMENT

• CONTROL

• SPECIAL INSTRUCTIONS