Instructions

ADD & AND

The arithmetic and logical operate instructions.

ADD Instruction

ADD is the LC-3's only arithmetic instruction. It has two modes: Register mode: ADD DR, SR1, SR2
  • Adds the values in SR1 and SR2, stores result in DR
    • Immediate mode: ADD DR, SR1, #imm5
  • Adds SR1 and a small constant (range: -16 to 15), stores in DR
  • The 5-bit immediate is sign-extended to 16 bits
    • ; Register mode
ADD R2, R0, R1    ; R2 = R0 + R1

; Immediate mode
ADD R3, R3, #1    ; R3 = R3 + 1 (increment)
ADD R4, R4, #-1   ; R4 = R4 - 1 (decrement)

; Copy a register
ADD R1, R0, #0    ; R1 = R0 + 0 = R0

    AND Instruction

    AND performs bitwise AND. Same two modes as ADD: Register mode: AND DR, SR1, SR2 Immediate mode: AND DR, SR1, #imm5
    ; Clear a register (very common pattern!)
AND R0, R0, #0    ; R0 = R0 & 0 = 0

; Mask the lower 4 bits
AND R1, R0, xF    ; Wait... we can't!
; imm5 is only 5 bits, so we'd use:
AND R1, R0, #15   ; R1 = R0 & 0x000F

    Both ADD and AND set the condition codes (N, Z, P) based on the result written to DR. This is important for branching.

    Exercise

    Write a program that sets R0 = 5 and R1 = 4, then stores their sum in R2. Convert the sum to an ASCII digit (add 48) and print it with OUT.

    1
    R0
    x00000
    R1
    x00000
    R2
    x00000
    R3
    x00000
    R4
    x00000
    R5
    x00000
    R6
    x00000
    R7
    x00000
    PCx3000
    CC
    NZP
    Quiz

    What does `AND R3, R3, #0` do?

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