Question

假设我们有号码510（0000 0001 1111 1110）这存储在2个位置

lowHalf 0000 0001＆amp; highHalf 1111 1110

如果我们想要执行右移，我们如何计算进位？所以结果将是

0000 0011 1111 1100

Answer 1

我建议右移前8位以查看是否有溢出，如果有溢出，则移动8位左边的位并加1。在我下面的例子中，我使用二进制掩码检查下半部分的第8位，使用AND操作码，我检查该位是否已设置。

.ORIG x3000

    LD R1, DATA         ; load the lower 8 bits into R1
    LD R3, MASK         ; load the mask into R3
    LEA R5, DATA        ; load the mem location of data into R5
    ADD R5, R5, #1      ; offset by 1 to get the upper 8 bits
    ADD R1, R1, R1      ; bit shift right the data in R1
    AND R4, R1, R3      ; check to see if we have overflowed
    BRz NO_CARRY        ; if bit[8] is set then we have a carry

CARRY
    NOT R3, R3          ; 2's complement
    ADD R3, R3, #1      ;
    ADD R1, R1, R3      ; remove bit[8] from the lower 8 bits
    LDR R3, R5, #0      ; load the value of the 8 upper bits into
            ; R3
    ADD R3, R3, R3      ; bit shift right the upper 8 bits
    ADD R3, R3, #1      ; add the carry bit to the upper bits
    ST R1, DATA         ; store the new value into DATA
    STR R3, R5, #0      ; store the new value for the upper bits
    BRnzp END

NO_CARRY
    LDR R3, R5, #0      ; load the value of the 8 upper bits into
                        ; R3
    ADD R3, R3, R3      ; bit shift right the upper 8 bits
    ST R1, DATA         ; store the new value into DATA
    STR R3, R5, #0      ; store the new value for the upper bits

END
    HALT            ; TRAP x25


; Variables 
DATA    .FILL b11111110
        .FILL b00000001

MASK    .FILL b0000000100000000

.END

LC3在双重世界上正确转变

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