Question

我知道连续的R-Type指令可能会导致危险，例如：

add $2, $2, $1
add $2, $2, $3

但可以连续输入I-Type指令吗？例如：

addi $2, $0, 10
addi $2, $0, 5

Answer 1

根据你的情况：

addi $2, $0, 10 
addi $2, $0, 5

您永远不会遇到数据危险，因为您在写入后永远不会读取值（写入后读取）

或许可以这样想：

$2 = $0 + 10
$2 = $0 + 5

你可以看到第二次计算中没有使用$ 2，而且没有更改$ 0，所以没有数据危险。

如果你这样做：

addi $2, $0, 10 # $2 = $0 + 10
addi $3, $2, 5  # $3 = $2 + 5

流水线操作不保证$ 2是在第二次计算期间读取时的预期值。

认为lw和sw也是I型指令;

RAW
    A Read After Write hazard occurs when, in the code as written, one instruction
    reads a location after an earlier instruction writes new data to it, but in the
     pipeline the write occurs after the read (so the instruction doing the read gets stale data).
WAR
    A Write After Read hazard is the reverse of a RAW: in the code a write occurs after a read,
     but the pipeline causes write to happen first.
WAW
    A Write After Write hazard is a situation in which two writes occur out of order. We normally
    only consider it a WAW hazard when there is no read in between; if there is, then we have a RAW
    and/or WAR hazard to resolve, and by the time we've gotten that straightened out the WAW has 
    likely taken care of itself.

http://www.cs.nmsu.edu/~pfeiffer/classes/473/notes/hazards.html

鉴于读取和写入数据的操作是I型指令并给出了这些潜在数据危害的定义，是的，I型指令仍然存在危险。

在MIPS中，I-Type指令会导致危险吗？

1 个答案: