Given the following code (main.s):

给定以下代码(main.s)：

        .text  
        .globl main

main:
        lui $t0, 0x1000
        lbu $t1, 2($t0)

        .data 
        .byte 0
        .byte 0
        .byte 133

When executed with

在执行时

cat << EOF | spim -bare -noexception
load "main.s"
run 0x400000
print \$9
EOF

SPIM yields Reg 9 = 0x00000000 (0). However, when adding just one more run:

SPIM得到REG 9=0x00000000(0)。但是，在仅添加一个管路时：

cat << EOF | spim -bare -noexception
load "main.s"
run 0x400000
run 0x400000
print \$9
EOF

SPIM suddenly yields the correct value of Reg 9 = 0x00000085 (133). This also happens if reinitialize is ran before the second run, but the weird stdin piping doesn't allow for that.

SPIM突然产生正确值REG 9=0x00000085(133)。如果在第二次运行之前运行重新初始化，也会发生这种情况，但奇怪的stdin管道不允许这样做。

Why does this happen?

这一切为什么要发生？

I'm using QtSpim, which, confusingly, has two bare machine options. 

我使用的是QtSpim，令人困惑的是，它有两个裸机选项。

• The first is a single check box for Bare Machine

  第一个是裸机的单个复选框

  
  
  
  
  • This option changes the location for .data (perhaps among other things) from 0x10010000 (default) to 0x10000000.
  
  
  
  



• The second is a push button for Bare Machine that reconfigures all the check box options.

  第二个按钮是Bare Machine的按钮，用于重新配置所有复选框选项。

  
  
  
  
  • It will select the Bare Machine check box, and also among the check boxes selected by the push button, is
  
  
  • Enable Delayed Loads.
  
  
  
  

I believe that -bare on Spim is the same as the push button on QtSpim, which includes both the "Bare Machine" .data location change, plus Delayed Loads and Delayed Branches.

我相信SPIM上的-Bare和QtSpim上的按钮是一样的，QtSpim上的按钮既包括“裸机”，也包括数据位置更改，加上延迟加载和延迟分支。

Delayed loads is a aspect of the original MIPS processor and with it the results of any load instruction do not appear in the target register for an additional cycle/instruction.

延迟加载是原始MIPS处理器的一个方面，因此任何加载指令的结果都不会出现在额外周期/指令的目标寄存器中。

So, since your program ends with a load instruction, and also is using the Delayed Load option, effectively, such program is too short to see the result of that final load operation.

因此，由于您的程序以加载指令结束，并且还有效地使用了延迟加载选项，因此这样的程序太短，无法看到最终加载操作的结果。

Somehow those results do appear if you run again without reinitialization — while I cannot reproduce using QtSpim (there's no "run" command that can be used twice), it would appear that this second run you're doing provides that extra one cycle needed to finish the (first run's) load.

不知何故，如果您在没有重新初始化的情况下再次运行，这些结果就会出现--虽然我不能使用QtSpim重现(没有可以使用两次的“run”命令)，但似乎您正在进行的第二次运行提供了完成(第一次运行)加载所需的额外一个周期。

To fix so that you see the 133/0x85 in $t1 on the first run, simply add one more instruction, e.g. addu $0, $0, $0 — this additional instruction to follow that final load and cover the extra cycle needed by the load with delayed loads option.

要修复以便在第一次运行时看到$T1中的133/0x85，只需再添加一条指令，例如Addu$0，$0，$0-这条额外的指令跟随在最终加载之后，并覆盖加载延迟加载选项所需的额外周期。

A nop pseudo instruction would normally be an acceptable way to fill a delay slot, but with the -bare option (at least from QtSpim), the check box Accept Pseudo Instructions is disabled, so the nop pseudo instruction is not available, have to do it using a real instruction.

NOP伪指令通常是一种可接受的填充延迟槽的方法，但使用-Bare选项(至少来自QtSpim)时，将禁用接受伪指令复选框，因此NOP伪指令不可用，必须使用真实指令。