MMIX TRAP指令每隔一次失效，打印次数异常问题咨询

Let's break down what's happening here and why you're seeing that odd alternating TRAP behavior:

First, interpret those TRAP outputs #

• The line 00000701 (TRAP) $255 = Fputs(StdOut,#c) = 0 is the successful case: you're correctly calling Fputs with the address of your "Hello World" string (#c) and the standard output stream. The return value 0 confirms the string was printed to the console without issues.
• The second line 00000701 (TRAP) $255 = Fputs(StdOut,#0) = 12 is the root problem: here, you're passing #0 (a null pointer) as the string argument to Fputs. The return value 12 is an error code (indicating an invalid pointer or argument for the output function), so nothing gets printed to the console during these iterations.

Why the alternating calls & only 3 valid prints? #

The core issue is that your loop logic loses track of the valid string address after a few iterations, leading to invalid Fputs calls for the remaining loops. Here are the most likely causes:

1. Register values aren't reset between iterations #

MMIX's Fputs TRAP call relies on specific registers for arguments and returns:

• You must load StdOut (value 1) into $1 and your string's address into $2 before every TRAP 0x701 call.
• If you don't re-set $2 to your string's address at the start of each loop iteration, $2 might get overwritten (either by the TRAP call itself or your loop counter logic) to #0, triggering the invalid calls.

2. Loop counter logic is broken #

You mentioned multiplying the times parameter by 3, but only seeing 3 prints. This suggests your loop only runs 3 valid iterations before switching to null pointer calls. Common mistakes here include:

• Miscalculating the total iterations: e.g., initializing the loop counter to times instead of 3*times.
• Using a scratch register (one that gets overwritten by TRAP calls) for your loop counter. Double-check MMIX's register preservation rules—some registers are not preserved across TRAP invocations.
• Incorrect termination condition: e.g., stopping the loop when the counter reaches 3 instead of 0, or incrementing/decrementing the counter incorrectly.

3. Accidental null pointer assignment #

It's possible your code has a misplaced instruction that sets the string address register to #0 after a few iterations—maybe a wrong register move or decrement operation targeting the wrong register.

Debugging steps to fix this #

• Check register state per iteration: Use the MMIX debugger to inspect the value of your string address register (e.g., $2) right before each TRAP call. If it's #0 when you expect your string's address, you've found where the value is being corrupted.
• Validate loop initialization: Confirm your loop counter is set to 3*times correctly. For example, if times is stored in $3, use MUL $4,$3,3 to set the total iterations, then use $4 as your loop counter.
• Re-initialize Fputs arguments every loop: Don't assume argument registers stay valid across iterations. At the start of each loop, reload $1 with StdOut and $2 with your string's address before executing the TRAP instruction.

内容的提问来源于stack exchange，提问作者Sancho Jimenez