求助：用两个8位寄存器实现Nand2Tetris课程的16位PC

Hey there! Let’s work through this Nand2Tetris problem—using two 8-bit Registers to mimic a 16-bit PC is totally doable, but your current code has a few key issues that are preventing it from working correctly. Let’s break down what’s wrong and how to fix it.

What’s Off in Your Current Code? #

Your setup has both registers hooked to the full 16-bit cout input, which means each register is trying to process all 16 bits instead of their assigned 8-bit slice. Additionally, sharing a single feedback signal between both registers breaks the state-retention logic, and you need to ensure both registers load new values at the exact same time.

Corrected Implementation #

Here’s how to adjust your code to make the two 8-bit registers work together as a single 16-bit PC:

// Low 8-bit Register: manages bits 0-7 of the 16-bit PC
Register(in=cout[0..7], out=pc_out[0..7], out=low_feedback, load=pc_load);

// High 8-bit Register: manages bits 8-15 of the 16-bit PC
Register(in=cout[8..15], out=pc_out[8..15], out=high_feedback, load=pc_load);

Key Changes Explained #

• Split the 16-bit input: Instead of feeding the full cout signal to both registers, split it into two 8-bit slices. The low register gets cout[0..7] (the least significant bits), and the high register gets cout[8..15] (the most significant bits).
• Synchronized load control: Use a single pc_load signal for both registers. This ensures that when you need to update the PC, both the high and low 8-bit sections update simultaneously—critical for maintaining a valid 16-bit value.
• Per-register feedback: Each register uses its own output as feedback (low_feedback for the low register, high_feedback for the high register). This lets each register retain its 8-bit state independently when pc_load is false, combining to keep the full 16-bit PC value intact.

Bonus: Adding PC Increment Logic (If Needed) #

If your PC needs to increment (a common requirement in Nand2Tetris), feed the combined pc_out[0..15] into a 16-bit incrementer, then pass that incremented value to the cout input of your registers. This way, when pc_load is triggered, the PC updates to the incremented value.

内容的提问来源于stack exchange，提问作者Gaben