x86汇编RIP寻址验证及GDB查看指定内存地址内容的问题

Hey there! Let's tackle your questions one by one—they're both super relevant for low-level debugging and x86 assembly analysis, so let's dive in.

1. How to view the content stored at 0xaddress(%register)? #

You've got two easy, reliable ways to do this in GDB:

• Let GDB compute the address directly:
  Use the x (examine) command with the exact addressing expression. For example, if you're working with 0x1234(%eax), run:

  x/wx 0x1234+$eax
  

  Breakdown of the format: w means examine 4 bytes (word-sized, ideal for 32-bit registers), x tells GDB to display the value in hexadecimal. Adjust the format as needed: use b for single bytes, h for 2-byte half-words, g for 8-byte words, or s to view a null-terminated string.

• Calculate the address manually first:

  1. Fetch the current value of the target register:

     info registers register
     

     (Replace register with the actual register name, like eax or rbp.)
  2. Add the 0xaddress offset to the register's value to get the full memory address.
  3. Examine that address with the x command, e.g.:

     x/wx 0xdeadbeef  # Swap this with your computed address
     

2. Understanding cmp 0x204d63(%rip),%eax and viewing its target value #

First, the big answer: your core understanding is correct, but there's a tiny precision detail to note.

In x86-64 RIP-relative addressing, disp(%rip) calculates the effective address as:

Effective Address = Current RIP value + disp

The key nuance: when the CPU executes the cmp instruction, %rip already points to the next instruction (not the start of the cmp itself). So the math is indeed %rip + 0x204d63—you just need to remember that %rip here refers to the address right after the cmp instruction, which is how the CPU handles it natively.

To view the value at this address in GDB:

• Simplest method (let GDB handle the math):
  When your breakpoint hits this cmp instruction, run:

  x/wx 0x204d63+$rip
  

  GDB uses the current $rip value (which matches what the CPU uses) to compute the effective address and show you the stored value immediately.

• Manual calculation method:

  1. Get the current %rip value:

     info registers rip
     

  2. Add 0x204d63 to this value to get the target memory address.
  3. Examine the address with:

     x/wx <your_computed_address>
     

• Bonus tip: If you use GDB's TUI mode (run layout asm to enable it), you can use print 0x204d63+$rip to get the effective address first, then examine it—this is handy if you want to note the address for later use.

内容的提问来源于stack exchange，提问作者9.1827E+11