来自C ASM的NodeJS按位运算符

Question

This is related to my previews question .

A complete ASM of the function in C is here .

My problem lies on:

00408091  |>  F6D3          ||NOT BL
00408093  |.  FEC3          ||INC BL
00408095  |.  02D3          ||ADD DL,BL

On javascript when I use the NOT bitwise operator on 0x35 it returns -36 instead of the expected 0xCA . Why is that?

  // cmp al, dl
  if (b <= a) {
    a -= b;
  } else {
    // problem lies here
    console.log(~b);
    a += b;
    a++;
  }

My nodejs code currently is:

for (var i = 0; i < hashed.length; i++) {
  var a = hashed[i];
  var b = seqX[i];
  var c = seqX[i+1];

  var stepIn = i+1;
  var stepOver = stepIn-1;

  // cmp BL, DL -- 0x63, 0x4e -- 0xf9, 0xc5 -- 0x75. 0x7a
  for (var j = internalRounds - 1; j > 0; j--, stepIn--, stepOver--) {
    if (seqX[(i*2)+1] <= a) {
      a -= seqX[(i*2)+1];
    } else {
      a += seqX[(i*2)+1];
      a++
    }
    // xor dl, bl -- 0x1c, 0xc0
    a ^= seqY[stepIn];
  }

  // cmp al, dl
  if (b <= a) {
    a -= b;
  } else {
    // problem lies here
    console.log(~b);
    a += b;
    a++;
  }

  // xor al, dl --- 0xd4, 0xb8
  a ^= seqY[i];

  // xor al, cl
  a ^= 0x6e;

  console.log(a.toString(16)); // I expect this to be 2.
  console.log('--------------------------');
}

Answer 1

The following sequence of instructions:

00408091  |>  F6D3          ||NOT BL
00408093  |.  FEC3          ||INC BL
00408095  |.  02D3          ||ADD DL,BL

Is treating the bytes in BL and DL as signed values and essentially subtracting BL from DL . The first two take the 2's complement negation of BL as a byte, then the third adds the negated BL to DL . Unless there are subsequent instructions that deal a certain way with the processor status flags, I'm not sure why it doesn't just say, SUB DL,BL .

So I would think this group of instructions would just translate into:

b -= a;

Rather than literally translating each instruction.

This depends somewhat on the context of the whole asm program, how BL and DL are treated. If they are treated as byte values consistentlyt, then the above should work.

Answer 2

That because javascript always operates with 32-bit signed integers. So you have to truncate result to 8 bits.

(~0x35) & 0xFF // results 0xCA

Answer 3

It looks like you have two problems. The first is that you are doing ~35 , not 0x35 (the first is decimal 35, the second hex 35).

The second is that the bitwise-not in javascript casts the argument to a 32-bit signed integer, whereas it appears that you expect it to be treated as an unsigned char (8-bits). You can fix this by taking the result of the not and doing a bitwise-and with 0xFF . So:

(~0x35) & 0xFF
> 202
((~0x35) & 0xFF).toString(16)
> "ca"