Memory Layout

• lecture

#lecture note based on 15-213 Introduction to Computer Systems

A process’s virtual memory, assuming it’s running one thread.

• int as memory address always writes byte location of first byte of whatever thing, regardless the size of the thing. (L3S46)
• byte order
  • Big Endian (Sun, network packet headers, etc.): least signifiant byte highest address
  • Little Endian (x86 (including those embedded in x86 machine code), iOS, etc.): least significant byte lowest address
  • Bi Endian (e.g. ARM): can be configured either way

Byte ordering example. say x = 0x12345678 and &x = 0x100

Big Endian
0x100  0x101  0x102  0x103
12     34     56     78

Little Endian
0x100  0x101  0x102  0x103
78     56     34     12

Why little endian makes sense - truncation doesn’t change address.

short foo (int x) {
	// return (short) x; can be rewritten as:
	return *(short*)&x;
	// not true if big endian
}

No byte ordering issue. Just go down the address (make sure to null terminate)