Representations in memory

Byte-Oriented Memory Organization

• Programs refer to data by address
  • Imagine all of RAM as an enormous array of bytes
  • An address is an index into that array
    • A pointer variable stores an address

• System provides a private address space to each "process"
  • A process is an instance of a program, being executed
  • An address space is one of those enormous arrays of bytes
  • Each program can see only its own code and data within its enormous array
  • "virtual memory" class에서 더 자세하게 배울거다

Machine Words

Any given computer has a "word size"

• Nominal size of integer-value data
  • and of addresses

• Until recently, most machines used 32 bits(4 bytes) as a word size

• Increasingly, machines have 64-bit word size

Addresses Always Specify Byte Locations

• Address of a word is address of the first byte in the word
• Addresses of successive words differ by 4(32-bit) or 8(64-bit)

Byte Ordering

So, how are the bytes within a multi-byte word ordered in memory?

Conventions

• Big Endian : LSB has higest address
• Little Endian : LSB has lowest address x86

Byte Ordering example

• variable x has 4-byte value of 0x01234567
• address given by &x is 0x100
  

Examining Data Representations

• Code to print byte representation of Data

typedef unsigned char *pointer;

void show_bytes(pointer start, size_t len) {
	size_t i;
    for (i = 0; i < len; i++) 
    	printf("%p\t0x%.2x\n", start + i, start[i]);
    printf("\n");
}

int a = 15213;
printf("int a = 15213;\n");
show_bytes( (pointer) &z, sizeof(int) );

결과

int a = 15213
0x7fffb7f71dbc	6d
0x7fffb7f71dbd	3b
0x7fffb7f71dbe	00
0x7fffb7f71dbf	00

=>Little Endian

15213
0011 1011 0110 1101
3B6D

Representing Strings

Strings in C

• represented by arrays of characters
• each character encoded in ASCII format

15213
31 38 32 31 33