Bits and Bytes CSCi 2021 : Machine Architecture and Organization

1. Bits and Bytes CSCi 2021: Machine Architecture and Organization Chapter 2.1

2. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary

3. 0 1 0 3.3V 2.8V 0.5V 0.0V Binary Representations: Why? Hardware + electronics can easily threshold voltages across transistor elements to set and detect 0’s and 1’s Easy to sequence them: 0 1 0 …

4. 15 14 10 13 11 12 0011 1111 1110 0000 0001 0010 1100 1101 1011 0100 0101 1010 0110 1001 0111 1000 3 8 8 4 5 7 6 6 F 4 2 5 2 A 1 9 D B 1 9 E 7 3 0 0 C Decimal Binary Hex Encoding Byte Values • Byte = 8 bits • Hexadecimal digit = 4 bits • Binary 000000002 to 111111112 • Decimal: 010 to 25510 (max: ?) • Hexadecimal 0016 to FF16 • Base 16 number representation • Use characters ‘0’ to ‘9’ and ‘A’ to ‘F’ • Write FA1D37B16 in C as • 0xFA1D37B • 0xfa1d37b • After the “x”, may be leading 0’s • 0x3 => 0x00…3 (how many? depends on how many bits)

5. Practice: Conversions • Don’t use Google • 8 bits • binary, hex, decimal A7 => 11000001 => 17610 => 75 =>

6. 00•••0 FF•••F • • • Byte-Oriented Memory Organization • Programs Refer to Virtual Addresses • Conceptually very large array of bytes • System provides address space private to particular “process” • Program being executed • Program can clobber its own data, but not that of others • Compiler + Run-Time System Control Allocation • Where different program objects should be stored • All allocation within single virtual address space

7. Machine Words • Machine Has “Word Size” • Nominal size of integer-valued data • Including addresses • Most current machines use 32 bits (4 bytes) words • Limits addresses to 4GB • Becoming too small for memory-intensive applications • High-end systems use 64 bits (8 bytes) words • Potential address space ≈ 1.8 X 1019bytes • Machines support multiple data formats • Fractions or multiples of word size • Always integral number of bytes

8. 0008 0012 0008 0004 0000 0000 32-bit Words 64-bit Words Bytes Addr. 0000 Addr = ?? 0001 0002 Addr = ?? 0003 0004 Addr = ?? 0005 0006 0007 0008 Addr = ?? 0009 0010 Addr = ?? 0011 0012 Addr = ?? 0013 0014 0015 Word-Oriented Memory Organization • Addresses Specify Byte Locations • Address of first byte in word • Addresses of successive words differ by 4 (32-bit) or 8 (64-bit)

9. Data Representations (bytes) Everything is binary to the machine … types are a higher level notion

10. Byte Ordering • How should bytes within a multi-byte word be ordered in memory? b1 b2 b3 b4 => which is stored at the lowest address? • Conventions • Big Endian: Sun, Internet • Least significant (rightmost) byte has highest address • Little Endian: x86 • Least significant (rightmost) byte has lowest address

11. 0x103 0x102 0x101 0x103 0x100 0x101 0x100 0x102 01 23 67 45 45 23 01 67 45 45 67 67 23 01 23 01 Byte Ordering Example • Big Endian • Least significant byte has highest address • Little Endian • Least significant byte has lowest address • Example • Variable x has 4-byte representation 0x01234567 • Address given by &x is 0x100 Big Endian Little Endian

12. Reading Byte-Reversed Listings • Disassembly • Text representation of binary machine code • Generated by program that reads the machine code • Example Fragment • Deciphering Numbers • Value: 0x12ab • Pad to 32 bits: 0x000012ab • Split into bytes: 00 00 12 ab • Reverse: ab 12 00 00 Address Instruction Code Assembly Rendition 8048365: 5b pop %ebx 8048366: 81 c3 ab 12 00 00 add $0x12ab,%ebx 804836c: 83 bb 28 00 00 00 00 cmpl $0x0,0x28(%ebx) Reverse: if binary is little endian

13. Thinking … • When does endian-ness matter? • Reading in a text file that contains a “4”? • Reading in a text file that contains a 4 in binary 000000…0100? • Sending an integer from one machine to another?

14. Examining Data Representations • Code to Print Byte Representation of Data • Casting pointer to unsigned char * creates byte array typedef unsigned char *pointer; void show_bytes(pointer start, intlen){ inti; for (i = 0; i < len; i++) printf(”%p\t0x%.2x\n",start+i, start[i]); printf("\n"); } Printf directives: %p: Print pointer %x: Print Hexadecimal

15. show_bytes Execution Example int a = 15213; printf("int a = 15213;\n"); show_bytes((pointer) &a, sizeof(int)); Result (Linux): int a = 15213; 0x11ffffcb8 0x6d 0x11ffffcb9 0x3b 0x11ffffcba 0x00 0x11ffffcbb 0x00 endian of this machine? if I say Linux – is this enough?

16. Please do not SIT on the LEFT (facing me) X

17. IA32 x86-64 IA32, x86-64 IA32, x86-64 Sun Sun 3B 00 00 3B 6D 00 93 00 6D C4 FF FF 00 C4 93 FF FF 00 00 6D 3B 00 00 6D 3B 00 00 00 Decimal: 15213 Binary: 0011 1011 0110 1101 Hex: 3 B 6 D Representing Integers int A = 15213; long int C = 15213; int B = -15213; Two’s complement representation (Covered later)

18. Representing Pointers int B = -15213; int *P = &B; Sun IA32 x86-64 Different compilers & machines assign different locations to data

19. Representing Strings char S[6] = "18243"; • Strings in C • Represented by array of characters • Each character encoded in ASCII format • Standard 7-bit encoding of character set • Character “0” has code 0x30 • Digit i has code ? • String should be null-terminated • Final character/byte = 0 • Compatibility • Byte ordering not an issue – why? • “you” are assigning each byte to a successive memory address! • Endian only applies to bytes within a word

20. Today: Bits, Bytes, and Integers • Representing information as bits • Bit-level manipulations • Integers • Representation: unsigned and signed • Conversion, casting • Expanding, truncating • Addition, negation, multiplication, shifting • Summary

21. Boolean Algebra • Developed by George Boole in 19th Century • Algebraic representation of logic • Encode “True” as 1 and “False” as 0 • And • A&B = 1 when both A=1 and B=1 • Or • A|B = 1 when either A=1 or B=1 • Not • ~A = 1 when A=0 • Exclusive-Or (Xor) • A^B = 1 when either A=1 or B=1, but not both

22. General Boolean Algebras • Operate on Bit Vectors • Operations applied bitwise • What are bit vectors good for? • compactly store information • Store 8 things (yes or no, taken or free) • “yes”, “no”, “yes”, “no”, …. • int, int, int, int, … 01101001 & 01010101 01000001 01101001 | 01010101 01111101 01101001 ^ 01010101 00111100 ~ 01010101 10101010 01000001 01111101 00111100 10101010

23. Bit-Level Operations in C • Operations &, |, ~, ^ Available in C • Apply to any “integral” data type • long, int, short, char, unsigned • View arguments as bit vectors • Arguments applied bit-wise • Examples (char data type) • ~0x41 = 0xBE • ~010000012=> 101111102 • ~0x00 = ? 0xFF ~000000002=> 111111112

24. Practice

25. Contrast: Logic Operations in C • Contrast to Logical Operators • &&, ||, ! • View 0 as “False” • Anything nonzero as “True” • Always return 0 or 1 • Early termination or short-circuit evaluation • Examples (char data type) • !0x41 => 0x00 • !0x00 => 0x01 • !!0x41 => 0x01 • 0x69 && 0x55 => 0x01 • 0x69 || 0x55 => 0x01 (don’t even eval 2nd argument) • p && *p (suppose p is a pointer?)

26. Argument x Argument x Arith. >> 2 Arith. >> 2 Log. >> 2 Log. >> 2 11101000 01100010 00010000 00011000 00011000 10100010 00010000 00101000 11101000 00101000 00010000 11101000 00101000 00010000 00011000 00011000 00011000 00010000 00010000 00011000 << 3 << 3 Shift Operations • Left Shift: x << y • Shift bit-vector x left y positions • Throw away extra bits on left • Fill with 0’s on right • Right Shift: x >> y • Shift bit-vector x right y positions • Throw away extra bits on right • Logical shift • Fill with 0’s on left • Arithmetic shift • Replicate most significant bit on right

27. Exercise • x=8 (logical shifts) • x>>2 • x>>3 • x<<2 • x<<3 • Interesting!

28. Next Time • Signed, unsigned integers • Casting, conversions • Chapter 2.2 – don’t worry about the mathematical notation • Have a great weekend