Data Representation. DRAM uses a single capacitor to store and a transistor to select. SRAM typically uses 6 transistors.

Transcription

1 Data Representation

2 Data Representation Goal: Store numbers, characters, sets, database records in the computer. What we got: Circuit that stores 2 voltages, one for logic ( volts) and one for logic (3.3 volts). DRAM uses a single capacitor to store and a transistor to select. SRAM typically uses 6 transistors. Definition: A bit is a unit of information. It is the amount of information needed to specify one of two equally likely choices. Example: Flipping a coin has 2 possible outcomes, heads or tails. The amount of info needed to specify the outcome is bit.

3 Storing Information Value Representation Value Representation Value Representation H T False True e-4 5 Use more bits for more items Three bits can represent 8 values:,,, N bits can represent 2 N values: N Can represent ,536 4,294,967, x 9 Approximately 65 thousand (64K where K=24) 4 billion 2 billion billion

4 Storing Information Most computers today use: Type bits name for storage unit Character 8-6 byte (ASCII) 6b Unicode (Java) Integers word (sometimes 8 or 6 bits) Reals word double-word

5 Character Representation Memory location for a character usually contains 8 bits: to (binary) x to xff (hexadecimal). Which characters? A, B, C,, Z, a, b, c,, z,,, 2,,9 Punctuation (,:{ ) Special (\n \O ) 2. Which bit patterns for which characters? Want a standard!!! Want a standard to help sort strings of characters.

6 Character Representation ASCII (American Standard Code for Information Interchange) Defines what character is represented by each sequence of bits. Examples: is 4 (hex) or 65 (decimal). It represents A. is 42 (hex) or 66 (decimal). It represents B. Different bit patterns are used for each different character that needs to be represented.

7 ASCII Properties ASCII has some nice properties. If the bit patterns are compared, (pretending they represent integers), then A < B 65 < 66 This is good, because it helps with sorting things into alphabetical order. But : a (6 hex) is different than A (4 hex) 8 (38 hex) is different than the integer 8 is 3 (hex) or 48 (decimal) 9 is 39 (hex) or 57 (decimal)

8 ASCII and Integer I/O Consider this program, what does it do? getc $t # get a digit add $t2, $t, $t putc $t2

9 ASCII and Integer I/O How to convert digits asciibiad:.word 48 # code for, 49 is, # do a syscall to get a char, move to $t sub $t2, $t, 48 # convert char for digit to num add $t3, $t2, $t2 add $t3, $t3, 48 # convert back to char putc $t3 The subtract takes the bias out of the char representation. The add puts the bias back in. This will only work right if the result is a single digit. Needed is an algorithm for translating character strings integer representation

10 Algorithm: Character string Integer Example: For Read 3 translate 3 to 3 Read 5 translate 5 to 5 integer = 3 x + 5 = 35 Read 4 translate 4 to 4 integer = 35 x + 4 = 354 Algorithm: asciibias = 48 integer = while there are more characters get character digit character asciibias integer integer x + digit

11 Algorithm: Integer Character string Example: For 354, figure out how many characters there are (3) For 354 div gives 3 translate 3 to 3 and print it out 354 mod gives div gives 5, translate 5 to 5 and print it out, 54 mod gives 4 4 div gives 4 translate 4 to 4 and print it out 4 mod gives, so your done

12 Character String / Integer Representation Compare: mystring:.asciiz 23 mynumber:.word is x3 2 x32 3 x33 \ x A series of four ASCII characters 23 = x7b = x7b = 7b A 32-bit 2 s complement integer

13 Integer Representation Assume our representation has a fixed number of bits n (e.g. 32).. Which 4 billion integers do we want? There are an infinite number of integers less than zero and an infinite number greater than zero. 2. What bit patterns should we select to represent each integer? Where the representation: Does not effect the result of calculation Does dramatically affect the ease of calculation 3. Convert to/from human-readable representation as needed.

14 Integer Representation Usual answers: 2 types. Represent and consecutive positive integers Unsigned integers 2. Represent positive and negative integers Signed magnitude One s complement Two s complement Biased Unsigned and two s complement the most common

15 Unsigned Integers Integer represented is binary value of bits:,, 2, Encodes only positive values and zero Range: to 2 n, for n bits Example: 4 bits, values to 5 n = 4, 2 4 is 5 [:5] = 6 = 2 4 different numbers 7 is 7 not represent able -3 not represent able Example: 32 bits = [: 4,294,967,295] 4,294,967,296 = 2 32 different numbers

16 Signed Magnitude Integers A human readable way of getting both positive and negative integers. Not well suited to hardware implementation. But used with floating point. Representation Use bit of integer to represent the sign of the integer Sign bit is msb: is +, is Rest of the integer is a magnitude, with same encoding as unsigned integers. To get the additive inverse of a number, just flip (invert, complement) the sign bit. Range: -(2 n- ) to 2 n- -

17 Signed Magnitude - Example 4 bits to to +7 Questions: is? -3 is? +2 is? [-7,, -,, +,,+7] = = 5 < 6 = 2 4 Why? What problems does this cause?

18 One s Complement Representation Historically important (in other words, not used today!!!) Early computers built by Semour Cray (while at CDC) were based on s complement integers. Positive integers use the same representation as unsigned. is is 7, etc Negation is done by taking a bitwise complement of the positive representation. Complement = Invert = Not = Flip = {, } A logical operation done on a single bit Top bit is sign bit

19 One s Complement Representation To get s complement of Take +: Complement each bit: Don t add or take away any bits. Examples: This must be a negative number. To find out which, find the additive inverse! is +3 must be? Properties of s complement: Any negative number will have a in the MSB There are 2 representations for, and

20 Two s Complement Variation on s complement that does not have 2 representations for. This makes the hardware that does arithmetic simpler and faster than the other representations. The negative values are all slid by one, eliminating the case. How to get 2 s complement representation: Positive: just as if unsigned binary Negative: Take the positive value Take the s complement of it Add

21 Two s Complement Example: (+5) (-5 in s complement) + (-5 in 2 s complement) To get the additive inverse of a 2 s complement integer,. Take the s complement 2. Add Number of integers represented: With 4 bits: [-8,,-,,+,,+7] = 8++7=6=2 4 numbers With 32 bits: [-23,,-,,+,,(23-)] = 23++(23-)=232 [ ,,-,,+,, ] ~ ±2B

22 A Little Bit on Adding Simple way of adding : Start at LSB, for each bit (working right to left) While the bit is a, change it to a. When a is encountered, change it to a and stop. Can combine with bit inversion to form 2 s complement. More generally, it s just like decimal!! + = + = + = 2, which is in binary, sum is, carry is. + + = 3, sum is, carry is. x +y + sum

23 A Little Bit on Adding Carry out Sum B A Carry in

24 Biased Representation An integer representation that skews the bit patterns so as to look just like unsigned but actually represent negative numbers. Example: 4-bit, with BIAS of 2 3 (8) (Excess 8) true value to be represented 3 Add in the bias +8 Unsigned value The bit pattern of 3 in biased-8 representation will be Suppose we were given a biased-8 representation as Unsigned represents 6 Subtract out the bias -8 True value represented -2 Operations on the biased numbers can be unsigned arithmetic but represent both positive and negative values How do you add two biased numbers? Subtract?

25 Biased Representation 25 in excess : 52,excess27 is: 2,excess3 is: 2,excess3 is: Where is the sign bit in excess notation? Used in floating-point exponents n-bit biased notation must specify the bias Choosing a bias: To get a ~ equal distribution of values above and below, the bias is usually 2 n- or 2 n-.

26 Sign Extension How to change a number with a smaller number of bits into the same number (same representation) with a larger number of bits? This must be done frequently by arithmetic units Unsigned: xxxxx xxxxx Copy the original integer into the LSBs, and put s elsewhere Sign/magnitude: sxxxxx sxxxxx Copy the original integer s magnitude into the LSBs Put the original sign into the MSB, put s elsewhere

27 Sign Extension s and 2 s complement: Sign Extension sxxxxx sssxxxxx Copy the original integer into the LSBs Take the MSB of the original and copy it elsewhere In 2 s complement, the MSB (sign bit) is the 2 n- place. It says subtract 2 n- from b n-2 b. Sign extending one bit Adds a 2 n place Changes the old sign bit to a +2 n- place -2 n + 2 n- = -2 n-, so the number stays the same

28 Sign Extension -2 in 8-bit 2 s complement: It s additive inverse is: