Tuesday, 1 September If you are still using the default password - - PowerPoint PPT Presentation
Tuesday, 1 September If you are still using the default password that was assigned when your account was created, CHANGE IT NOW! (It can be the same as your email password.) Tuesday, 1 September REMINDER: Department coffee/tea this
ASCII value Character ASCII value Character 48 ‘0’ 67 ‘C’ 49 ‘1’ ... ... 50 ‘2’ 90 ‘Z’ ... ... ... ... 57 ‘9’ 97 ‘a’ ... ... 98 ‘b’ 65 ‘A’ ... ... 66 ‘B’ 122 ‘z’
#include <stdio.h> main() { int c; /* input character--assumed to be a digit */ int digit; /* the decimal digit corresponding to c */ int value = 0; /* value of the input string as an int */ while ((c = getchar()) != '\n') { if (c < '0' || c > '9') { /* Check for error in input */ printf("Error--non-digit in input\n"); break; } digit = c - '0'; /* Convert ASCII to digit */ value = 10 * value + digit; /* Combine with previous digits */ } printf("Value of string: %d\n",value); }
rroos$./asciitoint 5082 Value of string: 5082 c = '5' (ASCII value 53) digit = 5 value = 5 c = '0' (ASCII value 48) digit = 0 value = 50 c = '8' (ASCII value 56) digit = 8 value = 508 c = '2' (ASCII value 50) digit = 2 value = 5082
int c, position; c = getchar(); /* read in a character, e.g, ‘A’ */ printf(“char = ‘%c’, ASCII value = %d\n”,c,c); position = c - ‘A’; printf(“position in alphabet: %d\n”,position);
char = ‘A’, ASCII value = 65 position in alphabet: 0
■ Which airplane has the best performance?
§1.6 Performance
■ Response time
■ How long it takes to do a task
■ Throughput
■ Total work done per unit time
■ e.g., tasks/transactions/… per hour
■ How are response time and throughput affected
■ Replacing the processor with a faster version? ■ Adding more processors?
■ We’ll focus on response time for now…
■ Define Performance = 1/Execution Time ■ “X is n time faster than Y” ■ Example: time taken to run a program
■ 10s on A, 15s on B ■ Execution TimeB / Execution TimeA
■ So A is 1.5 times faster than B
■ Operation of digital hardware governed by a
Clock (cycles) Data transfer and computation Update state Clock period
■ Clock period: duration of a clock cycle
■ e.g., 250ps = 0.25ns = 250×10–12s
■ Clock frequency (rate): cycles per second
■ e.g., 4.0GHz = 4000MHz = 4.0×109Hz
Hz = “Hertz” = “cycles per second”
■ Performance improved by
■ Reducing number of clock cycles ■ Increasing clock rate ■ Hardware designer must often trade off clock
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Aim for 6s CPU time
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Can do faster clock, but causes 1.2 × clock cycles
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■ Instruction Count for a program
■ Determined by program, ISA and compiler
■ Average cycles per instruction
■ Determined by CPU hardware ■ If different instructions have different CPI
■ Average CPI affected by instruction mix
ISA = “Instruction Set Architecture” CPI = “Cycles Per Instruction”
■ Computer A: Cycle Time = 250ps, CPI = 2.0 ■ Computer B: Cycle Time = 500ps, CPI = 1.2 ■ Same ISA ■ Which is faster, and by how much?
A is faster… …by this much
■ If different instruction classes take different
■ Weighted average CPI
Relative frequency
■ Alternative compiled code sequences using
Class A B C CPI for class 1 2 3 IC in sequence 1 2 1 2 IC in sequence 2 4 1 1
■ Sequence 1: IC = 5
■ Clock Cycles
■ Avg. CPI = 10/5 = 2.0
■ Sequence 2: IC = 6
■ Clock Cycles
■ Avg. CPI = 9/6 = 1.5
■ Performance depends on
■ Algorithm: affects IC, possibly CPI ■ Programming language: affects IC, CPI ■ Compiler: affects IC, CPI ■ Instruction set architecture: affects IC, CPI, Tc
Cycle time?