Basic Data Types 15 213 The course that gives CMU its Zip Integral Stored operated on in general registers Signed vs unsigned depends on instructions used Intel GAS Bytes C byte b 1 unsigned char word w 2 unsigned short double word l 4 unsigned int Structured Data Sept 20 2001 Topics Arrays Structs Unions Floating Point Stored operated on in floating point registers Intel GAS Bytes C Single s 4 float Double l 8 double Extended t 10 12 long double class08 ppt class08 ppt 2 Array Allocation CS 213 F 01 Array Access Basic Principle Basic Principle T A L Array of data type T and length L Contiguously allocated region of L sizeof T bytes T A L Array of data type T and length L Identifier A can be used as a pointer to starting element of the array int val 5 1 5 2 1 3 x 12 x 16 char string 12 x x x 12 Reference int val 5 x x 4 x 8 x 12 x 16 x 20 double a 4 x x 8 x 16 x 24 char p 3 x class08 ppt x 4 3 x 32 val 4 val val 1 val 2 val 5 val 1 val i x 4 x 8 Type Value int int int int int int int 3 x x 4 x 8 5 x 4i x 20 x 8 CS 213 F 01 class08 ppt 4 CS 213 F 01 Array Example Array Accessing Example typedef int zip dig 5 Computation zip dig cmu 1 5 2 1 3 zip dig mit 0 2 1 3 9 zip dig ucb 9 4 7 2 0 zip dig cmu 1 16 5 20 zip dig mit 24 0 36 56 9 52 7 64 36 3 48 4 60 3 32 1 44 9 1 28 2 40 zip dig ucb 2 Register edx contains starting address of array Register eax contains array index Desired digit at 4 eax edx Use memory reference edx eax 4 56 2 Memory Reference Code 0 68 int get digit zip dig z int dig return z dig 72 edx z eax dig movl edx eax 4 eax z dig 76 Notes Declaration zip dig cmu equivalent to int cmu 5 Example arrays were allocated in successive 20 byte blocks Not guaranteed to happen in general class08 ppt 5 CS 213 F 01 1 16 zip dig mit 5 20 0 36 zip dig ucb 24 2 40 9 56 2 28 1 44 4 60 1 64 32 48 68 36 52 56 0 72 76 Code Does Not Do Any Bounds Checking Reference Address Value Guaranteed mit 3 36 4 3 48 3 Yes mit 5 36 4 5 56 9 No mit 1 36 4 1 32 3 No cmu 15 16 4 15 76 No Out of range behavior implementation dependent No guaranteed relative allocation of different arrays class08 ppt 7 Original Source 9 2 CS 213 F 01 int zd2int zip dig z int i int zi 0 for i 0 i 5 i zi 10 zi z i return zi 3 3 7 6 Array Loop Example Referencing Examples zip dig cmu class08 ppt CS 213 F 01 int zd2int zip dig z int zi 0 int zend z 4 do zi 10 zi z z while z zend return zi Transformed Version As generated by GCC Eliminate loop variable i Convert array code to pointer code Express in do while form No need to test at entrance class08 ppt 8 CS 213 F 01 Nested Array Example Array Loop Implementation int zd2int zip dig z int zi 0 int zend z 4 do zi 10 zi z z while z zend return zi Registers ecx eax ebx z zi zend Computations 10 zi z implemented as z 2 zi 4 zi z increments by 4 zip dig pgh 4 zi 0 zend z 4 5 zi z z zi z 2 5 zi z zend if goto loop 9 96 116 136 156 Declaration zip dig pgh 4 equivalent to int pgh 4 5 Variable pgh denotes array of 4 elements Allocated contiguously Each element is an array of 5 int s Allocated contiguously Row Major ordering of all elements guaranteed class08 ppt CS 213 F 01 10 CS 213 F 01 Nested Array Row Access Nested Array Allocation Row Vectors Declaration T 1 5 2 0 6 1 5 2 1 3 1 5 2 1 7 1 5 2 2 1 76 ecx z xorl eax eax leal 16 ecx ebx L59 leal eax eax 4 edx movl ecx eax addl 4 ecx leal eax edx 2 eax cmpl ebx ecx jle L59 class08 ppt define PCOUNT 4 zip dig pgh PCOUNT 1 5 2 0 6 1 5 2 1 3 1 5 2 1 7 1 5 2 2 1 A R C Array of data type T R rows C columns Type T element requires K bytes a 0 0 a 0 C 1 A i is array of C elements Each element of type T Starting address A i C K a R 1 0 a R 1 C 1 int A R C Array Size A 0 R C K bytes Arrangement A 0 0 Row Major Ordering int A R C A A A A 0 0 1 1 0 C 1 0 C 1 A A R 1 R 1 0 C 1 A i A 0 C 1 A A i 0 A i C 4 A R 1 A A i R 1 C 1 0 A R 1 C 4 4 R C Bytes class08 ppt 11 CS 213 F 01 class08 ppt 12 CS 213 F 01 A R 1 C 1 Nested Array Row Access Code Nested Array Element Access Array Elements int get pgh zip int index return pgh index A i j is element of type T Address A i C j K A i j Row Vector pgh index is array of 5 int s Starting address pgh 20 index int A R C A 0 Code Computes and returns address Compute as pgh 4 index 4 index A 0 0 eax index leal eax eax 4 eax 5 index leal pgh eax 4 eax pgh 20 index A i A 0 C 1 A A i j A R 1 A i C 4 A R 1 0 A R 1 C 1 A R 1 C 4 A i C j 4 class08 ppt 13 CS 213 F 01 Nested Array Element Access Code Array Elements pgh index dig is int Address pgh 20 index 4 dig Code int get pgh digit int index int dig return pgh index dig Computes address pgh 4 dig 4 index 4 index movl performs memory reference ecx dig eax index leal 0 ecx 4 edx leal eax eax 4 eax movl pgh edx eax 4 eax class08 ppt 15 class08 ppt 14 Strange Referencing Examples zip dig pgh 4 1 5 2 0 6 1 5 2 1 3 1 5 2 1 7 …
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