Thread: casting large values to small registers using ptr
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Old Apr 22nd, 2008, 10:21 PM   #1
mrynit
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casting large values to small registers using ptr
Using [type size] ptr val truncates a bit range of an arguement value. [type size] = byte, word, dword, qword. This is used in intel and I believe not AT&T. My question is about around line 60. Why is there auto casting? I should have to explicitly say cast/truncate by using ptr. Is this just because I'm using Eclipse CDT IDE, linux and gcc with -masm=intel? Would some one please try running this under windows not with gcc.

C Syntax (Toggle Plain Text) 
  1. /* Auther:	mrynit
  2.  * Date:	4-22-05
  3.  * Class:	TCSS 372
  4.  * Assign:	ptroplab
  5.  * 
  6.  * System developed on:
  7.  * Ubuntu 7.10
  8.  * gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)
  9.  * using flag -masm=intel for intel syntax instead of AT&T.
  10.  * Eclipse Platform Version: 3.3.1.1
  11.  * 
  12.  */
  13.  
  14. #include <stdio.h>
  15.  
  16. /* All varieabls here are global so thay can be used in ASM. */
  17.  
  18. /* 'A'  I chose this value so it would be a printable character*/
  19. char c = 65; 
  20.  
  21. /* Max positive value of signed int. */
  22. int i = 2147483647;
  23. double d = 128.128;
  24.  
  25. /* holds value from the registers used for printing*/
  26. int int_holder1;
  27. int int_holder2;
  28. int int_holder3;
  29. int int_holder4;
  30.  
  31. int main(int argc, char* argv[])
  32. {
  33.  
  34. 	/* c is char 8 bits. moving a small value into a large or equal 
  35. 	 * register works fine 
  36. 	 */
  37. 	asm(
  38. 			".intel_syntax noprefix;"
  39. 			"mov al, c;" /* 8 bit low */
  40. 			"mov ah, c;" /* 8 bit high */
  41. 			"mov ax, c;" /* 16 bit */
  42. 			"mov eax, c;" /* 32 bit */
  43. 			"mov int_holder1, al;"
  44.  
  45. 			/* placed the values in the registers into int variables so the 
  46. 			 * register values can be printed to screen*/
  47. 			"mov int_holder2, ah;"
  48. 			"mov int_holder3, ax;"
  49. 			"mov int_holder4, eax;"
  50. 	);
  51.  
  52. 	printf("the value of c = %d\n", c);
  53. 	printf("moved value of i placed into al = %d\n", int_holder1);
  54. 	printf("moved value of i placed into ah = %d\n", int_holder2);
  55. 	printf("moved value of i placed into ax = %d\n", int_holder3);
  56. 	printf("moved value of i placed into eax = %d\n\n", int_holder4);
  57.  
  58.  
  59. 	/* moving a large value into a smaller register SHOULD not work.
  60. 	 * Why do these work? For some reason on my setup(see above) 
  61. 	 * "autocasting" is on  I don not have to use [size typte] ptr [var]
  62. 	 * I wonder if running this code in windows would work. 
  63. 	 */
  64. 	asm(
  65. 			"mov al, i;"			
  66. 			"mov ah, i;"			
  67. 			"mov ax, i;"			
  68. 			"mov eax, i;"
  69.  
  70. 			"mov int_holder1, al;"
  71. 			"mov int_holder2, ah;"
  72. 			"mov int_holder3, ax;"
  73. 			"mov int_holder4, eax;"
  74.  
  75. 	);
  76.  
  77. 	printf("the value of i = %d\n", i);
  78. 	printf("moved value of i placed into al = %d\n", int_holder1);
  79. 	printf("moved value of i placed into ah = %d\n", int_holder2);
  80. 	printf("moved value of i placed into ax = %d\n", int_holder3);
  81. 	printf("moved value of i placed into eax = %d\n\n", int_holder4);
  82.  
  83. 	/*However if i want to explicitly cast large value to small I can.
  84. 	 * I have no idea why this works.
  85. 	 */
  86. 	asm(
  87. 			"mov al, byte ptr i;"
  88. 			"mov ah, byte ptr i;"
  89. 			"mov ax, word ptr i;"
  90. 			"mov eax, dword ptr i;"
  91.  
  92. 			"mov int_holder1, al;"
  93. 			"mov int_holder2, ah;"
  94. 			"mov int_holder3, ax;"
  95. 			"mov int_holder4, eax;"
  96. 	);
  97.  
  98. 	printf("the value of i = %d\n", i);
  99. 	printf("Using ptr for cast");
  100. 	printf("moved value of i placed into al = %d\n", int_holder1);
  101. 	printf("moved value of i placed into ah = %d\n", int_holder2);
  102. 	printf("moved value of i placed into ax = %d\n", int_holder3);
  103. 	printf("moved value of i placed into eax = %d\n\n", int_holder4);
  104.  
  105.  
  106. 	/* Again moving large vales into small registers is done by auto 
  107. 	 * casting.
  108. 	 */
  109. 	asm(			
  110. 			"mov al, d;"			
  111. 			"mov ah, d;"			
  112. 			"mov ax, d;"			
  113. 			"mov eax, d;"
  114.  
  115. 			"mov int_holder1, al;"
  116. 			"mov int_holder2, ah;"
  117. 			"mov int_holder3, ax;"
  118. 			"mov int_holder4, eax;"
  119. 	);
  120.  
  121. 	printf("the value of d = %f\n", d);
  122. 	printf("moved value of d placed into al = %d\n", int_holder1);
  123. 	printf("moved value of d placed into ah = %d\n", int_holder2);
  124. 	printf("moved value of d placed into ax = %d\n", int_holder3);
  125. 	printf("moved value of d placed into eax = %d\n\n", int_holder4);
  126.  
  127. 	return 0;
  128. }

Here is my output.
(Toggle Plain Text) 
the value of c = 65
moved value of i placed into al = 65
moved value of i placed into ah = 0
moved value of i placed into ax = 65
moved value of i placed into eax = 65

the value of i = 2147483647
moved value of i placed into al = 255
moved value of i placed into ah = 255
moved value of i placed into ax = 65535
moved value of i placed into eax = 2147483647

the value of i = 2147483647
Using ptr for castmoved value of i placed into al = 255
moved value of i placed into ah = 255
moved value of i placed into ax = 65535
moved value of i placed into eax = 2147483647

the value of d = 128.128000
moved value of d placed into al = 106
moved value of d placed into ah = 188
moved value of d placed into ax = 48234
moved value of d placed into eax = -1821066134
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