bit-manipulation

I am trying to create a 48-bit integer value. I understand it may be possible to use a char array or struct, but I want to be able to do bit masking/manipulation and I'm not sure how that can be done. Currently the program uses a 16-bit uint and I need to change it to 48. It is a bytecode interpreter and I want to expand the memory addressing to 4GB. I could just use 64-bit, but that would waste a lot of space. Here is a sample of the code: unsigned int program[] = { 0x1064, 0x11C8, 0x2201, 0x0000 }; void decode( ) { instrNum = (program[i] & 0xF000) >> 12; //the instruction reg1 = (program[i]

This most likely has been asked and answered before, but my searches was futile. Question is about bits, bytes masks and checking. Say one have two "triggers" 0xC4 and 0xC5 : 196: 1100 0100 0xc4 197: 1100 0101 0xc5 The simple way of checking if var is either would be: if (var == 0xc5 || var == 0xc4) { } But sometimes one see this (or the like): if ( ((var ^ magic) & mask) == 0) { } My question is how to find magic and mask . What methods, procedures, tricks etc. is to be utilized to form these values and to assert if any exists? EDIT: To clarify. Yes, in this exact example the former would be

I'm trying to use unions to obtain the bit representation of float values, my code is currently as follows: union ufloat { float f; unsigned u; }; int main( ) { union ufloat u1; u1.f = 3.14159f; printf("u1.u : %f\n", u1.u); However anything I try to print gets printed as 0.0000000, instead of as bits (such as 0001 0110, or something similar), what is wrong in my code? Note that preferebly I would like to use unions to achieve this. There are a large number of ways to accomplish this. Understand that what you are really trying to do is simply output the bits in memory that make up a float .

I was looking at the below bit reversal code and just wondering how does one come up with these kind of things. (source : http://www.cl.cam.ac.uk/~am21/hakmemc.html ) /* reverse 8 bits (Schroeppel) */ unsigned reverse_8bits(unsigned41 a) { return ((a * 0x000202020202) /* 5 copies in 40 bits */ & 0x010884422010) /* where bits coincide with reverse repeated base 2^10 */ /* PDP-10: 041(6 bits):020420420020(35 bits) */ % 1023; /* casting out 2^10 - 1's */ } Can someone explain what does comment "where bits coincide with reverse repeated base 2^10" mean? Also how does "%1023" pull out the relevent

I need to create a method in C using bitwise operations which checks if x + y will overflow or not. I can only use a maximum of 20 of the following operations; ! ~ & ^ | + << >> Keep in mind I have to test for both negative and positive numbers. I've tried several times to make it work. Is my logic sound? I'm going by: if (x + y) is less than x, then it has overflowed. Based on that logic, I wrote this; int addOK(int x, int y) { int sum = x + y; int nx = ((~x) + 1); int check = (sum + nx)>>31; return !check; } Thank you! This should work, but it doesn't use only bitwise operator, but it work