Work Programming Misc Hex/Bin/Dec Conversions and Methods
BitMasks
#Bits Bit Pattern Hex Number
1 1 0x1
2 11 0x3
3 111 0x7
4 1111 0xf
5 11111 0x1f
6 111111 0x3f
7 1111111 0x7f
8 11111111 0xff
9 111111111 0x1ff
10 1111111111 0x3ff
11 11111111111 0x7ff
12 111111111111 0xfff
0 and 0 = 0 0 and 1 = 0 1 and 0 = 0 1 and 1 = 1 0 or 0 = 0 0 or 1 = 1 1 or 0 = 1 1 or 1 = 1 0 xor 0 = 0 0 xor 1 = 1 1 xor 0 = 1 1 xor 1 = 0 |
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BitShifts
To extract a field from an integer called, for instance, "instruction", and place it into the variable "field": 1) Three-bit field from bit 12 to bit 14, assuming that the first bit is bit 0. field = (instruction >> 12) & 0x7; This shifts the bits in "instruction" down by 12 bits, then uses a mask to zero out everything but the bits in the lowest three places (bits 0-2). After the shift, bits 12-14 are in locations 0-2. 2) Bits 8-11: field = (instruction >> 8) & 0xf; 3)Bits 2-7: field = (instruction >> 2) & 0x3f; Bits 13-15: field = (instruction >> 13) & 0x7; To insert a field into the integer called "instruction": 3-bit quantity to bits 10-12. instruction = (field & 0x7) << 10; The and ensures that "field" contains a 3-bit number (by zeroing out any other bits), and the shift puts the three-bit field into place. The variable "instruction" now contains only the three-bit field now in bits 10-12. To construct instructions out of many fields: Suppose we have a 3-bit opcode stored in the variable op, we have a 3-bit register stored in the variable r1, and we have another 3-bit register stored in the variable r2. Say we want them (respectively) in bits 15-13, 12-10, and 9-7. We would build up the instruction as follows: field = (instruction >> 8) & 0xf; instruction = ((op & 0x7) << 13) | ((r1 & 0x7) << 10) | ((r2 & 0x7) << 7); The ands ensure that each variable is only three bits wide, by masking out all other bits. The left-shifts put the values into place, according to the desired location. Since there is no overlap between the fields (we have ensured this by masking out any possible extra bits), the ors simply concatenate the fields together. (Taken from http://www.ee.umd.edu/courses/enee350h.F97/) |