I'm trying to create a program to add two IEEE 754 floating point numbers, I'm pretty far into the program but I've come to a halt in the bit shift section of the algorithm.

I have created a tmp variable to copy the mantissa(0's and 1's inside the char array to be shifted to the right). Then I copy the tmp variable into the previous variable in the spots needed... Any help or suggestions will be appreciated... here is my code that needs reworking.

``````if( e1 > e2 ){ // If exponent 1 is greater than exponent 2 shift exponent 2
k = e1 - e2;  // k = size of shift
for(i = 11; i < 40; i++)  // for loop for original array
for(j = 1; j < 30; j++)  // for loop for tmp array
tmp_bit_string[j] = bit_string2[i];  // copy original into tmp

*tmp_bit_string >>= k; //shift tmp

for(i = 11; i < 40; i++) //second loop to copy tmp back into original
for(j = 1; j < 30; j++)
tmp_bit_string[j] = bit_string2[i];
}

if( e2 > e1 ){  //same thing but opposite of previous loop
k = e2 - e1;
for(i = 11; i < 40; i++)
for(j = 1; j < 30; j++)
tmp_bit_string[j] = bit_string[i];

*tmp_bit_string >>= k;

for(i = 11; i < 40; i++)
for(j = 1; j < 30; j++)
tmp_bit_string[j] = bit_string[i];
}``````

Oh jeez, this is entirely the realm of assembly language. Why don't you use asm{} to get this done in like 15 lines of code or less? You can take advantage of the CPU zero and carry flags in byte shifts there.

## All 3 Replies

Oh jeez, this is entirely the realm of assembly language. Why don't you use asm{} to get this done in like 15 lines of code or less? You can take advantage of the CPU zero and carry flags in byte shifts there.

yeah, its for the class computer architecture... fun stuff.

but the teacher is allowing us to only use the shift and integer add operations in c to do the addition, so i dont think using asm would be allowed...

Not sure why it wouldn't be allowed in a class named for computer architecture, a realm completely dominated by MOSFETs and flip-flops, yet your limited to C of all things.

Anyways, while I am not entirely sure what you are asking, here is how I would bit shift across 2 chars:

``````unsigned char H, // High byte
L, // Low byte
T; // Temp byte

// To shift both of them to the left...

T = L & 0x80; // Grab the upper bit
// (to compensate for C not having a carry flag)
L <<= 1;      // Shift the low byte left by 1 bit. Lowest bit is now zero.
H <<= 1;      // Shift the high byte left by 1 bit.
H |= (T>0);   // If T is not zero, the expression
// will evaluate to 1. So this merges
// The high bit shifted by L into the low
// bit in H which should is currently zero.

// At this point, both H and L have been shifted left
// as one unit. You can do this across as many bytes as
// you need. You can optionally save the highest bit
// before it gets eaten up by the shifts by performing
// this test before the left shift operation...

int LostBit = ((H & 0x80)>0);

// Right shifting is about the same thing, only the
// opposite operations.

T = H & 0x01;   // Save the lowest bit of the highest byte
H >>= 1;        // High byte shift right
L >>= 1;        // Low byte shift right
L |= (T==0)?(0):(0x80);  // If the saved byte is not zero,
// merge H's lowest bit to L's
// highest bit.``````

Hope this helps you find a solution to your issue.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts learning and sharing knowledge.