Right shift using iteration in LC3

Question

So I am working on a program in LC3 and I am having a issue with masking. I am fine with creating a mask of all 1's of the appropriate length, fine shifting everything to the appropriate field, but I can't for the life of me shift things back over to the right once I am done doing the AND comparison. How do you right shift in LC3? I saw something on iteration comparison, but I don't know how to compare bits in different positions, and to compare them in the same position I would have to shift one to the right eventually anyway. Am I missing something?

Answer 1

To simulate right shift, you just need to make two masks, one for source bit and one for destination bit:

src_mask=0x04;  // read from bit position 2 
dst_mask=0x01;  // write to bit position 0

while(src) {  // or while src_mask, because that too will eventually go to zero
   if (src & src_mask) dst+=dst_mask;  // or dst|=dst_mask
   src &= (~src_mask);
   dst_mask=dst_mask+dst_mask;
   src_mask=src_mask+src_mask;
}

Answer 2

在这种情况下，您可能根本不需要右移-只需保存起始字段，然后再向左移，然后再从内存中将其还原即可。