Correction below - thanks Paul for pointing it out. Sent from my iPhone
> On Dec 23, 2015, at 7:02 PM, Will Senn <will.s...@gmail.com> wrote: > > > >> On 12/23/15 5:09 PM, Johnny Billquist wrote: >> >> As for your analysis: >> Your explanation of branches seems somewhat over complicated. The >> instruction is indeed in just 8 bits, while 8 bits are the offset. >> However, there is no need to mess things up with one-complement, or tricks >> like that. The offset is an 8-bit value. Sign extend to 16 bits. Multiply by >> 2, and add to the updated PC. Simple as that. >> (Note that I said "updated PC". The PC will contain the address of the >> instruction after the branch before you start doing the calculation for the >> branch destination.) > Johnny, > > Given Line 7 037760 100376 BPL WAIT > > 100376 is a BPL instruction and the offset (376) is to be multiplied by two > and added to the updated PC. That is, 376 * 2 is going to be added to 037762. > 376 represents a negative offset in two's complement notation - it has a one > in it's most significant bit, the left hand bit in 11 111 110. As a human, I > kind of need the number in some form that makes sense to me, so I convert it > to a positive quantity by first taking the one's complement and adding one. > This gives me the positive magnitude of the negative number -1. Multiplying > this by two is easy, it's -2, which when added to 037762 yields 037760 , > which is the correct location. Negative number -2. Multiplying this number by two is easy, it's -4, which when added to the updated PC 037762 yields 037756, the correct location. > This is how I did the math. If I understand you correctly, the machine > doesn't do it this way. I tried multiplying 376, 11 111 110 by shifting > left one, 11 111 100, and adding that to 037762's binary representation, but > that doesn't seem to make sense. Would you please elaborate? > > Thanks, > > Will _______________________________________________ Simh mailing list Simh@trailing-edge.com http://mailman.trailing-edge.com/mailman/listinfo/simh
