[Simh] Writeup of PDP-11 bootstrap loader analysis

[Will Senn]

Minor correction and last one, apologies for the spam. 037760, not 
037756 is the address for the BPL instruction, fixed below.

On 12/23/15 10:30 PM, Will Senn 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,
>
> OK. I think I figured out how to think about the BPL. Given the 
> machine code:
>
> 037760  100376
>
> It is a BPL instruction to the updated PC + (2 * offset). The offset 
> is given as 376, in binary, sign extended 1 111 111 111 111 110, 
> shifting it left to multiply by 2 (there's a carry out that we can 
> ignore) results in  1 111 111 111 111 100. Adding the result to the 
> updated PC, 037762, results in binary 0 011 111 111 110 010 (there's 
> another carry out we can ignore). The math looks like this:
>
>   1 111 111 111 111 100 (2 * offset 376 sign extended)
> + 0 011 111 111 110 010 (PC = 037762)
> -----------------------
>   0 011 111 111 101 110 (037756)
>
> I can see that this is a better, simpler explanation and more in line 
> with how the machine processes the instruction.
>
> Thanks,
>
> Will
>