Floating Point-How Important to Unix?
Ed G.
edgee at cyberpass.net
Sat Apr 11 12:40:35 AEST 1998
> I think that you will find that the compiler and assember always
> generate relative addressing for subroutines and jumps. Any call to an
> earlier address will generate a negative number, hence lots of 017xxxx
> numbers in the text image.
I am not an expert on PDP-11 op codes, so you may well be right about
this.
In response to your criticism, I looked up jmp and branch
instructions in the *Processor Handbook*. Based only on my quick
skim of the handbook, I don't think negative relative addresses would
be a problem because:
1. branch instructions are followed by a signed byte offset (-128,
127). This would not be a problem for my routine which only looks at
the first four bits of every word and would ignore the offset in the
odd byte.
2. jump instructions, which seem at first glance to be a problem
because they are followed by a 16 bit word, are not because they
always use absolute addressing, never relative and hence would never
be followed by a negative number.
Ed
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