Re: except what, is Vax addressing sane today

John Levine wrote:

According to MitchAlsup1 <mitchalsup@aol.com>:

In the days before <good> branch prediction having a conditional branch
after each instruction that could have an execution problem was an
extremely poor choice. Thus, exceptions were invented (circa 1958).

Before (good) branch predictors and exceptions, we did have the ability to fall through any forward branches, and assume backwards branches were taken, right?
With that approach, compilers were free to place all recovery code after the function itself, in which case something like
   add,ax,bx
    jo overflow_detected
   ;; ax is now good
would work quite well, i.e. costing "only" 8 cycles for the load of the two JO instruction bytes.

 Oh, it was worse than that.  There were instructions like "Divide or
Halt" which stopped the computer with an error light on a zero divide.

Sort of like "Halt_And_Catch_Fire"?

 

Many (most, nearly all) processor architectures have notoriously
bad exception delivery to a point of control that can deal with
the problem at hand.

 Some of us remember imprecise itnterrupts and the OS/360 S0C0
completion code.
 But you are in general right, it makes more sense to keep the computer
running in the normal case and provide slow ways to recover from
failures and do something else.
 

This was idea behind the single-byte INTO (Interrupt on Overflow) opcode:
   add ax,bx
   into
would cost just 4 clock cycles vs the 8 needed for the forward exception handler.
OTOH, you did need to either reload the INTO vector (same location as INT 4) every time you needed a possibly new/different handler.
I never saw any compiler using it and I never used ti in my own asm code.
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"