Falsies

[Ken Turkowski]

In article <1341 at brl-tgr.ARPA> cottrell at NBS-VMS.ARPA (COTTRELL, JAMES) writes:
>@ In article <139200010 at uiucdcsb> robison at uiucdcsb.Uiuc.ARPA writes:
>@ >> ... But no machine I know of has a `branch if true' instruxion...
>
>No, *I* wrote that. *He* responded.
>
>@ The 808x/Z80 families of microprocessors set the 'zero' flag in 'compare'
>@ instructions, and have 'jump if zero', 'jump if non-zero', as well as the
>@ corresponding conditional subroutine calls and and conditional returns.
>
>Don't you think I know that? Doesn't everybody? You missed the point 
>entirely! What I am saying is that at the assembly language level there
>is no notion of pure true or false as a primitive condition.

Wrong you are.  Look at the 68000 instruction set.  In it you'll find a
couple of instructions called DBT and DBF, which are "decrement and
branch on true" and "decrement and branch on false".  While not a pure
one condition branch (because of the decrement and test counter), the
assembly language does have a concept of absolute truth.
-- 
Ken Turkowski @ CADLINC, Menlo Park, CA
UUCP: {amd,decwrl,hplabs,seismo,spar}!turtlevax!ken
ARPA: turtlevax!ken at DECWRL.ARPA