Re: a bit of history, Stealing a Great Idea from the 6600

According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:

John Levine <johnl@taugh.com> writes:
[only 2 condition code bits]
The architecture paid for the reduction in branches with an increase
in other instructions; S/360 needed signed and unsigned versions of a
number of instructions, e.g.,
>
signed unsigned
A      AL
AR     ALR
S      SL
SR     SLR
C      CL
CR     CLR

There's more differences than the condition codes. A, AR, S and SR
trap on overflow, AL, ALR, SL and SLR don't. It is my impression that
trapping on fixed point overflow is not very useful, and it's easier
to do a jump on overflow in the few cases where you care, or the x86
INTO which you put after the arithmetic operaion to trap if the
overflow flag is set.

IBM then continued in their tradition by having < = > and sticky
overflow flags in Power (and a separate carry, sticky overflow and
overflow flag); this also requires signed and unsigned compares (cmp
and cmpl along with variants).

There's a fairly direct line from S/360 to ROMP to POWER so it's not
surprising.  ROMP had < = > carry overflow.

--
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John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
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