Re: Stealing a Great Idea from the 6600

On Thu, 25 Apr 2024 02:50:09 +0000, MitchAlsup1 wrote:

Lawrence D'Oliveiro wrote:
 

On Sat, 20 Apr 2024 18:06:22 -0600, John Savard wrote:

 

Since there was only one set of arithmetic instrucions, that meant
that when you wrote code to operate on unsigned values, you had to
remember that the normal names of the condition code values were
oriented around signed arithmetic.

 

I thought architectures typically had separate condition codes for
“carry” versus “overflow”. That way, you didn’t need signed versus
unsigned versions of add, subtract and compare; it was just a matter of
looking at the right condition codes on the result.

 
Maybe now with 4-or-5-bit condition codes yes,
But the early machines (360) with 2-bit codes were already constricted.

The DEC PDP-6, from around 1964, same time as the System/360, had separate
carry and overflow flags.