8 Feb
2022
8 Feb
'22
12:06 a.m.
On 2022-02-07 23:51, Robert Armstrong wrote:
The PDP-8 also have a carry. It's called the LINK register, which is
just one bit. But it's not flags in the same way or sense as on a PDP-11.
Not sure about the PDP-10, but I believe it's more similar to the PDP-8
than the PDP-11.
One thing about the condition code flags on the PDP-11 is that not only
arithmetic instructions modified them. A simple MOV for example, also
affects them. Basically anything that operates on data will affect the
condition codes. But sometimes in a weird way. As have been mentioned
before, MOV leaves the carry untouched, which is also the case for INC/DEC.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
The PDP-10 is
interesting (or strange) in that it uses skips to convey
the result of various tests and comparisons.� You don't use tests to set
PC flags and do conditional jumps on them (like nearly every other
architecture that comes to mind immediately).
It's worth pointing out that the PDP-10 _does_ have PC flags and instructions to
test them (just check my domain name!) but they're not used for things like
zero/negative tests. There is a carry flag of sorts (the APR overflow bit) but it was
almost never used. With 36 bits you didn't worry much about overflows :)