[Info-vax] Bliss was Re: Learning VMS application programming

[Johnny Billquist]

On 2014-09-11 21:52, glen herrmannsfeldt wrote:
> Johnny Billquist <bqt at softjar.se> wrote:
>
> (snip, someone wrote, regarding Macro-10)
>
>>> For me, I always found it funny that the instruction set had a
>>> jump instruction (spelled JUMP) that did not jump.
>>> I never understood why that form (other than just simplicity to
>>> implement in hardware).
>
>> Well, the JUMP doing no jump is totally in line with the instruction
>> set as a whole, and is very consistent. If you look at the opcodes
>> it is pretty obvious.
>
> Well, one question is why the hardware has the operation, and the
> other why the opcode has that name.

Right.

> The Motorola 6809 also has a full set of conditional branches:
>
> http://www.lomont.org/Software/Misc/CoCo/Lomont_6809.pdf
>
> but instead they have BRA (branch always) and BRN (branch never).

The PDP-10 just took another view on it, but equally meaningful. 
Instructions which are conditional have the condition appended to the 
end of the mnemonic. If there is no condition, it never is true.

The JUMP that always jump, is then logically JUMPA.
Same for all the others.

>> There is not only JUMP, you also have SKIP (which do not skip),
>> and so on.  Any conditional instruction have the "never" condition.
>> The PDP-10 is very consistent.
>
> But even more, no-one uses JUMPA but JRST instead.

Just tells your age... ;-)
JRST was the norm back from the KA and the KI (unless I misremember). On 
the KL JRST was no faster than JUMPA, as far as I recall.

	Johnny