3 Nov
2010
3 Nov
'10
12:36 p.m.
On 2010-11-01 00:02, ard at p850ug1.demon.co.uk (Tony Duell) wrote:
Cool. So you could add to it, index by it, and so on?
Indeed sounds nice. When did the machine appear?
Sorry. I was just being lazy, and trying to explain condition codes by
referring to what they are on the PDP-11.
To try and be more specific then: condition codes are bits that are
set/reset as a result of operations performed by the processed, and upon
which you can the make conditional branches/jumps on.
As opposed to, for example, a PDP-10, where you instead encoded the
condition within the instruction, along with a register to test upon.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
> On
2010-10-30 01:12,ard at p850ug1.demon.co.uk (Tony Duell) wrote:
>
> > >
>>> > >> > I have not seen anyone comment any of the other
things I listed as
>>> > >> > possible firsts on the PDP-11.
>>> > >> > Can anyone come up with an earlier machine that used
condition codes?
>>> > >> > How about general registers with addressing modes,
which is totally
>>> > >> > orthogonal? How about having the PC as a general
register?
> > > The Philips P800 series has the PC as a general register (register 0).
>
> Could you use it like any other register?
There were some restrictions. I
don't think you could shift it (at least
ont on the P850). But for many instructions it was just another register. > > >
There are 16 registers, some instructions can only use the first 8, others
> > > can use all 16. Addressing modes (simpler than the PDP11, I admit) are
> > > pretty much orthogonal.
>
> Sounds like the registers were atleast not as orthogonally used as on a
> PDP-11. If an instruction could take a register, it could take any
> register. And all addressing modes are valid (well, almost) anywhere.
I
think all P800 instructions that had addressing modes could use any
addressing mode. Any instruction with 4-bit register fields could use any
register. Any instruction with a 3 bit register field could use any of
the first 8 registers. And IIRC, like the PDP11, the addressing modes
commonly known as 'immediate' and 'absolute' were a couple of the other
addressing modes with the register specified as 0 (=PC, of course).
I guess the P800 wasn't totally ortogonal but it was a lot more
orthogonal than many other machines. > > >
What do you mean by condition codes here?
>
> The four low bits of PSW.
Err... I don;t think that's helpful. Quite a
lot of machines with a
status register have a 'low 4 bits' of it:-). But that doens't make them
condition codes. Similarly uf you hapopen to implement the same
functionality using other bits of a status registers, doesn't that make
them condition codes?
What I was asking was what fucntionality do you require of these
condtiion codes other than there being conditional jumps on carry, zero, etc? 
5 Nov
5 Nov
2:07 p.m.
> > > The Philips P800 series has the PC as
a general register (register 0).
>
> Could you use it like any other register?
There were some restrictions. I
don't think you could shift it (at least
ont on the P850). But for many instructions it was just another register. I guess the
P800 wasn't totally ortogonal but it was a lot more
orthogonal than many other machines.
Indeed sounds nice. When did the machine appear? > >
> What do you mean by condition codes here?
>
> The four low bits of PSW.
Err... I don;t think that's helpful. Quite a
lot of machines with a
status register have a 'low 4 bits' of it:-). But that doens't make them
condition codes. Similarly uf you hapopen to implement the same
functionality using other bits of a status registers, doesn't that make
them condition codes?
What I was asking was what fucntionality do you require of these
condtiion codes other than there being conditional jumps on carry, zero, etc?
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