[HECnet] How long has your 20 been up?

[Johnny Billquist]

On 2022-01-20 22:42, Thomas DeBellis wrote:
> Greenwich Mean Daylight Savings Time?  I don't recall that I knew that; 
> I had always thought of GMT being meridian based and other times being 
> based off that, but GMT DST really would change the equation, 
> particularly for those areas that don't do DST.

Well. That is the problem. There is no GMT DST. But if you are in 
Greenwich, of course the apply DST just as the rest of England. So time 
in Greenwich don't match GMT. Which is why it becomes 
bad/strange/tricky/confusing.

> So UTC makes sense, although I do confess that I did chuckle over frog 
> fulmination.  I spent some time in Italy studying for my second 
> bachelors and while most Frenchmen struck me as straight stand up folk, 
> there were the /others/...  My brother and I were in an overnight train 
> from Naples to Trieste and had two Australians in the sleeper with us 
> (it held six).  We were having a grand old time laughing and yacking 
> until who knows how late when the Parisian simply reached up and turned 
> out the light.  And so we went to bed...

Some French are just tricky, while others are perfectly fine. :-)

> I did the maintenance for the DN60 code, which was PDP-11 assembler, so 
> I ought to know better, but I guess I have just gotten too large word 
> width centric (36 or 64 bits).  You have enough bits so you just about 
> never need to get clever for OS and relating systems code.  However, one 
> of my favorite courses at WPI was Numerical Methods (we called it 
> "Nums"), so I pay attention to fractions.

Yeah. With 36 bits, you face the problem much less often than with 16. 
Not to mention 64... :-)

> The carry on the PDP-11 is another thing I forgot about.  The Microsoft 
> C library of the late 1980's handled 32 bit numbers as having the low 
> order 16 bits in the accumulator (AX) and the high order in the data 
> register (DX). ADD AX,/foo/ and ADC DX, 0 was an extremely common 
> sequence and unremarkable sequence in 8086/80286 code.

That sequence is simple on a PDP-11, but that just adds a 16-bit number 
to the 32-bit result. It becomes trickier on a PDP-11 when you want to 
add two 32-bit values, as the ADC on the PDP-11 *only* adds the carry. 
That means you get two additions hitting the high 16 bits, which in turn 
means you are not getting a simple result for the condition codes out of 
the whole thing. Which makes it even uglier when you want to go for even 
more than 32 bits...

> What surprised me was that INC (add one) didn't set carry, so you 
> couldn't do an INC AX - ADC DX, 0 sequence which would have been 
> faster.  I never did figure out how that was great.

Same on the PDP-11. INC and DEC does not affect the carry. Seems a very 
conscious choice. Not sure I agree with it, but that's the way it is.

> It's probably silly, but one of the things I really liked about PDP-11 
> assembler were unnamed forward and backward relative jumps (I.E., $:).  
> Of course, you could really get yourself into trouble when you started 
> numbering them in hairy code.  At least I did.

Not only jumps. These local labels can be used for anything, and yes, 
they are nice to have.

   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