Subject: A Show of Hands, Please!

From: [email-masked] (Jym Dyer)
Date: 22 Sep 92 10:24:57

=o= Who else out there kept seeing the popular abbreviation
for _Saturday_Night_Live_ and thought to themselves, "Skip on
Nonzero Link?"
    <_Jym_>

---

From: [email-masked] (Charles Lasner)
Date: Tue, 22 Sep 1992 18:32:02 GMT

>for _Saturday_Night_Live_ and thought to themselves, "Skip on

I certainly did.  SNL was Skip on Nonzero Link years before.  Also, Saturday
Night Live isn't the name of the show, it's just Saturday Night.  The SNL
buzzword came about later as the critics described the show, and also to
diffentiate it from the Howard Cosell show aka Saturday Night.  But, in any
case, the show is now, and always has been "live from New York, it's
Saturday Night!", not "from New York, it's Saturday Night Live".  The live
was merely the emphasis on the fact that in a virtually all-taped network
era, this was a live show, as in "It's on Saturday (late) Night!  (And it's
not just a Carson re-run.) Also, "It's live!"  (Unlike taped Carson shows
that are taped the same day for replay a few hours later in case they
screw up.)

Today, the "live" part is no novelty, considering shows like Leno, or even
Fox's Roc.  When it came out in the '70's, it did mean something though, and
the critics recognized that, so the (incorrect) abbreviation and name have
stuck.

In any case, SNL goes back to 1962 and the original plans for the PDP-5.

Here's an alternate story about 12-bit mnemonics:

There is an instruction on the LINC which is 0455 which is skip on the
low-order bit of the LINC's version of the MQ register being clear.  The
reason for such an instruction is the somewhat quirky way the DP instructions
are implemented on the LINC (and also an 11-bit multiply, etc.).  You need to
retrieve that bit to correct other values, etc., so it's clearly useful.

The LINC numbers the bits in a very un-PDP-8 way:

L 11 10 9 8 7 6 5 4 3 2 1 0

as opposed to PDP-8 standard of:

L 0 1 2 3 4 5 6 7 8 9 10 11

So, the reference is to bit[0], not bit[11].  The original name for the
register in question is the Z register, later renamed in the PDP-12 as the
Q register.  Of course, all bit references in the LINC-8 and PDP-12 are
PDP-8-style.

The original mnemonic for this instruction is ZZZ meaning skip on the
Z register bit Zero being Zero.

The replacement mnemonic for the same instruction as it applies to prevailing
LINC-8 and PDP-12 systems is QLZ meaning Q register Low-order is Zero is what
to skip on.

LINC mnemonics in general don't start with S for skip.  The stated condition
is what to skip on, and appending I to the instruction (which OR's in 0020,
not 0400 as in PDP-8 and means an entirely different thing) makes the
sense of the skip reverse, thus ZZZ I is skip if the low-order Z register
bit is set, not clear.

The LINC variants for our original example are LZE for skip on Link is ZEro
and to exactly replicate it LZE I for skip on Link ZEro Isn't true.

Note that in the LINC-8, the AC and LINC and Z register in question are part
of a separate CPU.  From the PDP-8's point of view, the LINC A register or
accumulator is a peripheral register, as is its memory buffer or B register,
and there are I/O instructions provided to access all of these registers.

In the PDP-12, there is only one set of registers, and the operations 
refer to the PDP-8 accumulator and MQ register, but the arithmetic operations
are identical to the original (separate) LINC processor operations in the
LINC-8's LINC CPU.  When the LINC CPU is running, the PDP-8 is operating in
100% DMA grant mode, and the BREAK light glows brightly.  The operation that
puts the PDP-8 into this mode has 0012 in the AC at the time.  Invariably
the lamps for bit[8] and bit[10] of the AC are blown out in LINC-8 systems.

To handle dual mode assembly of these distinctly different operations,
there are several assemblers for different systems such as OS/8's
user-written PAL12, P?S/8's PAL with the /8 or /9 switch enabled, LAP6-DIAL's
assembler for the PDP-12, and a PDP-10-based cross-assembler known as DIAL-10
(which is a "cousin" of PAL-10).

All of these systems use assembler directives PMODE and LMODE to change over
the assembler mode of the moment to suit either PDP-8 or LINC requirements:

	PMODE

	SNL

	LMODE

	LZE I

This would assemble the correct binary values for our mnemonics, but won't
run unless surrounded by the appropriate control instructions, but that's
an entirely different topic, etc. (PDP-12 needs a mode-change instruction
while LINC-8 needs a major amount of register initialization culminating with
a LINC CPU start operation, etc.)

cjl