Re: Polysemy in programming langs (was: Why does the meaning of words change?)

Ph. D. scripsit:

> As I recall, the shift was always one bit, but you could
> specify shifting only the AC or the AC and "link" bit as
> a unit.
Nope, not the PDP-8.  The assembler mnemonics were RAL/RAR (rotate AC
and link left/right) and RTL/RTR (rotate AC and link twice left/right).
The actual bitmap was:

bits 0-2: 111 (OPR)
bit 3: 0 (for Group 1)
bit 4: CLA (clear AC)
bit 5: CLL (clear link)
bit 6: CMA (complement AC)
bit 7: CML (complement link)
bit 8: rotate twice
bit 9: rotate left
bit 10: rotate right
bit 11: increment AC

The operations were done in left-to-right order: first clear, then
complement, then rotate, then increment, as required by the bits.
The effect of setting both the rotate-left and the rotate-right bits
was processor-dependent, as was the effect of setting the rotate-twice
bit without either of the other rotate bits.

Joe scripsit:

> How do you logically AND one memory location?  AND adjacent bits? (ie.
> 10111010 > (0)001100)
No, the AND instruction always used the AC as one of the operands, the
referenced memory instruction as the other, and put the result in the AC.

Memory reference was also interestingly restricted: memory was divided
into 128-word pages, and all memory references were either to the current
page or to page 0.  A reference, however, could be direct or indirect:
if indirect, the referenced location was a full 12-bit pointer to the
true location.  This allowed the packing of memory-referencing instructions
into 12 bits: 3 bits of operand, a direct/indirect bit, a current page/
page zero bit, and 7 bits of page-relative address.

ObConlangSorta:  it's been pointed out to me that the hymn is "Gladly
the Cross I'd bear", not "Surely"; I was crossing this mondegreen with
"Shirley, good Mrs. Murphy [for "goodness and mercy"], will follow me
all the days of my life."  For lots more on this topic, google for
"mondegreen".

--
Deshil Holles eamus.  Deshil Holles eamus.  Deshil Holles eamus.
Send us, bright one, light one, Horhorn, quickening, and wombfruit. (3x)
Hoopsa, boyaboy, hoopsa!  Hoopsa, boyaboy, hoopsa!  Hoopsa, boyaboy, hoopsa!
  -- Joyce, Ulysses, "Oxen of the Sun"       jcowan@reutershealth.com

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