<nallen05@gmail.com> wrote:
+---------------
| On Mar 10, 3:14 am, rpw3@rpw3.org (Rob Warnock) wrote:
| > As an ex-*PDP-10*[1] assembler programmer,  ;-}
| > I'd probably go for this: ...
| >      (defun pack (b01 b234 b5678)
| >        (dpb b5678 (byte 5 5)    
| >          (dpb b234 (byte 3 2)
| >            b01)))
...
| > Or the more-imperative equivalent:
| >       (defun pack (b01 b234 b5678)
| >         (setf (ldb (byte 3 2) b01) b234)
| >         (setf (ldb (byte 5 5) b01) b5678)
| >         b01)
...
| > [1] See CLHS LBD & DPB:
| >       Historically, the name ``ldb'' comes from a DEC PDP-10
| >         assembly language instruction meaning ``load byte.''
| >       Historically, the name ``dpb'' comes from a DEC PDP-10
| >         assembly language instruction meaning ``deposit byte.''
| 
| OMG what does any of this goat barf mean? ;-) I guess I've got some
| reading to do...
+---------------

A good quick reference on PDP-10 byte stuff is here:

    http://pdp10.nocrew.org/docs/instruction-set/Byte.html
    Byte instructions
    In the PDP-10 a "byte" is some number of contiguous bits within
    one word. A byte pointer is a quantity (which occupies a whole
    word) which describes the location of a byte. There are three
    parts to the description of a byte: the word (i.e., address) in
    which the byte occurs, the position of the byte within the word,
    and the length of the byte.

    A byte pointer has the following format:  ...

The Common Lisp LDB & DPB functions [together with BYTE, which
makes byte specifiers, though reversing the order of P & S]
were modelled on the PDP-10 byte instructions, though providing
a functional interface rather than a purely imperative one.

Very useful for low-level bit-banging of hardware registers
and protocol blocks...


-Rob

-----
Rob Warnock			<rpw3@rpw3.org>
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