Re: Can't Avoid That Shit Rust - Even On Gentoo

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

On 6 Oct 2024 06:26:32 GMT, rbowman wrote:

... as I recall you could twiddle octal locations in the opcodes.

Sounds PDP-11-ish. The instruction layout was such that, if an instruction
was expressed as an integer, register numbers and addressing modes fitted
neatly into particular octal digits.

Not sure if that was true of the PDP-10. The later VAX went all hex, of
course.

The PDP-10 instruction layout is

    1 1 1  1 1 2         3    3
0......8 9..2 3 4..7 890123456789012345
op code   AC  I  IX   address offset

All represented in octal.  The HALT instruction (a variant on the JRST) is a
good example:

HALT RESUME(17) ;RESUME is a symbolic address, let's say 654321

The assembly representation is

254 04 0 17 654321

The in-memory representation is 254217654321.  Patching from the front panel of
a 166, KA-10, or KI-10 processor is simple enough, if you know where your program
lives in memory (not a given, since the entire line is intended for multiuser
usage).

--
Rich Alderson   news@alderson.users.panix.com
      Audendum est, et veritas investiganda; quam etiamsi non assequamur,
  omnino tamen proprius, quam nunc sumus, ad eam perveniemus.
--Galen