On 10/27/10, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
I found that
octal on the PDP-11 with 16-bit words and 8 bit bytes
was unnecessarily confusing.
Except that the many of the PDP11 instructions are easy to decode if
written in octal (4 bit opcode, 3 bit addressing mde, 3 bit register
select, 3 bit addressing mode, 3 bit register select). I would hate to
have to work with PDP11 machine code in hex...
Agreed. When I first started working with PDP-11s, my supervisor
asked me for a PDP-11 program that would read the "window" register on
our product then write it back to the same address and loop (it was a
write-only register for each side - what each processor read was what
the other had previously written). I thought a second and recited,
"1000 slash 13700 lf 177300 lf 10037 lf 177300 lf 137 lf 1000 enter
1000 G" (or something substantially similar to that). He got annoyed
and told me to go sit down and write him a program. I told him to
turn around and type in what I was telling him. On the third
recitation, he started typing. Being two moves and a jump, it worked
the first time. He was gobsmacked that it was possible for a human to
spontaneously emit meaningful octal.
Given the spacing of the addressing modes, I wouldn't have wanted to
try it in hex without writing it down.
-ethan