[Simh] PDP-10 UUOs [was Re: pdp7/decsys bug?]

[Rich Alderson]

> Date: Sun, 03 Jun 2007 12:34:35 -0400
> From: Davis Johnson <davis at frizzen.com>

> Robert Armstrong wrote:

>>   TSS/8 uses unimplemented IOTs (i.e. 06xxx opcodes) as monitor calls.  With
>> the PDP-8 User Mode flag set, _all_ IOTs trap to the OS whether the hardware
>> actually implements them or not, and the TSS/8 designers simply picked some
>> unimplemented IOTs as the official way to call monitor functions.  Is that
>> what you're seeing??

> Didn't TOPS 10 do somthing similar? I remember from long time ago when I 
> was doing MACRO-10 that system calls were UUO (Unimplemented User 
> Opcode) instructions.

This is a feature of the PDP-6/PDP-10 architecture:  "Unimplemented User
Operations" (originally called "UnUsed Operations") trap to a defined location
in memory (user address space 40 & 41 for opcodes 001 to 037, exec address
space for opcodes 040 to 077 and any UUOs 100 and above).  Opcode 000 is always
an illegal operation; other opcodes can invoke specialized handlers.

Tops-10 (DEC) and ITS (MIT) both use the 040-077 range for system calls,
although they do not agree on which opcode invokes what function.  TENEX (BBN)
and Tops-20 (DEC, based on TENEX) use opcode 104 ("Jump to SYStem", or JSYS)
instead of assigning different functions to different opcodes.

I think that the TSS/8 developers were familiar with the PDP-10.

                                                                Rich Alderson