# Summary of known differences and limitations for w11a CPU and systems

This file descibes differences and limitations of the w11 CPU and systems.
The issues of the w11 CPU and systems are listed in a separate document
[README_known_issues.md](README_known_issues.md).

### Table of content

- [Known differences between w11a and KB11-C (11/70)](#user-content-diff)
- [Known limitations](#user-content-lim)

### <a id="diff">Known differences between w11a and KB11-C (11/70)</a>

- the `SPL` instruction in the 11/70 always fetched the next instruction
  regardless of pending device or even console interrupts. This is known
  as the infamous _spl bug_, see
  - https://minnie.tuhs.org/pipermail/tuhs/2006-September/002692.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002693.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002694.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002695.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002701.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002695.html
  - https://minnie.tuhs.org/pipermail/tuhs/2006-October/002702.html

  In the w11a the `SPL` has 11/70 semantics in kernel mode, thus next no 
  traps or interrupts, but in supervisor and user mode `SPL` really acts as 
  `NOOP`, so traps and interrupts are taken as for all other instructions.   
  **--> The w11a isn't bug compatible with the 11/70.**
- A 'red stack violation' looses PSW, a 0 is pushed in stack.
- The 'instruction complete flag' in `SSR0` is not implemented, it is 
  permanently '0', `SSR2` will not record vector addresses in case of a
  vector fetch fault. Recovery of vector fetch faults is therefore not
  possible, but only 11/45 and 11/70 supported this, no OS used that, and
  it's even unclear whether it can be practically used.
- the 11/70 maps the 18 bit UNIBUS address space into the upper part of
  the 22bit extended mode address space. With UNIBUS mapping enabled, this
  allowed to access via 17000000:17757777 the memory exactly as a UNIBUS
  device would see it. The w11a doesn't implement this remapping, an access
  in the range 17000000:17757777 causes a NXM fault.

All four points relate to very 11/70 specific behaviour, no operating system
depends on them, therefore they are considered acceptable implementation
differences.

### <a id="lim">Known limitations</a>

- some programs use timing loops based on the execution speed of the
  original processors. This can lead to spurious timeouts, especially
  in old test programs.  
  **--> a 'CPU throttle mechanism' will be added in a future version to 
  circumvent this for some old test codes.**
- the emulated I/O can lead to apparently slow device reaction times,
  especially when the server runs as normal user process. This can lead
  to timeout, again mostly in test programs.  
  **--> a 'watch dog' mechanism will be added in a future version which
  suspends the CPU when the server doesn't respond fast enough.**
