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OPEN-QUESTIONS.md

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Open Questions

Architecture

  1. Are registers N, P, Q, K, F, FA memory-mapped? (I have found no evidene of this being the case so far) (Note that configuration value 10 of the JMP instruction causes the value in the Q register to be saved in L(E), so the Q register must be emulated, see #149).
  2. If a program manually raises the flag for sequence 0, from where does sequence 0 begin executing? (See also the question about the TSP and index register 0, below).
  3. Was the U-memory ever fitted?
  4. If "trap on sequence change" is active and we change to sequence 41, is the TRAP sequence (42) activated? If so, what will it see in the E register when it begins to run?
  5. What sets the CODABO start point registers 03777710-03777717? Or are those the actual addresses (i.e. would execution begin at 03777710 rather than [03777710])?
  6. What constitutes an invalid address for the purposes of the PSAL alarm? For example, the gap between U-memory and V-memory? Does PSAL fire when P points to a location in S-memory but the S-memory is disabled on the console? Similarly for the other memories which can be disabled (T, U, V).
  7. If I write to a location which is in practice read-only (e.g. non-flip-flop V-memory, such as the plugboard or the shaft encoder register) does an alarm occur?
  8. If I use the MKC instruction to set the meta bit of an arithmetic unit register (register A for example), this apparently just sets the meta-bit in the M register (User guide page 5-23). What is the practical significance of this? Is this event detectable by the programmer?
  9. Are there any writable locations in the V-memory other than registers A, B, C, D, E? What are they, how do they behave?
  10. Is the plugboard which configures sequence priorities memory-mapped? If yes, how does it appear?
  11. If you press "RESET 0" and the defer bit is set in the "RESET 0" location, does a deferred addressing cycle occur (similarly for resets 1-7)?

Instructions

  1. Was the XEQ instruction implemented? This appears as a hand-written note on the opcode table of my scanned copy of the User handbook, apparently as opcode 2. But the document doesn't describe that instruction. Sketchpad does not appear to use it.

Start-Up

  1. Does it matter what the N register holds at start-up (e.g. after CODABO has been pressed)? It presumably won't matter if the hold bit is set or not, since the active sequence is 0.
  2. The manual states that "flip-flops" are reset by CODABO. Does this include zeroing all the flip-flops in V memory? Does this include registers N, P, Q, K, F, FA?

Peripherals

  1. The description of the audio system describes the existence of a microphone. Can its signal be read by a program?
  2. Is there surviving documentation on the magnetic tape unit? (It is not included in the edition of the User Handbook I have).
  3. Is there surviving documentation on the plotter? (It is not included in the edition of the User Handbook I have).
  4. If a TSD instruction causes "Dismiss and Wait", (because the buffer is busy for example) and the meta bit of the location which would have been used is set, and the trap circuit is set to "trap on metabit set on operand", does the flag for Unit 42 get raised?

Assembly Source

  1. How is a not-held instruction (overriding the default for LDE, ITE, JPX, JNX) indicated in the symbolic assembler input? I was unable to find an example.
  2. Is there a difference between the configuration values 34 and -3? Both seem to occur in symbolic code (e.g. the examples memo by H. Philip Peterson, 6M-5780, July 23, 1958).
  3. In the pipe construct "Z ₚ|ₜ Q" described in the User Handbook section 2-2.8 "SPECIAL SYMBOLS", are spaces allowed in the "ₚ" or "ₜ"?
  4. Symexes used only for index register values (User Handbook section 6-2.2) are assigned as "The lowest numerical index register value not already used". What does "used" mean? We currently assume it means "not already default-assigned", meaning that the assembler does not need to track which index registers are used via explicit equalities or numeric literals. (Otherwise, use of index register 0o52 would mean that the first value to be default-assigned would be 0o53, which isn't very helpful).
  5. In the Kleinrock network simulator source, why does the RC-word from the instruction assembled at 200150 end up first in the RC-block (at 206325), despite the fact that lots of other RC-word references (e.g. the one at 200003) precede it? In fact the words assembled at 200147 and 200151 also have RC-word references which are assigned in the obvious way.
  6. What did M4's ☛☛DEMO meta-command do?