PDP-8 Simple program V1

README.md

Scrolling MQ

A simple program for PDP-8/e. It should work on other PDP-8s, but the Extended Arthmetic Element is required as it uses the MQ register. It creates a scrolling effect on the MQ (Multiplier Quotient) by left shfiting it, after a given time.

How to run

• Set switches to 0000 and press LOAD ADDR
• Toggle in the 2nd column of the below "binary", pressing DEP (Deposit) after each one
• Set switches to 0002 and press LOAD ADDR
• Set indication selector to MQ
• Set switches to 7777, but do not press LOAD ADDR
• Press CLEAR, followed by CONT

How to enter values

The "binary" file is in octal notation. Each number specifies the position of 3 consecutive switches, like so:

Number	Switches
0	000
1	001
2	010
3	011
4	100
6	101
6	110
7	111

Remarks

The value of the SR (Switch Register) determines how long it takes for the MQ light to move. The smaller the value the less time it will take

assembly

CAC,  0             / Curent AC
NCAC, 0             / Negated current AC
      CLA CLL IAC   / Set AC = 1
      MQL           / MQ is now 1
LOOP, DCA CAC       / Despoit but don't clear AC
      TAD CAC
      CMA IAC       / Negate AC
      DCA NCAC      / Deposit before OSR
      OSR           / Get switches
      TAD NCAC      / SR - AC
      CLA SNA SZL   / Skip if AC != 0 and CLA
      JMP FIN
      TAD CAC       / Put back AC
      IAC           / Next AC
      JMP LOOP      / Try again
FIN,  CLL           / If we are here L != 0 (but AC == 0.. so great)
      MQL MQA       / Swap MQ and AC
      RAL           / Left shift MQ
      SZL           / Skip if didn't overflow, L == 0
      CLA CLL IAC   / Reset MQ to 1
      MQL MQA       / Put back MQ, AC still 0
      JMP LOOP

binary

0000 0000
0001 0000
0002 7301
0003 7421
0004 3000
0005 1000
0006 7041
0007 3001
0010 7404
0011 1001
0012 7650
0013 5017
0014 1000
0015 7001
0016 5004
0017 7300
0020 7521
0021 7004
0022 7430
0023 7301
0024 7521
0025 5004