In the early 1970s, I worked in the Small Systems Group at...

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Question by MarkR
Submitted on 3/29/2004
Related FAQ: PDP-8 Frequently Asked Questions (posted every other month)
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In the early 1970s, I worked in the Small Systems Group at DEC. Shortly after I joined, someone showed me a few instructions you could toggle in which would clear one 4K memory bank. They presented it as a puzzle - i.e. can you figure out how this works. I remember going home and puzzling over it for days, but eventually I figured it out. As an intellectual exercise, I was recently trying to remember the instructions. I think I've come close, but as I try to mentally work out how they'd execute, I don't think I've got it quite right. And I don't have a PDP-8 to try it out on. (Someday I'll get around to downloading a PDP-8 emulator.) In the meantime, I have two questions. First of all, is there anyone out there who remembers the correct instructions? And second of all, is there anyone out there who knows who in the world originally wrote this code? The way the code executes is pretty convoluted, and the code has always struck me as a brilliant hack. Anyway, here's the code I remember: 0004 TAD 5 0005 DCA I 10 0006 JMP 4 0007 ISZ 10 0010 10 Mark Rosenthal mbr@arlsoft.com

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Answer by wvdmark
Submitted on 5/31/2004
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I think it goes the following way: 0005 TAD 5 0006 DCA I 10 0007 JMP 5 0010 10 10 is autoincrement starts by putting DCA I 10 until end of memory, wraps around, until: 0005 DCA I 10 /* TAD gets replaced 0006 DCA I 10 0007 JMP 5 0010 5 0011 DCA I 10 .... 0005 DCA I 10 0006 0000 /* gets wiped out with 0 (no TAD) 0007 JMP 5 0010 6 0011 DCA I 10 .... 0005 DCA I 10 0006 0000 0007 0000 /* same as before 0010 7 /* drops thru 0011 DCA I 10 .... 0005 DCA I 10 0006 0000 0007 0000 0010 0000 0011 DCA I 10 .... now we only have DCA I 10 or 0000 in memory will eventually wipe out all DCA I 10. Haven't tried it out, my PDP-8 is dusty. Is this correct?

Answer by wvdmark
Submitted on 6/1/2004
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Sorry... It was supposed to be TAD 6

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