That OctalDecimal Table 383

+--------------- Rob Warnock I had been hastily going through my copy of the LGP-30 manual (not physical) to try and verify that, but I could not before posting. +---------------

I have a hard-copy LGP-30 maintenance manual at home. I'll look up the page number for you. But I'm 100% sure that the LGP-30 used "0123456789fgjkqw", because it was my very first computer!!! ;-}

Also, since the address field was not right-justified in the instruction, addresses as typed appeared to be incremented by 4, e.g., the first few locations in the machine were 0, 4, 8, j, 10, 14, 18, 1j pronounced "J-teen", 20, 24, 28, 2j "Twenty-J", etc.

By the way, I suspect the reason they used "fgjkqw" was that they'd already used "abcde" for instruction mnemonics... and the low-order four bits of the Flexowriter code for an instruction mnemonic *was* the 4-bit operation code for the instruction! So when they were picking characters to use for hexadecimals, they picked a *different* set of characters from the instruction mnemonics whose low-order four bits were binary 1010, 1011, 1100, 1101, 1110, & 1111. Here's a little table of hex codes, instruction mnemonic characters, and instruction descriptions:

hex instr. code mnem. 0 Z Stop conditional on console switches & track addr 1 B Bring load

4 I Input shift one 4- or 6-bit char from KBD-tape into AC 5 D Divide 6 N Multiply saves lower half of result, usually an integer 7 M Multiply saves upper half of result, usually a fraction

8 P Print shift one char out of AC to typewriter-tape 9 E Extract logical-AND memory to AC f U Unconditional branch g T Test and branch if AC negative

j H Hold store AC, leaving AC unchanged k C Clear store & clear accumulator (AC) q A Add memory to AC w S Subtract

Yes, this implies that instead of writing "a1f3j" (add location 0x1a3c) you could just as well write "q1u3h" -- it got read exactly the same.

-Rob

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