[pdp7-unix] A.out format and instruction encoding

Will Senn will.senn at gmail.com
Wed Oct 23 01:38:50 AEST 2019


All,
 
Is there a description of the pdp7-unix a.out format, somewhere? I’ve looked and didn’t see anything. I also tried to read the annotated as.s file, but can’t really grok the format from there. I am trying to understand the output of:
 
@ od system cat
000000: 000004 144144 040040 040040 040040 000000 000000 000000
000010: 000006 164164 171151 156040 040040 000000 000000 000000
000020: 000007 153145 171142 157141 162144 000000 000000 000000
000030: 000010 160160 164151 156040 040040 000000 000000 000000
000040: 000013 164164 171157 165164 040040 000000 000000 000000
000050: 000014 144151 163160 154141 171040 000000 000000 000000
000060: 000015 160160 164157 165164 040040 000000 000000 000000
…
and relate it back to cat.s
“ cat.s
   lac 017777 i
   sad d4
…
 
So far, I’m not seeing it. I’m sure it’s there, but not to my untrained eye. According to the pdp7 manual lac is opcode 20, but the user manual says opcodes are coded into the 4 msb. How can 20 octal (16 decimal) fit into 4 bits? Or sad which is 54 octal?
 
Thanks,
 
Will
 

Sent from my iPhone
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