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He said shell. Which is an equivalent to the -20's exec. Memory management of the -20 side...
He didn't say "monitor code".
Remember, Barb, the Tops-20 monitor has almost no command code built in:
SUBTTL Mini-EXEC -- Command Dispatch ;MINI-EXEC...
EXECT1: PBIN ;GET USER INPUT CAIN T1,.CHCRT ;FLUSH CR JRST EXECT1 CAIL 1,"A"+40 ;LC CHAR? CAILE 1,"Z"+40 SKIPA ;NO SUBI 1,40 ;YES, CONVERT CAIN T1,.CHLFD ;IGNORE BLANK LINE JRST EXECT0 MOVSI 4,-NXCMD ;SCAN COMMAND TABLE EXECT2: HLRZ 2,XCMD(4) ;GET KEY LETTER FOR THIS COMMAND CAMN 1,2 ;SAME AS JUST TYPED? JRST HRRZ 2,XCMD(4) ;YES, GET DISPATCH ADDRESS JRST 0(2) ;GO TO IT AOBJN 4,EXECT2 ;TRY ALL COMMANDS JRST EXERR ;NOT FOUND, INDICATE ERROR
;MINI-EXEC COMMANDS
XCMD: CM B,EBLT ;BLT SWP MON CM D,EDMP ;DUMP ON FILE CM E,GEX ;EXEC CM G,EGET ;GET FILE CM J,EJMP ;JUMP TO ADDRESS CM R,EREST ;RESET CM S,EST ;START NXCMD==.-XCMD
And the mini-EXEC can only be entered in two ways: An enabled wheel can run it by using the ^EQUIT command (rarely needed), or more likely the monitor is being installed, and the EXEC has not yet been loaded from tape to disk. So by far the most common mini-EXEC command sequence (where brackets surround monitor typeout) is
to load the next program (should be EXEC.EXE) from the install tape and start it.
The Unix shell, like the Tops-20 EXEC, is user-mode code that is loaded at the time it is needed by the kernel-monitor, and sits at the top of the user's process tree. The Unix shell has fewer commands built into it than EXEC, but if you include PCL (Programmable Command Language) into you EXEC you can even subsbreastute user-mode programs for built-in commands so that they don't get in your way.
-- Rich Alderson "ASCII ribbon "You get what anybody gets. You get a lifetime." x HTML mail and --rest, of the Endless postings