Path: utzoo!attcan!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.arch
Subject: Execute instructions
Keywords: writing return address to routine front
Message-ID: <11588@steinmetz.ge.com>
Date: 20 Jul 88 13:51:54 GMT
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 31


  The old GE600 series had two instructions called XEC (execute) and XED
(execute double). What these did was to fetch an operation from the
target location and execute it (or them) while leaving the IC pointing
at the XED instruction. Since the machine was able to apply IC
modification to any operation, jumps relative to the original
instruction were allowed.

  Here is how it could be used for a procedure call:
	xed	proc1		; call proc1


proc1:	even
	stc1	stak,di		; push on stack "stak" the IC+1 and
				; flags, IC still at XED
	jmp	*+1		; 1st instruction in proc1
	.
	.
	ldi	stak,i		; reload flags, skip if not needed
	jmp	stak,idc	; return to caller (I may not have the
				; modifier right on this one)

  What other machines have this type feature, and do people use it
anymore? It allowed the called program to save the registers modified,
was a one word calling sequence, and didn't destroy ANY registers (the
regular calls set the return in a register). It also saved the flags,
which could be preserved or not.
-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me