ado@elsie.UUCP (Arthur David Olson) (04/22/87)
> . . .the PDP-11 JSR instruction allows you to specify *any* of the eight > "GPRs" to hold the current PC contents, after pushing the old value of the > specified register onto the stack. (Among other purposes, this was used. . . > for passing the address of an in-line FORTRAN parameter address block. . . Yet another purpose: if you wanted to push register r0 onto the stack without changing the processor status word (as low-level interrupt handlers were wont to do), then instead of using the instruction mov r0,-(sp) you could use the instruction jsr r0,(pc) to turn the trick. You did clobber register zero--but you had just saved it away, yes? -- "We must choose: the stars or the starving. Which shall it be?" UUCP: ..seismo!elsie!ado ARPA: elsie!ado@seismo.CSS.GOV Elsie and Ado are trademarks of Borden, Inc. and Ampex.