MACRO-11 help

Pete Turnbull wrote:

Well, I'd say the "right thing to do" is to use absolute addressing modes. What modes 6/7 do is generate references which are relative to the instruction address. They're meant for things that will move if the code is moved (eg the target of a JSR). However, the address of a device register doesn't move, so although it's possible to refer to using modes 6/7, it breaks the position-independance of the code. If you move the code, you need to recalculate the address offset. Once assembled, the first part of Pat's code, despite using mode 6, is not PIC: 4 177564 XSR=177564 5 177566 XBUF=177566 6 7 001000 012767 000110 177566' MOV #110,XBUF 8 001006 105767 177564' L1: TSTB XSR 9 001012 100375 BPL L1 10 001014 012767 000064 177566' MOV #64,XBUF 11 001022 105767 177564' L2: TSTB XSR 12 001026 100375 BPL L2 13 001030 012767 000130 177566' MOV #130,XBUF The actual values in the finally-assembled code would be (if I've done my arithmetic correctly): 7 001000 012767 000110 176552 MOV #110,XBUF 8 001006 105767 176550 L1: TSTB XSR 9 001012 100375 BPL L1 10 001014 012767 000064 176536 MOV #64,XBUF 11 001022 105767 176534 L2: TSTB XSR 12 001026 100375 BPL L2 13 001030 012767 000130 17652 2 MOV #130,XBUF Note that the pointers to XSR and XBUF change according to their position in the code, because they're PC-relative (even if my PC-relative arithmetic is wrong, the differences are correct). Now if he'd written it using absolute addresses: 7 001000 012737 000110 177566 MOV #110,@#XBUF 8 001006 105737 177564 L1: TSTB @#XSR 9 001012 100375 BPL L1 10 001014 012737 000064 177566 MOV #64,@#XBUF 11 001022 105737 177564 L2: TSTB @#XSR 12 001026 100335 BPL L2 13 001030 012737 000130 177566 MOV #130,@#XBUF ... they don't change. Moreover, since there are no JMPs or JSRs, only (relative) BRanches, this is PIC. Now if he wanted to write it using a subroutine, and he used the simple method: 7 001000 012701 000110 MOV #'H', R1 8 001006 004537 JSR R5, @#PUT 9 001012 012701 000064 MOV #'4', R1 10 001014 004537 JSR R5, @#PUT 11 001022 012701 000130 MOV #'X', R1 12 001026 004537 JSR R5, @#PUT : : 35 001100 105767 177564 PUT: TSTB @#XSR 36 001104 100375 BPL PUT 37 001106 010137 177566 MOV R1, @#XBUF 38 001112 000205 RTS R5 That's hand-assembled, in a hurry, so don't rely on the opcodes being correct! but the point is that the subroutine is referred to by its absolute address, so the first part of the code can be moved but not the subroutine (because its address is fixed). However, if it were written using relative addressing, ie 8 001006 004567 JSR R5, PUT : 35 001100 105767 177564 PUT: TSTB @#XSR then the whole thing can be placed anywhere in memory without alteration (so long as the subroutine stays in the same position relative to the rest). Of course, the subroutine still uses absoute addresses for the registers. Does that make it clear what the point of the two types of addressing is? My example is a bit contrived; that's not really how I'd do it. Instead, I'd write a subroutine that wrote out a whole string, the string being stored immediately after the JSR, and returning to the word after the string. But that's another story. -- Pete Peter Turnbull Network Manager University of York