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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><!-- Nested and Recursive Macro Calls --><HTML><HEAD><TITLE>Nested and Recursive Macro Calls</TITLE></HEAD><BODY BACKGROUND="spiral.gif" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#990099" ALINK="#FF0000"><TABLE WIDTH="98%"><TR><TD WIDTH=60 VALIGN=BOTTOM NOWRAP><IMG SRC="transp.gif" ALT="" WIDTH=60 HEIGHT=20></TD><TD><!-- Ab hier Seitentext: --><BIG><BR><P><STRONG><U>III.11.7 Nested and Recursive Macro Calls</U></STRONG></P><P>Macro bodies may also contain macro calls, and so may the bodies of thosecalled macros, and so forth.<BR>If a macro call is seen throughout the expansion of a macro, the assemblerstarts immediately with the expansion of the called macro. For this, itsits expanded body lines are simply inserted into the expanded macro body ofthe calling macro, until the called macro is completely expanded. Then theexpansion of the calling macro is continued with the body line followingthe nested macro call.</P><P><BR><STRONG>Example 1:</STRONG><PRE> INSIDE MACRO SUBB A,R3 ENDM OUTSIDE MACRO MOV A,#42 INSIDE MOV R7,A ENDM</PRE><BLOCKQUOTE>In the body of the macro OUTSIDE, the macro ISIDE is called.If OUTSIDE is called (and the list mode is set to $GENONLY),one gets something like the following expansion:</BLOCKQUOTE><PRE> Line I Addr Code Source 15+ 1 0000 74 2A MOV A,#42 17+ 2 0002 9B SUBB A,R3 18+ 1 0003 FF MOV R7,A</PRE></P><P>Since macro calls can be nested to any depth (while there is free memory),the macro expansion level is shown in the I-column of the list file.Since macro and include file levels can be nested in arbitrary sequence anddepth, the nesting level is counted through all macro and include filelevels regardless. For better distinction, the character following theglobal line number is ':' for include file levels, and '+' for macro levels.</P><P>If macros are calling themselves, one speaks of recursive macro calls.In this case, there must be some stop criterion, to prevent the macro ofcalling itself over and over until the assembler is running out of memory!Here again, conditional assembly is the solution:</P><P><BR><STRONG>Example 2:</STRONG><BLOCKQUOTE>The macro COUNTDOWN is to define 16-bit constants from1 thru n in descending order in ROM. n can be passed tothe macro as a parameter:</BLOCKQUOTE><PRE> COUNTDOWN MACRO DEPTH IF DEPTH GT 0 DW DEPTH COUNTDOWN %DEPTH-1 ENDIF ENDM</PRE><BLOCKQUOTE>If COUNTDOWN is called like this,</BLOCKQUOTE><PRE> COUNTDOWN 7</PRE><BLOCKQUOTE>something like the following macro expansion results(in list mode $GENONLY/$CONDONLY):</BLOCKQUOTE><PRE> Line I Addr Code Source 16+ 1 0000 00 07 DW 7 19+ 2 0002 00 06 DW 6 22+ 3 0004 00 05 DW 5 25+ 4 0006 00 04 DW 4 28+ 5 0008 00 03 DW 3 31+ 6 000A 00 02 DW 2 34+ 7 000C 00 01 DW 1</PRE></P><P>After the Dark Ages, when the dust was settling and the sun broke throughthe gloom, computer science discovered the method of recursive programming.There was no doubt that this was the <EM>Solution</EM>!<BR>And the computer scientists started to explain this to the students.But it seemed that the students didn't get it. They always complainedthat recursive calculation of n! is a silly example indeed.<BR>All the scientists felt stronly that there was still something missing.After 10 more years of hard research work, they also found the <EM>Problem</EM>:</P><P><BR><STRONG>Example 3:</STRONG><BLOCKQUOTE><STRONG>The Towers of Hanoi</STRONG><BR><BR>There are three vertical sticks on the table. On stick 1there are n discs with different diameters and a hole inthe middle, the smallest disc on top, the biggest on the bottom.<BR><BR><CENTER><IMG SRC="hanoi.gif" ALT="The Towers of Hanoi" WIDTH=432 HEIGHT=163></CENTER><BR>The <EM>Problem</EM> is to transfer the tower of discs from stick 1to stick 2 with a minimum of moves. But only the topmostdisc on a tower may be moved at one time, and no disc maybe layed on a smaller disc. Stick 3 may be used for scratchpurposes. This is a <EM>Solution</EM> with <NOBR>ASEM-51</NOBR> macros:</BLOCKQUOTE><PRE> ;The Towers of Hanoi $GENONLY CONDONLY DISCS EQU 3 ;number of discs HANOI MACRO n, SOURCE, DESTINATION, SCRATCH IF n > 0 HANOI %(n-1), SOURCE, SCRATCH, DESTINATION ; move topmost disc from stick &SOURCE to stick &DESTINATION HANOI %(n-1), SCRATCH, DESTINATION, SOURCE ENDIF ENDM HANOI DISCS, 1, 2, 3 END</PRE><BLOCKQUOTE>The recursive macro HANOI generates an instruction manualfor the <EM>Problem</EM>, where the instructions appear as commentlines in the list file. The symbol DISCS must be set tothe desired number of discs. If HANOI is called like this,</BLOCKQUOTE><PRE> HANOI 3, 1, 2, 3</PRE><BLOCKQUOTE>the following "instruction manual" is generated:</BLOCKQUOTE><PRE> 27+ 3 ; move topmost disc from stick 1 to stick 2 35+ 2 ; move topmost disc from stick 1 to stick 3 44+ 3 ; move topmost disc from stick 2 to stick 3 53+ 1 ; move topmost disc from stick 1 to stick 2 64+ 3 ; move topmost disc from stick 3 to stick 1 72+ 2 ; move topmost disc from stick 3 to stick 2 81+ 3 ; move topmost disc from stick 1 to stick 2</PRE><BLOCKQUOTE>The GENONLY and CONDONLY controls ensure that the tabledoesn't contain all the macro calls and IF constructions.<BR><BR><STRONG>Exercise 1:</STRONG><BLOCKQUOTE>Modify the macro HANOI so that it is generating a movetable in ROM, which could directly be used as an inputfor an 8051-controlled robot-arm that really plays thegame with 3 real sticks and n real discs.</BLOCKQUOTE><STRONG>Exercise 2:</STRONG><BLOCKQUOTE>Prove that the minimum number of moves is 2<SMALL><SUP>n</SUP></SMALL> - 1. <IMG SRC="smile.gif" ALT="(smile)" WIDTH=15 HEIGHT=15></BLOCKQUOTE></BLOCKQUOTE></P></BIG><!-- Seitentext Ende --><P><BR><BR><CENTER><TABLE WIDTH="70%"><TR><TH><A HREF="contents.htm"><IMG SRC="home.gif" ALT="[contents]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="macros.htm"><IMG SRC="up.gif" ALT="[up]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="exitm.htm"><IMG SRC="back.gif" ALT="[back]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH> <TH><A HREF="nestdefs.htm"><IMG SRC="next.gif" ALT="[next]" BORDER=0 WIDTH=32 HEIGHT=32></A></TH></TR></TABLE></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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