williams-et-al-1983-apple-ii-computer-graphics

28 APPLE II COMPUTER GRAPHICSally prints the value returned by the subroutine. The subroutine in thisexample returns only garbage.The following alterations are those which have been made to Listing4-3:Lines changed: 1 (/), 7(/), 16(/), 17(/), 24(/)Lines added : 65, 134, 136, 172, 174,1 76, 1 78,2(/)5These differences are explained below.65 POKE 1, 76: POKE 11, : POKE 12, 3Line 65 places information into memory which is needed by the USRcommand. When BASIC encounters USR, it automatically jumps to location1 (/), and from there to the location of the subroutine which is to beused. The particular values POKEd here direct the computer to jump tolocation $3 'where lies the subroutine. Location 11 contains the secondbyte of the subroutine's addre\s, in this case $ 'and location 12 containsthe fi rst byte-$3. If the subroutine begins in a different location, say$C25, the values 37 ($25 = 2*16 + 5) and 192 ($Cf/) = 12*16 + (/))would be POKEd into locations 11 and 12, respectively.7 FOR L =768 TO 85The loop goes from 768 to 8(/)5 instead of to 8(/)2 because of the three extradata values which have been added in line 2(/)5. The effect of those threeextra values is to accept the character code which is passed by the USRcommand.134 PRINT "WHAT CHARACTER DO YOUWISH TO PRINT? 11136 GET C$:C = ASC (C$) + 128These lines GET the character that the user wishes to pri nt and convert it toits Apple code value.16 PRINT USR ( C)This line passes the selected character code to the subroutine and thenexecutes it. When the subroutine is fi nished, this statement also prints thevalue which is returned by the subroutine. In this case the value printed ismeaningless and it will not be printed if line 16(/) is changed to read 16X =USR(C).17 VTAB (22) : PRINT "ALL DONE"The VTAB has been changed to 22 to allow program line 172 to print onscreen line 23, and the GOTO has been removed since the program nolonger ends at this point.172 PRINT "ANOTHER CHARACTER? (Y/N) 11 : GET R$