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08B6: THE 'MERGE' CONTROL ROUTINE
Used by the routine at SAVE_ETC.
There are three main parts to this routine.
  • Load the data block into the work space.
  • Merge the lines of the new program into the old program.
  • Merge the new variables into the old variables.
Start therefore with the loading of the data block.
Input
IX Address of the header loaded from tape
Output
ME_CONTRL 08B6 LD C,(IX+$0B) Fetch the 'length' of the data block.
08B9 LD B,(IX+$0C)
08BC PUSH BC Save a copy of the 'length'.
08BD INC BC Now make 'length+1' locations available in the work space.
08BE RST $30
08BF LD (HL),$80 Place an end marker in the extra location.
08C1 EX DE,HL Move the 'start' pointer to the HL register pair.
08C2 POP DE Fetch the original 'length'.
08C3 PUSH HL Save a copy of the 'start'.
08C4 PUSH HL Now set the IX register pair for the actual load.
08C5 POP IX
08C7 SCF Signal 'LOAD'.
08C8 LD A,$FF Signal 'data block only'.
08CA CALL LD_BLOCK Load the data block.
The lines of the new program are merged with the lines of the old program.
08CD POP HL Fetch the 'start' of the new program.
08CE LD DE,($5C53) Initialise DE to the 'start' of the old program (PROG).
Enter a loop to deal with the lines of the new program.
ME_NEW_LP 08D2 LD A,(HL) Fetch a line number and test it.
08D3 AND $C0
08D5 JR NZ,ME_VAR_LP Jump when finished with all the lines.
Now enter an inner loop to deal with the lines of the old program.
ME_OLD_LP 08D7 LD A,(DE) Fetch the high line number byte and compare it. Jump forward if it does not match but in any case advance both pointers.
08D8 INC DE
08D9 CP (HL)
08DA INC HL
08DB JR NZ,ME_OLD_L1
08DD LD A,(DE) Repeat the comparison for the low line number bytes.
08DE CP (HL)
ME_OLD_L1 08DF DEC DE Now retreat the pointers.
08E0 DEC HL
08E1 JR NC,ME_NEW_L2 Jump forward if the correct place has been found for a line of the new program.
08E3 PUSH HL Otherwise find the address of the start of the next old line.
08E4 EX DE,HL
08E5 CALL NEXT_ONE
08E8 POP HL
08E9 JR ME_OLD_LP Go round the loop for each of the 'old lines'.
ME_NEW_L2 08EB CALL ME_ENTER Enter the 'new line' and go round the outer loop again.
08EE JR ME_NEW_LP
In a similar manner the variables of the new program are merged with the variables of the old program.
ME_VAR_LP 08F0 LD A,(HL) Fetch each variable name in turn and test it.
08F1 LD C,A
08F2 CP $80 Return when all the variables have been considered.
08F4 RET Z
08F5 PUSH HL Save the current new pointer.
08F6 LD HL,($5C4B) Fetch VARS (for the old program).
Now enter an inner loop to search the existing variables area.
ME_OLD_VP 08F9 LD A,(HL) Fetch each variable name and test it.
08FA CP $80
08FC JR Z,ME_VAR_L2 Jump forward once the end marker is found. (Make an 'addition'.)
08FE CP C Compare the names (first bytes).
08FF JR Z,ME_OLD_V2 Jump forward to consider it further, returning here if it proves not to match fully.
ME_OLD_V1 0901 PUSH BC Save the new variable's name whilst the next 'old variable' is located.
0902 CALL NEXT_ONE
0905 POP BC
0906 EX DE,HL Restore the pointer to the DE register pair and go round the loop again.
0907 JR ME_OLD_VP
The old and new variables match with respect to their first bytes but variables with long names will need to be matched fully.
ME_OLD_V2 0909 AND $E0 Consider bits 7, 6 and 5 only.
090B CP $A0 Accept all the variable types except 'long named variables'.
090D JR NZ,ME_VAR_L1
090F POP DE Make DE point to the first character of the 'new name'.
0910 PUSH DE
0911 PUSH HL Save the pointer to the 'old name'.
Enter a loop to compare the letters of the long names.
ME_OLD_V3 0912 INC HL Update both the 'old' and the 'new' pointers.
0913 INC DE
0914 LD A,(DE) Compare the two letters.
0915 CP (HL)
0916 JR NZ,ME_OLD_V4 Jump forward if the match fails.
0918 RLA Go round the loop until the 'last character' is found.
0919 JR NC,ME_OLD_V3
091B POP HL Fetch the pointer to the start of the 'old' name and jump forward - successful.
091C JR ME_VAR_L1
ME_OLD_V4 091E POP HL Fetch the pointer and jump back - unsuccessful.
091F JR ME_OLD_V1
Come here if the match was found.
ME_VAR_L1 0921 LD A,$FF Signal 'replace' variable.
And here if not. (A holds +80 - variable to be 'added'.)
ME_VAR_L2 0923 POP DE Fetch pointer to 'new' name.
0924 EX DE,HL Switch over the registers.
0925 INC A The zero flag is to be set if there is to be a 'replacement', reset for an 'addition'.
0926 SCF Signal 'handling variables'.
0927 CALL ME_ENTER Now make the entry.
092A JR ME_VAR_LP Go round the loop to consider the next new variable.
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