Routines
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13627: THE 'COMPARISON' OPERATIONS (offsets 9 to 14, 17 to 22)
The address of this routine is found in the table of addresses. It is called indirectly via fp_calc_2.
This subroutine is used to perform the twelve possible comparison operations (offsets 9 to 14 and 17 to 22: '<=', '>=', '<>', '>', '<' and '=' for numbers and strings respectively). The single operation offset is present in the B register at the start of the subroutine.
Input
B Operation offset (9 to 14, 17 to 22)
DE Address of the first byte of the second argument
HL Address of the first byte of the first argument
compare 13627 LD A,B The single offset goes to the A register.
13628 SUB 8 The range is now 1 to 6 and 9 to 14.
13630 BIT 2,A This range is changed to 0, 1, 2, 4, 5, 6, 8, 9, 10, 12, 13, 14.
13632 JR NZ,EX_OR_NOT
13634 DEC A
EX_OR_NOT 13635 RRCA Then reduced to 0 to 7 with carry set for 'greater than or equal to' and 'less than'; the operations with carry set are then treated as their complementary operation once their values have been exchanged.
13636 JR NC,NU_OR_STR
13638 PUSH AF
13639 PUSH HL
13640 CALL exchange
13643 POP DE
13644 EX DE,HL
13645 POP AF
NU_OR_STR 13646 BIT 2,A The numerical comparisons are now separated from the string comparisons by testing bit 2.
13648 JR NZ,STRINGS
13650 RRCA The numerical operations now have the range 0 to 1 with carry set for 'equal' and 'not equal'.
13651 PUSH AF Save the offset.
13652 CALL subtract The numbers are subtracted for the final tests.
13655 JR END_TESTS
STRINGS 13657 RRCA The string comparisons now have the range 2 to 3 with carry set for 'equal' and 'not equal'.
13658 PUSH AF Save the offset.
13659 CALL STK_FETCH The lengths and starting addresses of the strings are fetched from the calculator stack.
13662 PUSH DE
13663 PUSH BC
13664 CALL STK_FETCH
13667 POP HL The length of the second string.
BYTE_COMP 13668 LD A,H
13669 OR L
13670 EX (SP),HL
13671 LD A,B
13672 JR NZ,SEC_PLUS Jump unless the second string is null.
13674 OR C
SECND_LOW 13675 POP BC Here the second string is either null or less than the first.
13676 JR Z,BOTH_NULL
13678 POP AF
13679 CCF The carry is complemented to give the correct test results.
13680 JR STR_TEST
BOTH_NULL 13682 POP AF Here the carry is used as it stands.
13683 JR STR_TEST
SEC_PLUS 13685 OR C
13686 JR Z,FRST_LESS The first string is now null, the second not.
13688 LD A,(DE) Neither string is null, so their next bytes are compared.
13689 SUB (HL)
13690 JR C,FRST_LESS Jump if the first byte is less.
13692 JR NZ,SECND_LOW Jump if the second byte is less.
13694 DEC BC The bytes are equal; so the lengths are decremented and a jump is made to BYTE_COMP to compare the next bytes of the reduced strings.
13695 INC DE
13696 INC HL
13697 EX (SP),HL
13698 DEC HL
13699 JR BYTE_COMP
FRST_LESS 13701 POP BC
13702 POP AF
13703 AND A The carry is cleared here for the correct test results.
STR_TEST 13704 PUSH AF For the string tests, a zero is put on to the calculator stack.
13705 RST 40
13706 DEFB 160 stk_zero
13707 DEFB 56 end_calc
END_TESTS 13708 POP AF These three tests, called as needed, give the correct results for all twelve comparisons. The initial carry is set for 'not equal' and 'equal', and the final carry is set for 'greater than', 'less than' and 'equal'.
13709 PUSH AF
13710 CALL C,f_not
13713 POP AF
13714 PUSH AF
13715 CALL NC,greater_0
13718 POP AF
13719 RRCA
13720 CALL NC,f_not
13723 RET Finished.
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