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13422: THE 'UNARY MINUS' OPERATION (offset 27)
Used by the routine at subtract.
The address of this routine is found in the table of addresses. It is called via the calculator literal 27 by the routines at CD_PRMS1, get_argt, cos, atn, asn and acs. It is also called indirectly via fp_calc_2.
This subroutine performs its unary operation by changing the sign of the 'last value' on the calculator stack.
Zero is simply returned unchanged. Full five byte floating-point numbers have their sign bit manipulated so that it ends up reset (for 'abs') or changed (for 'negate'). 'Small integers' have their sign byte set to zero (for 'abs') or changed (for 'negate').
HL Address of the first byte of the number
negate 13422 CALL TEST_ZERO If the number is zero, the subroutine returns leaving 00 00 00 00 00 unchanged.
13425 RET C
13426 LD B,0 B is set to 0 for 'negate'.
This entry point is used by the routine at abs.
NEG_TEST 13428 LD A,(HL) If the first byte is zero, the jump is made to deal with a 'small integer'.
13429 AND A
13432 INC HL Point to the second byte.
13433 LD A,B Get 255 for 'abs', 0 for 'negate'.
13434 AND 128 Now 128 for 'abs', 0 for 'negate'.
13436 OR (HL) This sets bit 7 for 'abs', but changes nothing for 'negate'.
13437 RLA Now bit 7 is changed, leading to bit 7 of byte 2 reset for 'abs', and simply changed for 'negate'.
13438 CCF
13439 RRA
13440 LD (HL),A The new second byte is stored.
13441 DEC HL HL points to the first byte again.
13442 RET Finished.
The 'integer case' does a similar operation with the sign byte.
13444 PUSH HL Save pointer to the number in HL.
13445 CALL INT_FETCH Fetch the sign in C, the number in DE.
13448 POP HL Restore the pointer to the number in HL.
13449 LD A,B Get 255 for 'abs', 0 for 'negate'.
13450 OR C Now 255 for 'abs', no change for 'negate'.
13451 CPL Now 0 for 'abs', and a changed byte for 'negate'; store it in C.
13452 LD C,A
13453 CALL INT_STORE Store result on the stack.
13456 POP DE Return STKEND to DE.
13457 RET
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