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2F9B: THE 'PREPARE TO ADD' SUBROUTINE
Used by the routine at addition.
This subroutine is the first of four subroutines that are used by the main arithmetic operation routines - subtract, addition, multiply and division.
This particular subroutine prepares a floating-point number for addition, mainly by replacing the sign bit with a true numerical bit 1, and negating the number (two's complement) if it is negative. The exponent is returned in the A register and the first byte is set to +00 for a positive number and +FF for a negative number.
Input
HL Address of the first byte of the number
Output
A Exponent byte
PREP_ADD 2F9B LD A,(HL) Transfer the exponent to A.
2F9C LD (HL),\$00 Presume a positive number.
2F9E AND A If the number is zero then the preparation is already finished.
2F9F RET Z
2FA0 INC HL Now point to the sign byte.
2FA1 BIT 7,(HL) Set the zero flag for positive number.
2FA3 SET 7,(HL) Restore the true numeric bit.
2FA5 DEC HL Point to the first byte again.
2FA6 RET Z Positive numbers have been prepared, but negative numbers need to be two's complemented.
2FA7 PUSH BC Save any earlier exponent.
2FA8 LD BC,\$0005 There are 5 bytes to be handled.
2FAB ADD HL,BC Point one past the last byte.
2FAC LD B,C Transfer the 5 to B.
2FAD LD C,A Save the exponent in C.
2FAE SCF Set carry flag for negation.
NEG_BYTE 2FAF DEC HL Point to each byte in turn.
2FB0 LD A,(HL) Get each byte.
2FB1 CPL One's complement the byte.